Symposium SF06—Recent Advances in Structural Materials from Bulk to Nanoscale

The studies on the mechanical behavior of nanoporous gold (np-Au) have been intensively conducted to apply it to practical applications by the benefit of the high surface-to-volume ratio and chemical inertness. There is a phenomenon of “smaller is stronger” in nanoporous gold because of nano-sized constituent material, ligament, so mechanical properties increase with decreasing ligament size which called ligament-size effect. However, the analysis of mechanical behavior of np-Au is more required with the state of removed grain boundaries and cracks. In addition, the study on the creep, time-dependent deformation, is rare although it could be important factor for durability and life-time for materials. We investigated the mechanical properties of np-Au in tension, compression, and creep with ligament-size effect. Four np-Au samples with ligament sizes ranging from 30 nm to 986 nm were prepared without grain boundaries and cracks by severe thermal annealing and selecting the micro-sized gauge region. Regarding the tensile and compressive tests, we confirmed important things related with tension-compression asymmetry: (1) the higher yield strengths in tension than in compression and (2) the greater size effect exponent n in compression that in tension while there is no ligament-size effect beyond ligament size of 402 nm in compression. We discuss the tension-compression asymmetry and ligament-size effect by the effect of loading mode on deformation behavior and dislocation activities in the ligaments. Regarding the creep tests, we additionally prepared the np-Au samples coated with thin amorphous alumina layer by ALD (atomic layer deposition) process to figure out the creep behavior of np-Au. The nanoindentation creep tests with spherical tip were performed for uncoated and coated np-Au samples in a single grain region. We found the ligament-size dependent creep in the range of ligament size of 103 nm to 986 nm; the increase of total creep strain and creep rate with increasing the ligament size while 30 nm np-Au did not follow this trend, but exhibited the greatest creep. We discuss the dominant creep mechanism and dislocation mechanism for respective ligament size for analyzing the ligament-size dependent creep.

Microstructural features such as twins and grain boundaries directly impact mechanical behavior in metals and when engineered can push properties to new extremes. Coupled with the complexities of printed architectures, additive manufacturing has opened a new realm of microstructural engineering as it pushes the conditions of material processing into an unexplored realm. We have recently developed a new method for additive manufacturing called Hydrogel Infusion (HI) which converts hydrogels printed via vat polymerization into various metals and metal alloys through a calcining and reduction process. This technique introduces a new opportunity for microstructural engineering in architected materials as the combustion synthesis during processing provides a unique environment for nucleation and growth of the constituent material. Additionally, HI can produce microlattices with critical dimensions of less than 50 µm and grain sizes ranging from several hunderds of nanometers to tens of microns, establishing a new space for printed metallic structures and enabling for fundamental investigations of nano-plasticity.

This work highlights the highly twinned microstructures of metals and alloys produced by HI. Through EBSD mapping, nanoindentation tests, and other nanomechanical experiments, we delve into the role of various microstructural features on the enhanced mechanical behavior of our fabricated copper, nickel, and copper-nickel alloys. We also examine the impact of these microstructurally heightened properties on the structural performance of the microlattices. This work provides direct insight to the dominant plasticity mechanisms in materials produced by this novel technique, advances our understanding of the relationship between highly twinned microstructures and material properties, and demonstrates a unique method for materials engineering through microstructure.

Nanoarchitectures that combine the nanomechanical size effects with conventional structural engineering offer the opportunity to establish a new kind of property-structure-process relationship. In spite of the successful demonstration of the proof-of-concept, scalable, and facile fabrication techniques enabling macroscopically producing nanoarchitectures at a low cost are required to utilize these unique materials for specific applications. Unlike conventional nanofabrication techniques, Proximity-field nanoPatterning (PnP), one of the interference lithographic processes, is capable of simultaneously obtaining the high spatial resolution and scalable fabrication in synthesizing such nanoarchitectures in the form of an inch-scale film. We reported continuous and ordered thin-shell (<60 nm) oxide nanoarchitectures fabricated by PnP and material conversion techniques, and applied them to inch-scale multifunctional nanocomposite film. The Al₂O₃ nanoarchitectures evenly embedded in the polymeric matrix are capable of maximizing load transfer between oxide reinforcement and matrix, achieving metal-like hardness (~1.3 GPa). In addition, the transmittance of nanocomposite films is nearly unchanged even after 20,000 bending motions (>82.0 % at 550 nm), showing high transparency and flexibility. The nanocomposite film reinforced by thin-shell oxide nanoarchitectures can be a promising option for the protective film of foldable and flexible optodevices.

Templated porous metals with well defined, periodic pore structure on the submicrometer length scale offer an opportunity to study the effects of ligament dimensions and pore architecture on mechanical behavior at critical length scales. Furthermore, they can act as hosts for secondary ceramic phases to prepare interpenetrating metal–ceramic networks that can combine high strength, toughness, and hardness. Here, we present the synthesis, structural and mechanical characterization of colloidal-crystal templated, three-dimensionally ordered macroporous (3DOM) tungsten and 3DOM tungsten–silicon oxycarbide nanocomposites. The 3DOM tungsten materials contain periodic, interconnected macropores ~300 nm in diameter and tungsten struts tens of nanometers in diameter. Micropillar compression tests below the brittle–ductile transition temperature show failure at 50 MPa contact pressure at 30 °C, implying a ligament yield strength of approximately 6.1 GPa for a structure with 5% relative density. In situ SEM frustum indentation tests indicate local compressive strains of approximately 2% at failure at 30 °C. Increased sustained contact pressure is observed at 225 °C. The 3DOM W–SiOC nanocomposites contained crystalline tungsten and amorphous silicon oxycarbide phases, both of which formed continuous and interpenetrating networks, with some discrete free carbon nanodomains present. They maintained their periodic structure up to 1000 °C. In situ SEM micropillar compression tests demonstrated that the 3DOM W–SiOC material could sustain a maximum average stress of 1.1 GPa, a factor of 22 greater than the 3DOM W matrix, resulting in a specific strength of 640 MPa/(Mg/m³) at 30 °C. The deformation mode of 3DOM W–SiOC exhibited a transition from fracture-dominated deformation at low temperature to plastic deformation above 425 °C.

Nanoscale welding yields low-temperature and dissimilar metal sinterability. Nanoscale structures have drawn great attention in research for the past decades due to the different mechanical, optical, and other properties they can present and render to bulk materials. Additive manufacturing has embodied nanostructures for their interesting properties targeting optimum reliability. To better understand the effect of nanostructures in AM applications in terms of necking formation, electrical conductivity, and optical properties, this study explores the material interdiffusion of hierarchical nanostructures fabricated in metal powders and films during low-temperature sintering. For that, both nanoporous copper (NPC) powders and nanoporous gold (NPG) thin films of similar ligament sizes were initially obtained by chemically dealloying Cu₃₃Al₆₇ gas atomized powders and Ag₆₀Au₄₀ thin films as precursors, respectively. NPC powders were deposited on NPG films and sintered at 400 °C in reducing atmosphere (H₂/Ar). FIB cross-sections and SEM images show the necking formation in between particles through ligaments, and EDS images attest the material interdiffusion along cross-sectional interface. Low temperature sinterability in nanostructures is accredited to the thermal instability of ligaments [1], as equivalent to the Gibbs-Thomson effect in metal nanoparticles of similar size. To confirm the conductivity of NPC powders, the same were cast in a mold and sintered in both inert and reducing atmosphere up to 500 °C. Electrical conductivity was remarkably evident on temperatures as low as 200-300 °C. These studies and results shed a light on promising uses of NPC towards metal and metal-polymer 3D printing.

[1] Hakamada, M., Mabuchi, M. Thermal coarsening of nanoporous gold: Melting or recrystallization. Journal of Materials Research 24, 301–304 (2009). https://doi.org/10.1557/JMR.2009.0037

Natural materials derive their unique combination of strength and toughness from material interactions at the micro-and nanoscale, but it has been challenging to fabricate and experimentally quantify materials at these length scales. In this work, we demonstrate a strategy to study microscale toughness in a novel nanoarchitected material. We use two-photon lithography to create polymeric microscale bouligand-style beams with sub-micron features in a single-edge notch three-point bend setup and combine it with in-situ nanomechanical testing to analyze their fracture characteristics. We quantify the crack growth resistance as a function of architecture and density using the J-integral method. The specimens demonstrate a significant enhancement in energy dissipation with increasing twist angles in samples with ~80% relative density and decreasing twist angles in samples with ~50% relative density. Compared to a solid (non-architected) beam, we observe delayed crack initiation, higher normalized J toughness, and greater energy dissipated during fracture in the Bouligand samples, illustrating the role of nanoarchitecture in enhancing the damage tolerance of these materials. The results of this study provide fundamental insight into the effect of architectural tortuosity on crack propagation, energy dissipation, and toughening mechanisms at the micro-and nanoscale. Importantly, this work illustrates how to create microscale fracture specimens and understand fracture processes when the sample size is comparable to the fracture process zone size of the material. This combined understanding of the effect of architecture and specimen length scale on toughness is crucial to developing new methods to engineer stronger, tougher materials at the macroscale.

Nanostructured materials are greatly attractive due to their unique material properties with high specific surface area and low relative density. Especially, the nanostructured materials can be designed to have novel mechanical performances. Various methods to realize the enhanced mechanical performances with relatively low density using low-dimensional nanostructures have been proposed. However, low-dimensional based nanostructured materials are not suitable for practical applications because of their size limitations.
Here, we have suggested a strategy to overcome the elastic limit of ceramic material with a uniform large area, and aligned 3D nanoshell structures via proximity field nanopatterning (PnP). 3D nanostructured polymer templates are prepared by PnP, which includes an optical phase mask to generate 3D interference patterns in a photoresist film. We have selected 3D nanostructured zinc oxide (ZnO), which is well known piezoelectric ceramic material. ZnO nanoshell is deposited on the surface of the 3D polymer template using atomic layer deposition (ALD). After that, the 3D polymer template is removed by a thermal annealing process to generate a 3D ZnO nanoshell structure. The measured piezoelectric coefficient of 3D ZnO reaches up to 9.2 pm/V, which is analyzed through piezoresponse force microscopy (PFM). Also, the nanopillar compression test result shows that the elastic limit of our 3D ZnO (10%) is significantly improved than that of the reported bulk ZnO (3%). The 3D ZnO can be prepared in a large area (1 inch²) with improved mechanical performances. Therefore, we believe that our 3D nanostructures can scale nanoscience and nanotechnology into real applications.

Stainless steel is widely used in the industry because of its very high corrosion resistance and excellent mechanical strength. Copper shows a very high electric and thermal conductivity. The joining of both materials offers the potential to provide a combination of the benefits resulting from their unique properties. However, to bond these dissimilar materials mostly techniques like welding are used which typically require high temperatures leading to residual stress, solidification cracking and reduced electric and thermal conductivity.[1] Many of these problems are related to the melting of the metals and formation of an intermetallic phase at their interfaces.

In this abstract, a room temperature technique will be discussed in detail, where we demonstrate a mechanically stable galvanic copper deposition on stainless steel surfaces with extremely high interface conductivity for electrical current as well as for heat.

The copper deposition technique contains two steps. First, a special surface preparation on the stainless steel where a surface structure, the nanoscale sculpturing, full of mechanical hooks is created. The nanoscale sculpturing process has been developed for aluminium and other metals like zinc and titan to provide a microscale rough surfaces for mechanical interlocking.[2][3] Recently, it has been transferred to stainless steel for a mechanical interlocking with polymers.[4]
And second, the controlled galvanic copper deposition allowing the nucleation of copper islands within the hooking structure and further growth of a microscale galvanic copper thin film. Due to the formed hooks by the nanoscale sculpturing process, the deposited copper is mechanically stable interlocked with the stainless steel surface.

This microscale galvanic copper thin film allows now the application of standard bonding technologies onto copper (like soldering or the usage of conductive glues) and thus finally on the stainless steel.
In order to evaluate the quality of the electrical conductivity and mechanical strength of the connection, bonding with a commercially available conductive silver adhesive is applied for the copper thin film on the stainless steel samples. The results were compared to sandblasted stainless steel which was attached to the conductive adhesive as well. While in the here applied nanoscale sculpturing process the electrical conductivity reaches the limit of the conductive adhesive, the sandblasted samples show a decrease by orders of magnitude in electrical conductivity. In addition, the strength of the new copper stainless steel joints is increased by around 50% compared to the sandblasted samples.

References
[1] G. R. Joshi and V. J. Badheka, “Microstructures and Properties of Copper to Stainless Steel Joints by Hybrid FSW,” Metallogr. Microstruct. Anal., vol. 6, no. 6, pp. 470–480, 2017, doi: 10.1007/s13632-017-0398-x.
[2] H. Qiu, C. Schmidt, J. Carstensen, S. Kaps, and R. Adelung, “Nanoscale-Sculptured Al Microparticles as Universal Hierarchical Adhesion Promoters,” Phys. Status Solidi - Rapid Res. Lett., vol. 2100296, pp. 1–5, 2021, doi: 10.1002/pssr.202100296.
[3] M. Baytekin-Gerngross, M. D. Gerngross, J. Carstensen, and R. Adelung, “Making metal surfaces strong, resistant, and multifunctional by nanoscale-sculpturing,” Nanoscale Horizons, vol. 1, no. 6, pp. 467–472, 2016, doi: 10.1039/c6nh00140h.
[4] C. O. Kalu et al., “Formation of micro-mechanical interlocking sites by nanoscale sculpturing for composites or hybrid materials with stainless steel,” J. Mater. Res., vol. 35, no. 23–24, pp. 3145–3156, 2020, doi: 10.1557/jmr.2020.284.

Al alloys can exhibit very high-strengths due to nano-scale precipitates that typically form during post synthesis heat treatments. A key limitation of many of these alloys, however, is their lack of thermal stability at moderate temperature (e.g., T > 200 °C) as well as limited processing flexibility. Specifically, the number of Al alloys systems that can be synthesized via Additive Manufacturing (AM) techniques is relatively small due to issues with hot-cracking. Here we discuss the development of Al-Ce-based alloys for AM processing that show both excellent processing flexibility and nano-scaled microstructures that exhibit enhanced thermal stability relative to precipitation-strengthened Al alloys. We have used a combinatorial approach to print bulk samples over large composition ranges using multiple feedstock powders. The compositional dependence of the mechanical behavior can be rapidly characterized using hardness testing to correlate with the phase stability and microstructural evolution. The rate-controlling deformation mechanisms for the different compositions and structures can then be identified. This approach helps to accelerate the design and development of alloys for additive manufacturing.

The influence of post-fabrication heat treatment on the hydrogen-assisted failure in Inconel 718 fabricated by laser powder-bed-fusion was investigated through a series of tensile experiments. Samples subjected to two different heat treatments, viz. direct aging (DA) and homogenization plus aging (HA), were examined. Detailed microstructural analysis revealed that the DA sample exhibited a solidification substructure decorated with a high density of dislocations and precipitates while such a substructure was absent in the HA sample. The DA sample exhibited a comparatively higher strength, but a lower resistance to hydrogen embrittlement. Through a statistical analysis of the hydrogen-assisted cracks, it was revealed that the severe hydrogen embrittlement of the DA sample was attributed to the significant hydrogen-assisted intergranular cracks that occurred without the aid of slip localization. These results are discussed in terms of the changes in microstructure upon heat treatments, and their influences on the hydrogen trapping sites.

High surface area substrates are often used to explore the effect of size and structure on the properties of the nanomaterials. One of the most popular templates is anodized aluminum oxide (AAO), which has been used for the fabrication of a wide variety of advanced nanomaterials and devices. The geometry of AAO is well studied and controllable by varying the voltage applied and/or the electrolyte used. However, the utilization of the AAO as a template for large scale nanomaterial synthesis is limited due to the brittleness of the material, high anodization currents, and low temperatures required for synthesis. These technical challenges are exacerbated for wider pores as the pore diameter and required anodization voltage are proportional.
In this talk, we present a modification of Thompson’s method to fabricate highly ordered AAO on standard 76 mm Silicon wafers. Si was used due to its compatibility with standard semiconductor processing tools. High purity Al is deposited onto the Si wafer and is polished flat using a newly developed quiescent electropolishing technique. Next the Al-coated wafer goes through any number of standard lithography processes defining regions on the wafer to be anodized. The lithography process allows for these regions to have nearly any shape compatible with the technique. Anodization is conducted either through a one or two step process. The diameter of the pores is tunable between ~40 nm to ~450 nm and the length is tunable between ~500 nm - > 50μm. Throughout the talk we discuss the effects of the voltage, electrolyte and substrate temperatures, and the type of electrolyte (i.e. oxalic acid, phosphoric acid, etc.) on the dimensions and ordering of the pores.
This platform opens new opportunities to utilize standard semiconductor processes for nanomaterial synthesis at the wafer scale with highly tunable dimensions. Given the integration of silicon wafers with standard semic-conductor manufacturing processes we expect this process to have significant impacts on the adoption of nanomaterials into next generation devices. Several areas of research including, energy storage, thermoelectrics, photonics, etc. benefit from the use of nanomaterials where reducing sizes leads to dramatic increases in the various figures of merit for each field.

There is a strong interest in increasing the high temperature performance of metallic alloys used in extreme conditions, such as those found in many energy generations and propulsion systems. Microstructure refinement has long been considered a potential route for improvement; however, nanocrystalline (NC) metals and alloys (with grain size less than 100 nm) exhibit significant microstructure instability at high temperatures and even at room temperatures under, e.g., cyclic loads. Thermodynamic (solute segregation to reduce grain boundary energy) and/or kinetic approaches (solute drag, Zener pinning, etc., to pin grain boundaries) can be used to suppress grain growth through the addition of alloying elements and second phases. Some NC alloys such as Cu-10at%Ta fabricated using far from equilibrium processing techniques have displayed stability of their microstructures mostly through kinetic mechanisms at various thermo-mechanical loading conditions. However, the low melting point of Cu makes Cu alloys ill-suited for advanced applications compared to Ni and Ti. Hence, we plan to investigate a new class of NC Ni-Y alloys with stable microstructures under thermomechanical loads.
In particular, the interplay between matrix and dispersoids in alloys with NC and hierarchical, i.e., multiple characteristic length scales, microstructures, is of significant importance to understand the mechanical behavior of these materials under thermomechanical loading. High voltage X-ray diffraction measurements performed in-situ during thermomechanical loading of samples at beamline 1-ID of the Advanced Photon Source (APS) were used to characterize the evolution of both matrix and dispersoids as functions of macroscopic stress and strain in the samples. Powder consolidation at high temperatures was used to produce NC Ni-based samples alloys with intermetallic precipitates (the precipitates are expected to be ~ 4 nm in size with ~ 10 nm spacing). The NC alloys were then compared to hierarchical CG alloys with similar compositions with an ultrafine eutectic phase (~150-300 nm average lamellar spacing) synthesized using conventional arc-melting techniques in an inert atmosphere. Additionally, analysis on Cu-10Ta alloys (with precipitates of sizes 3-32 nm and grain sizes of 170 nm) synthesized by powder consolidation through equal channel angular extrusion (ECAE) processing provided a basis for comparison of properties between Ni and Cu based alloys.
Small-angle X-ray scattering (SAXS) analysis was performed to characterize dispersoid size and spacing distributions while wide-angle X-ray scattering (WAXS) was performed to quantify the evolution of lattice strains in both dispersoid and matrix as functions of macroscopic stresses and strains applied to the samples, so that their mechanical interactions, e.g., stress partitioning and its evolution with plastic strain, could be quantified and used to elucidate basic deformation mechanisms in these novel materials. This research is based upon work supported by the National Science Foundation under Grant DMR/MMN # 1810431, resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357, and guidance and help of the beamline scientist, Dr. Jun-Sang Park.

Alloying elements that i) segregate toward grain boundary or ii) facilitate the generation of low-energy boundaries (e.g. low-angle grain boundary, twin boundary) have shown to be effective approaches for stabilizing nanocrystalline materials. However, in practice, many of these successes were limited by the formation of intermetallic compound or elemental precipitates at elevated temperatures. Therefore, the temperature at which secondary phases starts to form and the attendant changes on its mechanical behavior are crucial information in designing stable nanostructured metals with exceptional strength.
Here we report the microstructural and mechanical property of sputter deposited, freestanding Ni-Mo-W alloy thin films with Mo concentration above the solubility limit (20 at.%). The as-deposited films exhibit single-phase solid solution with grains filled with nanoscale twin boundaries and possess high tensile strength of approximately 3 GPa. Upon post-annealing, grain growth and nucleation of MoNi phase was observed. Interestingly, films retained its tensile strength of 2.5-3.0 GPa even after heat treatment up to 700°C, while the elastic modulus started to increase at temperature above 500°C. We discuss the influence that precipitate can impart, including the changes in tensile and fracture properties.

Magnesium-Lithium alloys have long been investigated for their potential to realize ultralightweight structural materials, however efforts have been limited by a combination of low strength and thermal stability at low homologous temperatures. We report the implementation of severe plastic deformation (specifically, high pressure torsion (HPT)) to refine the grains structures of Mg-3.5Li-0.6Ca (wt%) alloy to the sub-micrometer regime via the imposition of hydrostatic pressure and strain at ambient temperatures. The grain refinement results in increased hardness and strength while Mg₂Ca nanoprecipitates further strengthen the alloy and hinder grain boundary movement at elevated temperatures. Recrystallization curves as a function of strain applied during HPT with concurrent microstructural and hardness evolution are reported.

Solid-state dissimilar bulk joining of additively manufactured maraging steel and commercially available martensitic stainless steel (AISI410) is accomplished through electrically assisted pressure joining (EAPJ). The additively manufactured cylindrical maraging steel specimen is fabricated to have a porous layer on the joining end, while another end is printed as a perfectly solid matrix. The porous layer significantly increases the maximum joining temperature by the locally increased electrical resistance and dramatically enhances the interfacial diffusion thickness. The effect of the porous layer on the microstructural evolution and mechanical properties is characterized by the EBSD and quasi-static tensile test, respectively. The present study demonstrates that bulk joining of additively manufactured components and conventional components can be more easily and effectively achieved with the use of an additively manufactured porous layer.

Additive manufacturing (AM) is an emerging technology due to its superiority in complex and customized design. However, in general, additively manufactured (AMed) materials inherently possess certain undesirable microstructural features, including anisotropy, hot-cracking, and inclusions, to name a few. While previous studies have mainly focused on bulk-scale mechanical testing to correlate apparent mechanical properties with microstructure, these attempts cannot fully explain the independent effect of various defects (e.g., grain/phase boundary, inclusion, pore, micro-cracks).
In this presentation, we will report the effect of titanium nitride (TiN) inclusion on the mechanical properties of AMed Inconel 718 through multi-scale mechanical characterization. First, bulk-scale tensile testing is performed to understand the macroscopic mechanical behavior of AMed Inconel 718. Each test shows a discrepancy in elongation and yield strength, which suggests that TiN inclusions highly affect the mechanical properties of AMed Inconel 718. Based on strain map analysis, X-ray tomography imaging, and finite element analysis, we suggest that the presence of titanium nitride inclusions can lead to significant strain localization, which invokes premature failure. Second, meso-scale testing is performed to maximize the effect of TiN inclusions on mechanical properties by reducing the overall volume of the specimen. Finally, the effect of a single TiN inclusion on the deformation behavior is being investigated via micro-scale experiments. Micro-scale tensile bars are fabricated utilizing femtosecond laser machining followed by focused ion beam milling, and in-situ scanning electron microscopy (SEM) tensile testing is performed using a nanoindenter. Micro-scale testing also provides a direct comparison of the mechanical behavior of specimens with and without TiN inclusions.

In this study, when different bending stresses were applied to cracked specimens of high strength steel (SKD11: HV550) immersed in 0.057M solution of acetic acid (CH3COOH), elastic waves were detected and frequency characteristics were analyzed using time-frequency analysis. The dominant frequency band by the tensile test was approximately 103 kHz, and in the acetic acid solution without stress were approximately 32 kHz and 101 kHz. The dominant frequency bands of the crack specimens in which cracks propagated were three groups of about 30-40 kHz(F1), about 60-85 kHz(F2), and about 100-110kHz(F3). The elastic wave was obtained by corrosion, pitting, crack initiation, and propagation, but no elastic wave was detected during the hydrogen aggregation time. The dominant frequency bands of the crack specimen without crack propagation were two groups of approximately 28-33 kHz(F1) and approximately 94-109 kHz(F3). This is the same as the dominant frequency band in the acetic acid solution under non-stress. The fracture surface showed many traces of pitting and corrosion regardless of the applied stress, and microcracks were observed in Cr carbide.

The effect of testing temperature on mechanical properties and deformation mechanisms was investigated by tensile testing of fine-grained magnesium samples under a range of temperatures (i.e. room temperature 20 °C, subfreezing temperatures 0 °C, −30 °C, and −60 °C) and microstructural observations of both untested and tested samples using electron backscatter diffraction. The results suggest that mechanical properties and deformation mechanisms were significantly affected by the variation of testing temperature. Strength increased linearly and ductility decreased linearly with the decrease of testing temperature. Although twinning is generally known to be athermal, this experimental study shows that tensile twinning is the dominating deformation mechanism at room temperature, and that twinning is negligible and dislocation operation is the primary deformation mechanism at 0 °C, −30 °C, and −60 °C.

It is well-known that the nanoscale structures on materials can improve mechanical properties. Since it is difficult to make nanoscale structures by mechanical deformation, a few brittle materials are used for nanoscale structures using lithography and etching. However, previous researches empirically showed that brittle materials behaved as if they were ductile materials under extremely low force. No brittle fracture but ductile deformation occurred even when they were scratched with sharp tips. The previous studies, however, showed only phenomena and no quantitative studies on ductile deformation. In this study, we applied the extremely low force on a few brittle materials such as single-crystal silicon, germanium and gallium-arsenide using a nanoscratch tester, analyzed mechanical behavior of the ductile deformation, and determined the transition point of the ductile deformation and the brittle fracture. First, we measured the lateral force and depth while applying the linear increasing normal force through moving a sharp tip along the lateral direction. The ductile deformation occurred at extremely low force and the first brittle fracture occurred in the brittle materials over a certain force varying according to the brittle materials. The variation of the forces is related to the mechanical properties, such as the toughness, fracture strength, and elasticity. The rapid change of lateral force was observed when the first brittle fracture occurred, which meant that the transition point could be determined by observing the change of lateral force. Finally, nanoscale structures were made on the brittle materials by ductile deformation, and we verified that the nanoscale structures can be made on brittle materials based on mechanical deformation.

Ceria nanoparticles (CeO₂ NPs) are used as an abrasive for silicon dioxide (SiO₂) chemical mechanical planarization (CMP) because of the strong chemical interaction between Ce³⁺ ions of Ceria NPs and hydrated silicate species on the surface of SiO₂ films. However, there is an issue from the low concentration of Ce³⁺ ions in Ceria NPs for CMP application. Herein, we present a facile method for engineering the interface of ceria NPs with high Ce³⁺ ion concentrations for improved adsorption with silicate anions. The Ce³⁺ content of ceria NPs increases as the particle size of the basic NPs reduces. The Freundlich model matches the adsorption isotherm, and the constants of adsorption capacity (K_F) and adsorption intensity (1/n) show that the adsorption affinity for silicate anions increased as Ce³⁺ concentration increase. The chemical adsorption between ceria NPs and silicate anions increase as Ce³⁺ concentration increase, resulting in a high removal rate of SiO₂ during CMP.

The research and industrial interests on aerogels are growing rapidly due to the increasing demand for high-performance thermal insulation materials. More recently, polymer-based aerogels have drawn much attention, but to date have primarily focused on those from the phenolic-group. While the candidates for polymer aerogels are very limited, the primary method for processing has been freeze drying. Challenges associated with freeze dried aerogels arise in the resultant morphology and properties which are inferior to those processed using super critical drying (SCD). This work presents SCD process optimization and process-structure-property studies on polymer nano-composite based aerogels. This work investigates the formation of gels via non-solvent induced phase separation and polymer crystallization approach, which aims to control solvent/non-solvent ratios as well as undercooling temperatures. The subsequent precursor gels are optimized to retain a semi-crystalline three-dimensional network during the SCD process. The mechanisms of solvent exchange and diffusion during initial gelation and aerogel processing have been investigated and will be presented. In addition, this unique processing approach is also used to enhance blending for polymer-nanofillers based aerogels. Current work has shown that the composite precursor gels and aerogels exhibit enhanced mechanical and thermal performance. A fundamental study regarding these materials will be presented here.

SiC monofilament reinforced titanium MMCs offer dramatic weight savings and improved mechanical performance at elevated temperatures, compared to monolithic aerospace alloys¹. Their implementation in aerospace applications will enable significant reductions in CO₂ emissions.
UK based company, TISICS, manufactures a 140 µm diameter SiC monofilament (SM3256) via a high-speed chemical vapour deposition (CVD) process². These monofilaments consist of stoichiometric SiC deposited onto a 15 µm diameter W core, with a bilayer coating deposited on top containing: an inner SiC_x layer, and an outer amorphous C layer. The exceptional mechanical properties, narrow strength distribution, and the reinforcing performance of the coated fibres in composites of SM3256 is dependent on the complex microstructures, more precisely at the interfaces between the sub-components, present in the monofilament.
At loads exceeding the expected composite performance, it is desirable that the fracture of the monofilaments initiates at the SiC/W reaction zone (at the interface between the SiC and W core). Furthermore, the activation of the toughening mechanisms in composites requires the debonding of the coating layers to arrest crack propagation.
In this presentation, we investigate the interfaces within SM3256 that are critical to its performance. Complimentary use of Tip Enhanced Raman Spectroscopy (TERS), HRTEM, SEM and confocal Raman spectroscopy were employed to explore the complex microstructures and phases within the SiC bulk and its interfaces. Investigation of the SiC_x coating using TERS and HRTEM enabled amorphous and crystalline nanostructures within the sub-micrometre coating layer to be analysed at nanoscale resolution for the first time. In the SiC-W reaction zone, distinct phases together with nano-voids were observed and examined in detail using SEM and HRTEM. The nano-voids could be acting as fracture initiation sites.
References
1. Rahman, K. M., Vorontsov, V. A., Flitcroft, S. M. & Dye, D. A High Strength Ti–SiC Metal Matrix Composite. Advanced Engineering Materials 19, (2017).
2. Rix, M. v., Baker, M., Whiting, M. J., Durman, R. P. & Shatwell, R. A. An improved silicon carbide monofilament for the reinforcement of metal matrix composites. Minerals, Metals and Materials Series 317–324 (2017).

Chabazite (CHA), one of the most common zeolite framework types, has a remarkable capacity to accommodate a wide range of different cations within the unique CHA framework. This has led to CHA being applied extensively in ion exchange, and studied for highly selective gas sorption, most notably through a trapdoor mechanism. Here, we report the systematic study of a series of six chabazite zeolites (i.e. K-CHA, Cs-CHA, Ca-CHA, Ba-CHA, Sr-CHA and Zn-CHA) obtained by subjecting the parent chabazite (KNaCHA) to exchange operations with cations of different valences and atomic radii. These samples were examined using numerous techniques and it was found that the differences in valence and size between extra-framework cations exert a significant effect on the abundance of these cations positioned in the framework, resulting in differing nitrogen sorption ability measured in the synthesised chabazite zeolites. These findings will help to understand how the zeolite counter-cation affects the ability of the CHA material to selectively sequester and separate gases through the use of the trapdoor mechanism.

Over the last decade, high entropy oxides (HEOx) have captured attention due to their unique optical, thermal, magnetic, and electric properties since 2015. Stabilized by high configurational entropies, HEOx (usually composed of at least five different elements) can provide more design flexibility to meet present demands. The study of this new material system forms a new playground due to the potential of developing materials with outstanding performance. Here, we process the growth of epitaxial CrMnFeCoNiO thin films on SrTiO₃, MgAl₂O₄, MgO substrates by pulsed laser deposition, aiming for the exploration of new HEOx epitaxial thin film system. With the aid of x-ray diffraction (XRD), vibrating sample magnetometer (VSM), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) analysis, we unravel the enhanced low-temperature ferrimagnetic coupling as well as the variation of strain-induced magnetic anisotropy behavior. First, high resolution XRD is utilized to analyze the heteroepitaxy, film quality and lattice strain. Then, from VSM results, a feature of increasing ferrimagnetism is discovered at low temperature. Furthermore, strain-induced perpendicular magnetic anisotropy is uncovered in CrMnFeCoNiO/STO system. In addition, the local environment of each cation and coupling among magnetic ions are revealed by XAS-XMCD. With the above analyses, a design of next-generation high entropy magnetic system is provided.

In this study, the use of Ni-Ti-Cu-Zr(-Si) alloy as a metallic glass precursor for a pseudoelastic alloy, following thermoplastic forming, is investigated. The addition of Si enhanced the glass forming ability such that a fully amorphous sample with a thickness of ~450 µm was obtained with Si containing alloy. The addition of Si effectively stabilizes the supercooled liquid by significantly increasing the activation energy of glass transition. The incubation time increased with the addition of Si. The parameters relevant to pseudoelastic effects, such as the maximum indentation depth (D_max) and remnant indentation depth (D_rem), remained almost constant. However, the remnant depth ratio increased slightly with the addition of Si, which means a slight deterioration of pseudoelastic effect. The fabrication of millimeter-scale Ni-Ti-Cu-Zr-Si metallic glass was achieved via the hot pressing of overlapped as-cast plates. Through the study of pseudoelastic alloys using bulk amorphous alloys as precursors, the possibility of expanding the application of pseudoelastic alloys was explored.

Neutron grating interferometry (nGI) is a unique, nondestructive analysis technique for materials research, providing three forms of image contrast, namely transmission, differential phase, and dark field images. Most notably, the dark field image contrast is generated by the small angle neutron scattering (SANS) of material structure on a length scale, represented by the autocorrelation length (ξ) of the nGI [1]. ξ is proportional to the product of the neutron wavelength and sample-detector distance divided by the interference fringe period. Scanning dark field images over a broad range of ξ allows one to map out the pair correlation function and quantitatively assess the structural parameters of the microstructure such as size, concentration, etc. In this study, we report on the use of neutron grating interferometry (nGI) to ascertain the microstructural properties of steel samples produced by metal additive manufacturing (MAM). The dark-field images generated by nGI on our MAM samples were shown to be sensitive to the microstructure (defects, void, etc). Specifically, the selective laser melted (SLM) SS316 tensile specimens were prepared, and tensile strain was applied perpendicular or parallel to the build direction at three different tensile levels of 0 %, 75 %, and 100 % of fracture. The SS316 compression rods manufactured by SLM and directed energy deposition (DED) methods were prepared with four compression levels of 10 %, 20 %, 30 %, and 40 % of the initial rod length. Two types of nGI setups were used, Far-field interferometer (FFI) and Talbot-Lau interferometer (TLI). The microstructure behavior according to the mechanical deformation of tension and compression that was observed by nGIs can represent the mechanical properties and further predict features such as deformation, fracture, etc. We will present the nGI analysis of these MAM samples to demonstrate how the method can be used to explore hierarchical structures.
[1] Adam J.Brooks et al, Neutron interferometry detection of early crack formation caused by bending fatigue in additively manufactured SS316 dogbones, Materials & Design Volume 140, 15 February 2018, Pages 420-430

In this study, the authors create various open structures that reflect the atomic-level structural database, and design porous structures with higher Young's modulus. Firstly, the atomic positions were imported from Materials Project, an open access materials database. Then, we modeled the open structures by considering the atomic positions as vertices and connecting them with circular 1D beams. Young's modulus of the open structures was obtained by compression tests along the x-, y- and z-axis using the FEM tool. Through this process, open structures were modeled by various atomic structures belonging to different spacegroup. Among these lots of structures, authors introduce elastically stiff structures. We find which geometric factors influence the mechanical properties in open structures and discuss how these factors affect to make higher Young’s modulus.

This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. 2020R1A5A1019131).

Over the past three years, we have developed new hardware, software and testing methods to conduct nanoindentation testing at the very high strain rates. To date, indentation strain rates as high as 10⁴/s have been achieved during the initial stages of indentation contact with a Berkovich indenter, with the potential to go even higher. At the heart of the new testing system is a laser interferometer that measures indenter displacements with sub-nanometer resolution at data acquisition rates in excess of 1 MHz. High data rates are essential since the loading of the indenter usually lasts no longer than a few hundred microseconds. The new high strain rate system also incorporates a very high stiffness hexapod for precise sample positioning and alignment, and various hardware modifications that provide for the measurement of indentation load at speeds commensurate with the displacement measurements. Various testing methods have been explored, including impact tests in which the indenter is accelerated to velocities up to 0.5 m/s before contacting the specimen surface, and step load tests in which the indenter starts in contact with the specimen and is then step loaded to a high value in a relatively short period of time. Results demonstrating the capabilities and limitations of the system are presented and discussed based on experiments conducted in fused silica (as a model hard material) and aluminum (as a model soft metal). New methods for analyzing the nanoindentation data to extract hardness as a function of strain rate are also discussed.

Dislocation patterns and cell wall formation are key to understanding the mechanical properties of the materials in which they spontaneously arise, and yet the processing and self-assembly mechanisms leading to their formation continue to baffle experts. Tantalum subjected to shock loading has radical changes to its microstructures as function of grain orientation: cell walls are found in grains oriented in <001> but are suppressed in grains with <011> orientation. Here we use transmission electron microscopy (TEM) and discrete dislocation dynamics (DDD) to demonstrate a new mechanism that is key to the pattern-formation process: a coupling reaction between pseudo-dipolar mixed dislocations.
We present the first large-scale DDD simulations exhibiting self-organization of dislocation networks into cell walls in deformed BCC metal (Tantalum) persisting at strain 20%. The simulation analysis captures several important features of the dislocation cell pattern effect observed in experiments.

Graphene, a 2D material, has been rapidly gaining steam for material strengthening and protection applications at the nanoscale. Despite its single atomic thinness, it is thought to improve pre-existing desirable mechanical, electrical or optical properties, among numerous others. It is therefore highly desirable to accurately measure and understand the structure and properties of graphene commensurate on other materials.
For mechanical property characterization, nanoindentation has been widely adopted for measuring modulus and hardness of thin and thick films, and more recently for measuring the stiffness and strength of suspended 2D materials. In both cases, a measured load-displacement response must be fitted to an analytical model for extraction of mechanical properties. The Oliver-Pharr method, for example, is one of the most widely used for elastic modulus and hardness extraction using load-displacement for bulk materials supported on rigid substrates. In contrast, the mechanical properties of 2D materials are extracted using free-standing indentation techniques (FSI), where classical solutions for suspended clamped membranes are adopted for modeling their load-displacement behavior. However, the existing indentation models can not currently capture atomically thin materials on a substrate, hence the urgent need for new models capable of explaining the behavior of such systems.
In this study, we report on the mechanical property enhancement of graphene-covered copper (Cu) foils through nanoindentation and subsequent high-resolution transmission electron microscopy (TEM) techniques. We observe a clear enhancement of elastic modulus of copper when it is covered by graphene, in the range of 5 to 10% compared to graphene-free regions. No significant enhancement of hardness was measured. We present an equal-displacement model explaining the nanoindentation behavior observed for the graphene-copper system. Our simplified model takes the limit of frictionless sliding of the graphene within the indented region due to shear lag, a concept inspired by the mechanics of composite materials. It uses of both the Oliver-Pharr method for the behavior of copper in addition to the FSI model based on classical clamped membrane solutions for graphene. We also take cross sectional TEM samples of indented regions and compare the plastic behavior of graphene-covered and graphene-free regions. We observe more scattered plastic activity in the former, as opposed to the more tip-localized plasticity observed in the latter.

Nanoscale metal-graphene nanolayered composite is known to have ultra high strength due to its ability to effectively block dislocations from penetrating through the metal-graphene interface. The same graphene interface can simultaneously serve as a barrier interface for deflecting the fatigue cracks that are generated under cyclic bendings. Cu-graphene composite with repeat layer spacing of 100 nm was tested for bending fatigue at 1.6% and 3.1% strain up to 1,000,000 cycles that indicated ~5 times enhancement in robustness against fatigue induced damage in comparison to the conventional Cu only thin film. Fatigue induced cracks that are generated within the Cu layer were stopped by the graphene interface, which was confirmed using transmission electron microscopy images acquired ex-situ as well as during in-situ tensile test. Molecular dynamics simulations for uniaxial tension of Cu-graphene showed limited accumulation of dislocations at the film/substrate interface, which makes the fatigue induced crack formation and propagation through thickness of the film difficult in this materials system. Robustness against bending fatigue-induced damage makes this material well-suited for the flexible electrode application, and methods for enhancing the scalability of fabrication will also be discussed.

Al alloys have been widely used in the automobile and aerospace industry due to their lightweight property. Among them, the Al-Zn-Mg alloy is attracting attention due to its high strength. The main strengthening mechanism of Al-Zn-Mg alloy is known as precipitation hardening. The precipitation sequence of this ternary system has been reported as follows: Super-saturated solid solution → GP zones → η’ phase → η phase. The η phase possesses the space group of P6₃/mmc and is known to have 15 different orientation relationships with Al. Among the 15 differently oriented η precipitates, η_1, η₂, and η₄ have the highest proportion and the rest except for these are known to exist less than 1%. In this study, η phase is studied thoroughly at atomic-scale using probe aberration-corrected FEI Themis Z microscope and Ab-initio calculation. Besides, 3-dimensional morphology of η phase was observed using 3-D tomography with the transmission electron microscope. Although the orientation relationship of η precipitates and Al matrix was reported, the atomic-scale observation of η₄ has not been done yet. In this study, η₄ precipitate, which occupies about 10% of the total precipitates, was observed in atomic scale and it was confirmed that it has a pseudo-periodic step-like interfacial structure with Mg and Zn segregation. We compared the interface energy of the simulated flat interface and experimentally observed the unique interface using ab-initio calculation. Consequently, the experimentally observed unique interface was energetically favored. We will further discuss the origin of this unique interface formation.

Dislocation cross-slip is a thermally activated process and a key mechanism responsible for many important phenomena in metal plasticity, including dislocation multiplication, pattern formation, dynamic recovery, and strain hardening. So far, the efforts on predicting the cross-slip rate from atomistic models has been focused on determining the activation enthalpy of cross-slip under stress, while the entropic effects were usually neglected. We show that the rate predicted based only on activation enthalpy underestimates the cross-slip rate by 4-orders of magnitude when compared to direct molecular dynamics (MD) simulations. We further show that the harmonic transition state theory (HTST) can still be used for accurate prediction of cross slip rate in comparison with MD, provided that a set of corrections are made to account for anharmonic effects such as soft modes (in the example case of FCC nickel). The result is method that successfully predicts the cross-slip rate over a much wider range of stress and temperature conditions than accessible by direct MD simulations. We find that thermal expansion and thermal softening effects contribute significantly to the activation entropy and the cross-slip rate; this effect is linked to a non-negligible local volume expansion during cross-slip under stress.

We recently proposed the adaptive incrementally affine method as a novel homogenization scheme that enables accurate prediction of the nonlinear mechanical response of particulate-reinforced composites under cyclic or multiaxial loading conditions. However, being a mean-field homogenization method that relies on the solution of Eshelby’s inclusion problem, the aforementioned method does not provide accurate predictions when the shape of the reinforcement has a high aspect ratio or the interaction among reinforcements becomes significant. Herein, we propose a combined theoretical and data-driven approach in which the homogenization method is followed by transfer learning to enhance the prediction of the nonlinear mechanical response of particle/fiber-reinforced composites. We trained a deep neural network (DNN) with a massive stress-strain curve dataset of a composite subjected to uniaxial loading in the elasto-plastic regime based on the adaptive incrementally affine homogenization method; then, we fine-tuned the pre-trained DNN via transfer learning with a relatively small dataset based on time-consuming three-dimensional (3D) finite element analyses (FEA). The transfer learning approach exhibited better predictive performance than the DNN directly trained only with the FEA dataset did, for a wide range of reinforcement volume fractions and shapes. With transfer learning, the DNN exploits the knowledge learned from the homogenization theory to perform a new learning task on a small dataset from 3D FEA. The combined theoretical and data-driven approach proposed herein can be extended to other composite analyses when experiments or simulations require excessive time and cost for a large dataset and the applicability of the homogenization theory is limited.

Ternary and higher-order borides remain an underexplored area in the search for hard materials. The difficulties associated with purely systematic experimental investigations have largely hindered the consideration of such complex phases. Here, traditional design rules are merged with computation-based methods in an effort to direct synthetic efforts, addressing this challenge. Ternary transition metal borides, like Sc₂Ru₅B₄, were selected to demonstrate this approach. The target phases were first prepared using arc melting, and the crystal structures were resolved with single crystal X-ray diffraction. Vickers microhardness indentation revealed that these compounds are hard materials, and the mechanical properties can be enhanced by substituting denser, isoelectronic transition metals into the original structures following an empirical understanding of hardness in borides. Analyzing the ensuing density of states (DOS) curves indicates that the Fermi levels nevertheless fall in unfavorable positions requiring a change in the electron count to optimize the electronic structures. The subsequent synthesis of the transition metal substituted compounds alters the electron count moving the Fermi levels from the peaks. At the same time, Vickers indentation measurements, combined with DFT-level stress-strain calculations and a bonding analysis, show shifting the Fermi level reduces the occupation of antibonding interactions, which also increases the hardness. These data suggest that electron density, Fermi level position, and chemical bonding are all essential markers when developing high hardness materials.

Though twinning plays an equally vital role to that of dislocation slips in accommodating deformation in the hexagonal close-packed crystal structure, its atomic-level nucleation mechanism has long been under debate. We performed large-scale molecular dynamics simulations to study the deformation process in magnesium and titanium. A close interrelation between martensitic phase transition and twinning was found. Various twins were observed to form through reversible martensitic phase transformations via evanescent intermediate phases. Finally, twin-twin interactions and dislocation-twin interactions will be discussed.

Recently, nanoscale shape memory alloys (SMAs) have attracted special attention because of their unique characteristics. In particular, previous studies have focused on the phenomenon that the signal of shape memory behavior disappears below the critical size. In this study, we suggested a novel method to examine the size effect of shape memory alloys at the nanoscale by fabricating free-standing nanoparticles based on liquid-liquid phase separation in Ni-Ti-Gd system. We systematically performed thermal analysis and in situ mechanical tests in scanning electron microscope to understand martensitic phase transformation behaviors and carefully compared with the data of molecular dynamic simulation to analyze the mechanical behavior of the nanoparticles. Interestingly, the isotropic polycrystalline NiTi-based shape memory alloy particles show size-dependence of recoverable strain which shows decreasing recoverable strain as the particle size increases until the critical size. The origin was a size-dependent transition temperature studied by molecular dynamics simulation. This study provides a theoretical basis to fabricate isotropic polycrystalline shape memory alloy nanoparticles and novel guidelines for studying the size effect of shape memory alloys. We also expect that the results provide an effective guideline for constructing the size-stress-temperature phase diagram, which can be used as an indicator that can determine the other one when two of the three factors, transition temperature, stress, and size are determined. Finally, we expect the study can accelerate the design and practical uses of shape memory alloys not only through composition but also through size.

Strain engineering especially in epitaxial films of complex oxides plays an important role in determining structural and physical characteristics. With the epitaxial strain, even paraelectric perovskite oxides can possess ferroelectricity that does not exist in a bulk state [1–3]. Density functional theory (DFT) has predicted that the ferroelectricity could be induced in SrMnO₃, which is paraelectric cubic phase in bulk, exhibiting a high spontaneous polarization driven by off-centered Mn⁴⁺ ion under tensile strain larger than 1% [1]. Moreover, an abrupt increase of spontaneous polarization exists around 2% misfit strain. The SrMnO₃ (SMO) films have been experimentally studied grown on various substrates [4–8]; however, complicated growth condition and consequent crystalline characteristics depending on the substrate has been an obstacle to investigate a misfit strain effect onto the induced ferroelectricity of SrMnO₃ films. Here, we report in-situ strain engineering of SrMnO₃ film using a piezoelectric substrate, e.g., 0.72Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (PMN-PT), in order to manipulate the spontaneous polarization in an SrMnO₃ epitaxial thin film while applying voltage to the PMN-PT substrate. In this work, we show that epitaxially grown SrMnO₃ films on (110)-oriented PMN-PT possess the spontaneous polarization along [110] direction which can be modulated by the epitaxial strain transfered from the piezoelectric strain according to applied voltage on substrate.
The SrMnO₃ films with a thickness of approximately 100 nm were deposited onto La_0.7S_r0.3MnO₃ (LSMO) / (110)-oriented PMN-PT substrates using pulsed laser deposition (PLD) technique. The LSMO layers with a thickness of 20 nm were grown directly on the PMN-PT substrate to play roles of a bottom electrode as well as a buffer layer to reduce the large lattice mismatch between SMO and PMN-PT (~5.39%). The substrate temperature was kept at 700°C and oxygen partial pressure was controlled in a range from 1 to 200 mTorr. The Pt top electrodes with a size of 100 μm were deposited using an e-beam evaporator. We will discuss spontaneous polarization and crystallinity of films with respect to the misfit strain of films.
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In the hierarchically-arranged crystallographic structure of reduced activation ferritic/martensitic (RAFM) steel, the smallest but the most abundant microstructural unit, i.e., the lath, plays an important role in the dislocation plasticity. Due to the multi-scale complexity in the lath-martensitic microstructure, miniaturized mechanical characterization at the very fine scale is required to understand the deformation mechanism associated with the laths. In this study, the uniaxial micro-compression tests combined with rigorous crystallographic analysis were performed to figure out the plastic deformation mechanism of lath boundary sliding in RAFM steels. These experimental results were further interpreted by conducting molecular dynamics simulation to find the underlying dislocation mechanism in different types of plastic deformation responses. We found the amount of lath boundary sliding is controlled by the crystallographic orientation of the lath boundary plane, the direction of Burgers vectors of the interfacial dislocations, and the magnitude of resolved shear stress on the lath boundary plane. Also, the effect of normal stress on the lath boundary plane was investigated.

The plastic deformation kinetics of ultrafine grained (ufg) and nanocrystalline (nc) metal thin films have been investigated using an in situ TEM nanomechanical testing technique. This technique allows for simultaneous observation of the active deformation mechanisms and measurement of stress relaxation and true activation volume. Experiments have been conducted on gold (Au) and aluminum (Al) thin films with varying thicknesses, grain sizes, and texture. Activation volume measurements range from 5 – 15 b³ for ufg Au specimens with an average grain size of 150 nm. Activation volume values for nc Al decreased from 25 – 35 b³ to 5 – 10 b³ as the average grain size decreases from 74 to 47 nm. Experimentally measured activation volume values are compared with values determined from atomistic simulations for different unit dislocation processes, using a model relating activation volume and grain size. Based on these comparisons, it is determined that diffusive processes, such as grain boundary disconnection climb, are most likely the rate-controlling mechanisms that govern deformation of these nc/ufg metal thin films, as opposed to surface and grain boundary dislocation nucleation.

Classical continuum theory fails to predict micron-sized structural behavior due to an elastic size effect. Various higher-order deformation theories, such as couple stress theory, have been developed to simulate stiffening of structures with decreasing size. In this work, micro-scale cantilever bending experiments are conducted and an increase in the effective elastic modulus is observed with decrease in beam thickness.
Microstructural and mechanical characterization of the polycrystalline copper plate revealed that the crystal consists of non-textured equiaxed grains with an average grain size of 2.3 µm, and the measured elastic modulus was 112 GPa. Micro-scale cantilevers with thicknesses ranging from 1.6 µm to 8.6 µm were fabricated within the copper plate using femtosecond laser machining followed by focused ion beam milling. A nanoindenter equipped with a wedge tip was utilized to apply line load on the fabricated samples. Finite element analysis was performed to exclude the effect of substrate deformation for calculating pure beam stiffnesses. The measured effective elastic modulus increases from 94 GPa to 215 GPa with decreasing thickness.
Couple stress theory is used to model the size effect. In the couple stress theory, a length scale parameter is introduced as an additional material property in addition to Lamé constants. An optimization problem was formulated to find the length scale parameter that minimizes the deformation difference between the experiment result and finite element analysis. The length scale parameter determined by solving the optimization problem is 0.78 µm.
The physical origin of the length scale parameter is discussed by considering mobile dislocations escaping via the free surfaces. Based on experiment results, it is suggested that the length scale parameter in metals is dependent on the microstructure of the crystal. As the couple stress theory is the simplest higher-order deformation theory, it has the advantage of low computational cost for analyzing structures with arbitrary geometry. Due to this advantage, FEA simulations based on the couple stress theory have the potential to bridge between micro/nano-scale (e.g. molecular dynamics, dislocation dynamics) simulations and bulk simulations, and can be widely used in the design and analysis of micro-scale devices.

Laser nitriding is known as one of the methods that can easily improve corrosion properties through surface alloying or microstructural alteration by forming nitride on the surface. Therefore, studies on the reinforcement of corrosion properties through laser nitriding on the surface of various materials are being conducted, but studies on the mechanism of the interface between nitride and matrix are insufficient. In this study, we performed the laser nitriding on the surface of NAK80 steel alloy and analyzed the growth behavior of nitride by interfacial energy between matrix and interface through Gibbsian interfacial free energy calculation.
During laser nitriding, Al in the alloy causes the formation of many faceted AlN precipitates in the subsurface reigon. The atom-probe tomography and transmission electron microscope analysis identified AlN facets with three different crystallographic orientations, and their relationships with respect to martensite matrix were [-2110]_AlN || [-111]_Fe, [2-111]_AlN || [010]_Fe, and [-2110]_AlN || [001]_Fe. Among these, [-2110]_AlN || [-111]_Fe exhibited a basal plane of AlN at approximately -5 degree with respect to the {110} Fe plane, suggesting that a tilted basal plane was chosen to reduce the interfacial energy between AlN and the matrix. The mismatch parameters of each facet direction were calculated as 6.19%, 4.03%, and 0.86% for [-2110]_AlN || [-111]_Fe, [2-111]_AlN || [010]_Fe, and [-2110]_AlN || [001]_Fe, respectively.
Consequently, it was confirmed that the interface with a large degree of mismatch parameter had high interfacial energy. To resolve the high energy, the precipitates had a tilted basal plane or more elements were segregated in the interface between matrix and precipitates. The results of the interfacial energy analysis suggest a direction for considering the efficient formation of the precipitates to be formed in order to improve the corrosion properties.

While nanocrystalline copper-tantalum (Cu-Ta) has documented thermomechanical stability and strength, there have been far fewer investigations regarding the use of other Group VB transition metal solute elements, e.g., vanadium and niobium (Nb). This work explores the stabilizing and strengthening effect of Nb on nanocrystalline Cu. While nanocrystalline Cu-Nb alloys have comparable strength to Cu-Ta at ambient temperature, mechanical tests reveal Cu-Nb alloys to be only half as strong as Cu-Ta alloys when at temperature. To explain this loss of strength, we use transmission electron microscopy and atom probe tomography to elucidate the role that solute choice has on microstructure and strengthening mechanisms. Divergent solute-contaminant interactions, mainly involving oxygen (O), are the major differences uncovered between the two alloys. In the case of Ta, a pentoxide (i.e., Ta₂O₅) is the only equilibrium configuration, while Nb can form multiple stable oxides including a monoxide (NbO) and a dioxide (NbO₂) in addition to a pentoxide (Nb₂O₅). These different oxides create different extents of possible coherency with the matrix. The consequences of these different oxide structures on nanocrystalline stability and strength in Cu-based alloys are discussed, with implications on solute selection for the broader nanocrystalline community.

Thin films are widely used as functional and structural elements in micro-electronic devices. Therefore, understanding the mechanical behavior of thin films at different length scales and environmental conditions is essential for the design of reliable devices. However, it is difficult to precisely measure the properties of small-scale materials with the methods that are employed for bulk materials; probing micro/nano scale samples is challenged by the inherent difficulties associated with fabricating and handling of extremely small specimens. For several decades, much work has been dedicated to characterizing the mechanical behavior of thin metal films and it has been observed that the mechanical properties of thin films can be very different to those of their bulk counterparts. In addition, passivated metal thin films showed interesting features such as higher strain hardening rate and enhanced failure strain.
In this presentation, the effect of carbon addition and passivation on the microstructure and mechanical behavior of freestanding Al thin films will be discussed. Tensile specimens with sub-micron thickness were fabricated via sputter deposition followed by standard Si-based microfabrication techniques. Mechanical characterization was carried out by performing micro-tensile tests and membrane deflection tests. Supersaturated Al-C thin films containing 9.8at% C exhibit yield strength of about 400 MPa, over three times the yield strength of the Al thin films. This strengthening can be explained by the role of interstitial C atoms which block dislocations movement inside the Al matrix. In addition, an upper yield followed by drop in stress was observed in Al-C thin films which implies the formation of Cottrell atmosphere in supersaturated Al-C thin films. For passivated films, a large increase in the failure strain was observed when an ultra-thin Si₃N₄ layer (< 10 nm) is deposited, which is attributed to delay in the strain localization due to constraint imposed by the passivation layer. Ongoing efforts utilizing high-throughput combinatorial experiments to identify the effect of carbon content on the mechanical behavior will also be introduced.

Mechanical treatments of surfaces to create compressive residual stress gradients and plastic deformation are commonly used to improve the mechanical properties of engineering alloys. Commercial Ti alloys contain complex grain structures based on heat treatments, however heat treatments that involve recrystallization commonly decrease the residual stresses formed from processes such as shot peening. In this study we develop ultra-fine grain structures in three difference Ti alloys with colony, basket weave, and lamellar precipitates. All base microstructures were peened to control residual stresses on the order of 200 to 800 MPa to depths of about 200 microns; the surface nanoindentation hardness after peening was 3-4 GPa (depending on the alloy). Nanoindentation was used in conjunction with x-ray diffraction to quantify the residual stresses. A thermal treatment schedule between 700-1000K for 20-30 minutes was sufficient to recrystallize the top 2-5 microns of the surface, which lowered residual stresses but formed a grain size between 350 and 700 nm. The resulting hardness increased over 1 GPa (from 4-5 GPa). This increase in surface hardness was achieved with no decrease in bulk hardness; and the compressive residual stress appears to stabilize the alpha phase in commercial beta Ti alloys, allowing for controlled precipitation and gradients in microstructure with no changes in bulk chemistry.

Plastic deformation in crystals is mediated by the motion of line defects known as dislocations. For decades, dislocation activity has been treated as a homogeneous, smooth continuous process. However, it is now recognized that plasticity can be determined by long-range correlated and intermittent collective dislocation processes, known as avalanches. Here we demonstrate in body-centered cubic Nb how the long-range and scale-free dynamics at room temperature are progressively quenched out with decreasing temperature, eventually revealing intermittency with a characteristic length scale that approaches the Burgers vector itself. Plasticity is shown to be bimodal across the studied temperature regime, with conventional thermally-activated smooth plastic flow (‘mild’) coexisting with sporadic bursts (‘wild’) controlled by athermal screw dislocation activity, thereby violating the classical notion of temperature-dependent screw dislocation motion at low temperatures. An abrupt increase of the athermal avalanche component is identified at the critical temperature of the material. Our results indicate that plasticity at any scale can be understood in terms of the coexistence of these mild and wild modes of deformation, which could help design better alloys by suppressing one of the two modes in desired temperature windows.

Amorphous silica deforms viscoplastically under e-beam irradiation as is common for brittle glasses at elevated temperatures. In this study, we show that how we can precisely control the mechanical shaping of brittle amorphous silica at nanoscale by engineered electron-matter interaction without heating. We observed a ductile superplastic deformation of amorphous silica under a focused scanning electron-beam with low acceleration voltage from few to tens of kV during in-situ compression tests and unique dependencies of acceleration voltage and beam current in it. By simulating the electron-matter interaction, we prove that the deformation of amorphous silica strongly depends on the volume where the inelastic scattering occurs. Finally, by systematically controlling the electron-matter interaction volume, we can mechanically shape the brittle amorphous silica at room temperature to a level comparable to existing glass craft technique at high temperature. This study also presents the key mechanism of athermal viscoplastic deformation under our e-beam condition based on experiment and DFT simulation.

The glide of a dislocation on a selected crystallographic plane is facilitated depending on the dislocation core configuration. In pure Ti, the dislocation motion has been described as jerky, gliding during an extended period at high velocity on the {10-10} prismatic plane and cross-slipping slowly during a limited period on the {10-11} pyramidal plane. The rate of cross-slip is an important parameter that influences the ductility and strength of the material. The study of the temperature dependence of dislocation core configurations will allow us to know which glide plane is favored at a given temperature, hence, allowing us to predict the rate of cross-slip. We’ve computed the temperature dependence of the dislocation core Gibbs free energy for different core configurations to study the thermodynamic stability of these configurations. These computations have been done using molecular dynamics simulations using nonequilibrium free energy calculation methods, among which the Frenkel-Ladd path and the Reversible scaling methods, and a MEAM potential describing the properties of Ti. Different ground state core configurations are reached by applying a non-Schmid stress on the simulation cell, and the changes in the structures of the cores are characterized using an elastic stability parameter. It is shown that at low temperature the prism configuration is favored but as the temperature increases, a wider range of core structures become accessible through thermal fluctuation.
The authors gratefully acknowledge funding from the U.S. Oce of Naval Research under grants N00014-16-S-BA10.

Typical plastic anisotropy characterizations of metal alloys require repetitive mechanical tests in various loading directions and stress states. To efficiently characterize and predict plastic anisotropy without extensive mechanical tests, crystal plasticity finite element method (CPFEM) simulations using initial microstructural data from EBSD and XRD measurements are performed and compared with experiments. It is shown that CPFEM model incorporating the texture and grain morphology of various aluminum alloys captures anisotropic mechanical behavior and r-values reasonably well. In addition, ~70,000 crystal plasticity data were generated to train a neural network model that instantly and accurately predicts normalized yield stresses and r-values from the initial texture. The model is then used to provide anisotropy parameters of various yield models. This capability provides fast and accurate prediction of material’s anisotropic response without involving extensive experimental characterization or expensive computational calculations.

In this study, anisotropic plasticity is derived from spherical and Knoop indentation responses using neural networks (NNs) and finite element (FE) simulations. We present an FE–NN model that inversely solves the problem of leading the mechanical properties of anisotropic materials from indentation data composed of the load-depth curve, pile-up/sink-in, and residual in-plane displacement fields. Hyperparameter-tuned NNs are trained using a database created with the help of parametric studies for experimentally verified FE simulations of indentation. The real-life indentation data are converted into parameters of anisotropic yield criterion and isotropic hardening through the trained NN. The predictive performance of the FE–NN model is evaluated, followed by an in-depth discussion on the influence of each mechanical parameter on the indentation responses. The predicted results are shown to be in good agreement with uniaxial plastic curves measured in the rolling, diagonal, and vertical directions. The predictive performance results demonstrate that the proposed FE–NN modeling approach is robust and effective in capturing anisotropic plastic flows from indentation data. Furthermore, we propose that a more accurate and stable inverse analysis can be achieved without requiring additional indentation information other than the load–depth curves by introducing the Knoop indenter. By noting the characteristics of the Knoop indenter whose load–depth curve varies with the rotation angle about the indentation axis in contrast to the spherical indenter, the potential of the Knoop indenter is presented, which has not drawn attention in the inverse analysis.

Martensitic phase transformation has received great interest owing to its broad academic and technological relevance. A representative class of materials utilizing the martensitic phase transformation is shape-memory alloys (SMAs). SMAs are widely used in many applications by virtue of their unique shape-memory effect and superelasticity, which can be controlled by reversible temperature- and stress-induced phase transformations, respectively. As applications of SMAs recently entered the arena of smart materials for micro- and nano-electromechanical systems (MEMS/NEMS) and nano-composite materials with exceptional mechanical properties, their distinctive phase transformation and deformation behaviors at the small-scale have attracted particular attention. For example, a noticeable common characteristic of phase transformations in nanoscale SMAs is an over-stabilization of the austenite phase with respect to the martensite phase. If the system size approaches the nanometer scale, a decrease of the transformation temperature is commonly observed for various kinds of SMAs regardless of the boundary conditions, e.g., nanocrystalline SMAs, shape-memory nano-precipitates embedded in a stiff non-transforming matrix, and freestanding shape-memory nanoparticles.
In this study, detailed mechanisms of the phase transformation and deformation behaviors at the nanoscale are investigated using atomistic simulation techniques such as the molecular dynamics. For this purpose, interatomic potentials capable of reproducing the martensitic phase transformation were developed based on the modified embedded-atom method (MEAM) model. As a fitting method of potential parameters, the force-matching method based on the density functional theory (DFT) calculation was used. The resulting interatomic potentials reproduce accurately the temperature- and stress-induced martensitic phase transformations of NiTi SMAs as well as various fundamental physical properties. Subsequent molecular dynamics simulations verified that developed potentials can be successfully applied to provide insights into the atomic details of the phase transformation and deformation behaviors of various SMA-related materials.

The multiscale modeling has developed to investigate size-dependent plasticity by coupling conventional continuum crystal plasticity finite element (CPFEM) with a discrete defect (DD) modeling. While the detailed defect microstructure and short-range interaction is handled by DD framework, the long-range elastic interactions with image stress field are calculated by CPFEM (using commercial package(ABAQUS) with user-subroutine), accounting for complex boundary conditions. The proposed model could account for complex multi-physical phenomena, which would play an important role in obtaining a fundamental understanding of deformation mechanism at small scale. Applications of developed concurrent coupled model includes dislocation nucleation at a crack tip, nanoindentation, hydrogen embrittlement at submicron length scale. The developed model will shed light on fundamental investigation of “defect-controlled” mechanical behaviors in crystalline materials, such as plasticity and damage and fracture.

The simulation of oxides, and hydroxides is important to guide in the design of cement, ceramics, composites and glasses. Here we introduce an extension of the Interface force field (IFF) for seven oxides and two hydroxides (NiO, CaO, MgO, β-Ni(OH)₂, β-Ca(OH)₂, α-Al₂O_3, α-Cr₂O₃, α-Fe₂O₃, and SiO₂) using a only non-bonded potential that involves Coulomb and Lennard-Jones terms. Care is taken that the atomic charges remain close to verifiable internal dipole moments even though covalent bonding contributions are disregarded. The non-bonded models, even though simplistic, are inherently reactive, enabling changes in morphology and defects. All models achieve quantitative agreement with experiment in lattice parameters (<1% deviation) and in bulk modulus (<10% deviation). The models are also tested for surface energy (key property for reproducing interfacial interactios) and compared with experimental and DFT sources. The IFF models can be used with existing parameters for other oxides, metals, solvents, polymers and biomolecules. The oxides/hydroxide parameters are available using both 12-6 and 9-6 Lennard-Jones potentials to achieve compatibility with CHARMM, CVFF, AMBER, OPLS-AA, PCFF, and COMPASS. The models can be used to simulate from length scale of 1nm³ to 1µm³ systems. The present models outperfoms popular atomistic models (Pedone et. al. and ReaxFF) to simulate oxides either in speed, relaibility or compatibility. Further improvement in the models include development of complete bonded potential with either harmonic or Morse bonds.

In this work, the AE technique is coupled with in-situ micromechanical testing inside a scanning electron microscope (SEM) to study the deformation of Ni microcrystals under monotoic compression and cyclic loading. By continuously recording of the generated AE signals simultaneously with the load-displacement measurements, correlations were found between the AE wave characteristics and different deformation stages. AE analysis in both time and frequency domains was employed to identify different active sources and probe the underlying deformation kinetics which are not discernible from the load-displacement curves solely. In addition, statistics associated with strain bursts in single crystal Ni microcrystal are discussed in detail.

There is a great interest in glass systems for a variety of applications such as electronic devices, displays, and coatings for orthopedic implants. Surface damage is a major issue in each of these applications, and it creates many problems for glass makers, suppliers, and end users. The cracking behavior that governs glass surface damage is not yet completely understood. In many cases, the nucleation of these cracks results from a sharp contact loading. To simulate sharp contact loading conditions, nanoindentation with triangular pyramidal indenters with centerline-to-face angles in the range of cube corner (35.3°) to Berkovich (65.3°) was performed on glass samples, and the resulting crack initiation and propagation was investigated. Two different glass structures have been analyzed - fused silica and soda lime glass - as representative of anomalous and normal glasses, respectively. Fused silica is thought to deform by a process dominated by densification, whereas soda lime glass deforms primarily by shearing processes. Due to these different deformation processes, it is anticipated that the different stress fields for each material around the contact area result in different cracking morphologies. Nanoindentation at various loads (e.g., 3 mN - 20 N) was used to identify a load dependency of the cracking behavior. Surface cracking was documented using atomic force microscopy, scanning electron microscopy, and laser confocal microscopy. Results show that cracking is enhanced by sharper indenters and higher loads. However, there exists a distinct crack initiation threshold, below which no cracking occurs. The threshold depends on the glass system as well as the indenter geometry.

Potential energy landscape (PEL) is considered one of the most promising ways to understand the atomistic mechanisms of inelastic deformations in glassy materials. However, the evolution of the entire PEL under loading strain is still not clear. Here, we use molecular dynamics simulations and high-throughput energy barrier calculations to determine the PEL evolution as a function of strain under finite temperature, presented as the “energy-strain landscape.” Based on this new picture, we discover a strain-independent quantity “eigen-barrier” to characterize the reversibility of shear-transformation (ST) events. A model is developed to determine the hysteresis stress-strain loop of individual independent ST events as a function of temperature, thus predicting the temperature dependence of the elastic limit at low temperatures. Contrary to the common expectation that the elastic limit decreases with increasing temperature, we show that the elastic limit can increase with increasing temperature. This work provides a quantitative description of the strain dependence of the PEL and brings a new perspective to understand the inelasticity properties of glasses.

In this work, we introduce reverted austenite with various stacking fault energies in a PH17-4 stainless steel to study its mechanical effects via integrated in-situ scanning electron microscopy and in-situ synchrotron diffraction tests, as well as nanoindentation experiments. These experiments demonstrate that the proper tuning of size, stability, and chemistry of austenite can be an effective measure to overcome the strength-ductility tradeoff in martensitic stainless steels. Most intriguingly, we observe that the austenite does not always lead to softening when introduced in the highly-defected body centered tetragonal (BCT) martensitic matrix. The causes and consequences of this observation are discussed critically, leading to design considerations for martensitic stainless steels.

Numerical finite element method (FEM) calculation results reveal that the effective strain differs between the ductile Zr-based and brittle Hf-based bulk metallic glasses (BMGs) during rolling at room temperature. The effective strain applied to BMG plates plays a key role for enabling malleability of BMGs. The current results demonstrated that the deformation mechanism of ductile Zr-based bulk metallic glass can be explained by perceptibility of multiple shear bands formation, however, the deformation mechanism of brittle Hf-based bulk metallic glass is represented by uniformly distribution of effective strain through overall specimen rather than localized into shear band. The variation of stress and the distribution of effective strain change significantly near the surface region of a Hf-based bulk metallic glass plate with increasing number of rolling passes. Under elasto-plastic deformation by cold rolling, the brittle Hf-based BMG has a thickness strain (ε_t) of -0.012, and the neutral effective strain (ε_eff) is 0.011 at the thickness direction, respectively. We present experimental confirmation that when the applied effective strain can be adjusted to below 1.1% during elasto-plastic deformation then even the brittle as-cast Hf-based BMG can be deformed up to 44% thickness reduction and 23% width expansion after multi-pass cold rolling without fracture triggered by localization of shear stress.

In order to reduce the fossil fuel consumption and greenhouse gas emissions, there is a continued demand for the new generation lightweight materials for better performance. Titanium alloys have attracted significant amount of attention in aerospace, automobile, chemical and bio-medical industries, due to the combination of great properties, such as high specific strength, great toughness, excellent corrosion resistance and good biocompatibility. The mechanical property of the titanium alloys is highly dependent on the microstructure, more specifically the HCP structure alpha precipitate microstructure in the BCC structure beta phase matrix. Recent studies have shown that various fine-scale alpha microstructures can be achieved in titanium alloys with the assistance of different metastable phases, such as hexagonal structure omega phase. For example, our previous study in a metastable beta Ti-5Al-5Mo-5V-3Cr (wt%, Ti5553) has shown that alpha microstructure of three different size scales, namely refined, more-refined and super-refined alpha, can be generated via the selection of different roles of nanoscale omega phase particles.
This presentation is mainly focused on the use of analytical electron microscopy techniques to investigate the fundamental mechanisms in the development of hierarchical microstructure in the metastable beta titanium alloys. An example of designing high-strength titanium alloys for aerospace application via microstructure engineering will be introduced. In this study, the factor of mechanical treatment has been taken into consideration for microstructure engineering in a metastable beta Ti5553 alloy. For the first time, a novel highly-indexed {10 9 3}<3-3-1> type deformation twin has been characterized in the cold-rolled Ti5553. The substructure of this novel high-indexed deformation twin has been studied using aberration-corrected scanning transmission electron microscopy. Nanoscale hexagonal omega phase and orthorhombic alpha double prime phase has been observed in the interior of highly-indexed deformation twin together with nanoscale omega phase layer at the twin/matrix interface. The response of cold-rolled Ti5553 to the subsequent heat treatment was then studied via scanning electron microscopy and transmission electron microscopy. Hierarchical alpha precipitate microstructure composed of coarse alpha plates, alpha sub-layers and fine-scale alpha precipitates has been observed. It is speculated that the coarse alpha plates may form from the pre-formed {10 9 3}<3 -3 -1> twin boundary via the assistance of twin boundary omega phase; alpha sub-layers and fine-scale alpha precipitates may be related to the substructure in the interior of {10 9 3}<3 -3 -1> twin including alpha double prime phase, omega phase and dislocations. The influence of highly-indexed deformation twin and pre-formed omega phase on the formation of hierarchical microstructure in the metastable beta titanium alloys will be discussed in detail. This work was supported by the National Science Foundation under the contract No. 2122272.

Ni-based superalloys are critical to manufacture aeroengine parts. Alloy 718 is one the main representatives of this type of material due to its excellent combination of properties, including elevated strength up to 650°C. Changes in the thermo-mechanical processing routes affect micro- to nanoscale strengthening factors. Direct ageing, for instance, generates different configurations of γ′ and γ′′ precipitates when compared to conventional ageing, contributing to a 10% increment in yield strength [1]. Other processing conditions, such as cooling rate from forging also impact the microstructure, where air cooling enables accelerated formation of γ" precipitates along dislocation channels [2]. However, a systematic approach to compare differences between ageing treatments, and processing heterogeneities within industrially forged materials was lacking.
Atom probe microscopy, as a sub-nanometre resolution characterization technique, that provides a plethora of information regarding precipitates and atomic clustering [3]. By systematically examining the evolution of the nanoscale precipitates and hardness during conventional versus direct ageing [4], we determine that highly strained Alloy 718 enables faster γ" precipitation at temperatures higher than predicted by existing time-temperature-transformation diagrams. We also provide a thorough microstructure-properties-processing analysis in multiple regions of a forged turbine disk at varying length scales, allowing future developments in modelling and microstructural engineering.
[1] F. Theska, A. Stanojevic, B. Oberwinkler, S.P. Ringer, S. Primig, Acta Mater. 156 (2018) 116–124.
[2] F. Theska, K. Nomoto, F. Godor, B. Oberwinkler, A. Stanojevic, S.P. Ringer, S. Primig, Acta Mater. 188 (2020) 492–503.
[3] B. Gault, M.P. Moody, J.M. Cairney, S.P. Ringer, Atom Probe Microscopy, Springer Science & Business Media, 2012.
[4] V.V. Rielli, F. Theska, F. Godor, A. Stanojevic, B. Oberwinkler, S. Primig, Mater. Des. 205 (2021) 109762.

As the need for high-efficiency power generation is growing for environmental-friendly development, materials or machine parts for the extreme environment have been developed. For the high-temperature materials, Ni-based superalloys with γ-γ' complex structure have been utilized, since they exhibit higher working temperatures than other conventional materials, such as steels. At the same time, since it is used under high temperature and oxidizing atmospheres, further property improvement is also needed.
Meanwhile, since cobalt has a higher melting temperature than nickel, the Co-based superalloy is drawing big attention as the next generation. Therefore, in this research, we will report a novel Co-based superalloy. In particular, the superalloy can heal the damage formed during operation by itself, as well as exhibit improvement of the mechanical property at high temperatures. For the self-healing property, we introduced boron as a metastable “Healing core”, the interstitial atoms, that can be easily segregated and heal the damaged part. By alloying the healing core, the superalloy exhibits a longer life-cycle at high temperatures as well as the enhanced mechanical property itself. This work will give us a new idea to develop self-healing alloys by introducing boron as metastable healing cores. Finally, this work will give us a guideline to develop self-healable metallic materials.

The commercially pure Ti (grade 4) and Ti-15Mo alloy in a metastable β solution treated condition were successfully processed by a novel severe plastic deformation technique referred to as rotational constrained bending (RCB) at the temperature of 300 °C. This technique introduces a bending deformation to the sample in the form of a bar, by its passing through the special constrained bending die. The investigation of the influence of the increasing number of passes on the microstructure homogenisation, texture and dislocation density evolution was followed by precise microhardness mapping and XRD. Microstructural observations in the central part of the billet cross-section were performed by the combination of electron backscatter diffraction and a special technique of automated crystal orientation mapping in TEM. The repetitive RCB processing leads to the gradual microstructure homogenisation and grain refinement in both cp-Ti and Ti-15Mo samples. The heterogeneous distribution of microhardness observed after a single pass was gradually transformed to almost uniform one in the whole cross-section after 10 and 6 passes in cp-Ti and Ti-15Mo alloy, respectively. The initial grain size of both samples decreased below 1 μm. Moreover, RCB processing significantly increases both the microhardness and the strength of both materials. The contribution of the grain size, the dislocation density and the texture to the room temperature proof stress is discussed in detail.

The ternary-based alloy (Fe1-x-yMnxCry) has not only superior mechanical properties, based on an attractive combination of extraordinarily high strength, toughness, and long-term environmental stability in highly corrosive environments. We show here that the environmental instability caused by the rapid electrochemical corrosion kinetics on the surface of conventional high Mn-bearing ferrous alloys could be overcome by a combination of high Mn-low Cr-balanced Fe and their intriguing synergistic interactions. These interactions can be summarized by a three-step sequential process: (i) (Mn,Fe)-dissolution with accelerated reaction kinetics, leaving a Cr-enriched surface; (ii) precipitation of Fe-oxide substituted partially by oxidized-Cr from the underlying matrix; (iii) formation of a bi-layer structure consisting of inner α-Fe2-xCrxO3/outer FexMn3-xO4. In this work, the changes in corrosion and surface-inhihbiting behaviors of Fe1-x-yMnxCry alloy and conventional steel alloys with increasing immersion in two aqueous environments were interpreted based on the characteristics of the corrosion scale formed on their surfaces.

Al-Zn-Mg-(Cu) (Al 7XXX) is an ultra-high strength class of aluminum alloy which is used for various applications like aerospace and automobile industry[1]. These alloys are mainly strengthened through precipitation of nanoscale metastable Guinier-Preston (GP) zones and eta’ precipitates which are key in controlling the strength as well as other properties. The composition, size and structure of these nanoparticles are equally important , which can further influence the properties like corrosion resistance[2]. These clusters and precipitate formation require heat treatment or high temperature bakes to form high density of precipitates which is not only time consuming but high in cost as well. A novel technique of controlled cyclic deformation at room temperature is sufficient to mediate dynamic precipitation of solute clusters which also enhance the material properties[3]. Here, we investigate the 7XXX series of aluminum alloys, one with copper and another with little or no copper using the Atom Probe Tomography technique (APT) after subjecting them to cyclic ageing. The APT is a powerful analytical characterization tool at the atomic scale for various characterization including local chemical composition and 3D morphology of individual grain boundaries and precipitates[4]. The objective of this work is to carry out cyclic loading fatigue tests and look at the composition and structure of clusters and compare them to the ones formed by conventional Natural ageing (NA) phenomenon after Solution Heat Treatment (SHT).
Preliminary results show a strong correlation between hardening and the clusters formed while the clusters formed immediately after cyclic ageing seem quite unstable rendering a poor mechanical response. A 10 day NA allows the clusters to stabilize in structure and size as confirmed by APT results which enhances the mechanical properties. A severe reduction in ductility (fracture strain) is observed for cyclic aged samples which can be understood by studying the microstructure using correlative TEM and APT.

References
[1] T. F. Chung et al., “Transmission electron microscopy investigation of separated nucleation and in-situ nucleation in AA7050 aluminium alloy,” Acta Mater., 2018.
[2] Y. L. Wang, Y. Y. Song, H. C. Jiang, Z. M. Li, D. Zhang, and L. J. Rong, “Variation of nanoparticle fraction and compositions in two-stage double peaks aging precipitation of Al−Zn−Mg alloy,” Nanoscale Res. Lett., 2018.
[3] W. Sun et al., “Precipitation strengthening of aluminum alloys by room-temperature cyclic plasticity,” Science (80-. )., 2019.
[4] D. Blavette, A. Bostel, J. M. Sarrau, B. Deconihout, and A. Menand, “An atom probe for three-dimensional tomography,” Nature, 1993.

In an effort to increase the operating life of the material, many studies have developed a process for repairing damage to the material. Welding, a typical repair process, is widely used due to the advantages of material saving and excellent efficiency, but after the repair process, the welded part has significantly weaker physical properties compared to the base material. Recently, in order to overcome such disadvantages, a study on a mechanism capable of repairing damage before microcracks occur, that is, a resetting process study has been reported. The main mechanism of this resetting process used the transformation-induced plasticity of Martensite – Austenite, and the retained austenite fraction and shape before and after stress is applied are important control factors. The retained austenite fraction is strongly dependent on the heat treatment temperature and time, and the austenite shape depends on the existing prior austenite and martensite (packet, block, lath) grain sizes. In this study, the effect of prior austenite and martensite refinement via cyclic heat treatment on resetting heat treatment conditions and properties was analyzed. Finally, the microstructure and resetting conditions were optimized, and this study will be a guideline for the development of resettable alloys.

Ti and its alloys are often found due to their properties, such as high strength and biocompatibility. In particular, it is predicted that use of high strength beta-titanium alloys will expand in the industries of electronic devices and IT where human-friendly and high strength and lightweight are emerging. Therefore, we designed human-friendly Ti-Mo-Fe alloy and applied powder metallurgy technology to develop high-strength beta-titanium over 800MPa. Mo added as an alloying element suitable of decrease the modulus of bcc Ti without compromising the strength of Ti alloy, changes in microstructural properties, phase transformation, and mechanical properties under the control of the Mo equivalent([Mo]eq) are studied. In this poster, Ti-Mo-Fe alloys produced by powder metallurgy and manufactured with different Mo content were evaluated in conjunction with phase transformation and elastic modulus. As a result mechanical properties such as hardness, tensile and compressive strength were found to be excellent than Ti-Mo-Fe Ingot.

Magnesium (Mg) alloys have been under immense research during recent years owning to its low density and high specific strength. However, the vast applications of Mg alloys have been impeded due to its low ductility and poor formability at room temperature (RT), which can be attributed to the high mechanical anisotropy, limited number of deformation modes and strong basal texture evolved after primary processing. Workability of Mg substantially increases at elevated temperatures as non-basal slip becomes available by thermal activation, which can also lead to the dynamic recrystallization (DRX) process resulting in enhanced formability in Mg alloys. On the other hand, hot processing issues such as, high energy consumption and low productivity of Mg alloy sheets; limit their applications in several fields as compared to other materials. In this study, accordingly, the kinetics of dynamic recrystallization (DRX) of AZ31-0.5Ca magnesium alloy during warm rolling was studied. Isothermal compression tests were employed to evaluate the DRX in AZ31-0.5Ca. The results suggested the addition of Ca in AZ31 significantly enhanced the DRX behavior as compared to Ca free AZ31. The presence of (Mg,Al)₂Ca particles in AZ31-0.5Ca acted as a barrier to dislocation motion, resulting in the nucleation of DRXed grains in this area. Moreover, fully DRXed structure was obtained at the reductions of 50%, where majority of the grains were found to be stress free with Kernel average misorientation < 1°. The enhanced workability in Ca added AZ31 alloy was explained on the basis of lower values of Z-parameters received as a result of higher reductions which, in turn, were responsible for lower critical strains for DRX.

Recently, surface crack nondamaging technology has been developed to render harmless surface cracks that can reduce the fatigue limit of metals by 20–60%. This is due to the effects of compressive residual stress induced by shot peening. This study evaluated the characteristics depending on steel with the different carbon content using surface crack nondamaging technology. First, the fatigue limit according to the size of microcracks was evaluated. Next, Using the threshold stress intensity factor and the residual stress distribution by shot peening (SP), the As dependence of the harmless maximum crack depth and the As dependence of crack depth which lowers the fatigue limit of non-SP smooth materials by 25% or 50% were evaluated. The relationship between minimum detectable crack depth via nondestructive inspection (a_NDI1 and a_NDI2)and harmless maximum crack depth (a_hlm), assuming that nondestructive inspection could detect the crack with sufficient probability were discussed.

304L stainless steel (SS304L) is a chromium- and nickel-containing austenitic stainless steel with the mechanical strength and highly corrosion-resistant material, such as automotive exhaust systems, gas boiler tubes and nuclear power plants in various venues be used. However, when SS304L is exposed to a temperature range of 425°C to 870°C, the precipitation of chromium carbide occurs and chromium depletion zone is formed adjacent to the grain boundary. Maintenance via corrosion removal using surface cleaning techniques is necessitated for maintenance of parts and materials. The most well-known abrasive blasting techniques are shot blasting and sand blasting. However, abrasive blasting can induce severe plastic deformation and surface defects, as well as reduce the corrosion resistance due to the application of abrasive particles with high kinetic energy against the workpieces. Moreover, airborne abrasive particles (sand) can deteriorate the operating environment and increase the health risk of workers. In this study, a high-power Nd:YAG laser is used to remove a corrosion layer on the surface of 304L stainless steel as an efficient and eco-friendly method than the conventional method. In order to increase the efficiency and productivity of rust removal technology through the laser surface cleaning (LSC) process, microstructure and mechanical characteristics according to laser conditions are analyzed and optimized.

Duplex stainless steel, which shows that strong general corrosion resistance and high strength, is widely used as a structural material in industrial fields. Although duplex steels show high performance in terms of mechanical properties, embrittlement caused by the α′ phase and sigma phase which reduces the integrity of structural materials. Therefore, it is important that selecting optimal alloy composition to prevent α′ phase and sigma phase precipitation.
In the Fe-Cr system, it has α phase and α′ phase at about 800K or less, and has a sigma phase and α phase or a sigma phase and α′ phase at about 800K or more according to the alloy composition. The precipitation phenomenon in many studies through the spinodal decomposition process of α phase and α′ phase was simulated using the Cahn-Hilliard equation, and many simulations and experimental studies have been conducted on the sigma phase precipitation. However, a physical model that can simultaneously consider α-α′ spinodal decomposition and sigma phase precipitation through a single physical model has not yet been proposed. In this study, a simulation framework that can predict the microstructural change of Fe-Cr under a given temperature and composition is presented without prior assumptions.
We simulate the behavior of precipitation for α′ phase and sigma phase using calculation of phase diagram(CALPHAD)-based phase-field model of Fe-Cr system. Also, we conduct quantitative analysis for microstructural characteristics such as average size, precipitate number density and phase fraction according to sigma phase transformation. Moreover, to improve numerical stability and to accelerate simulation, we applied methods that semi-implicit Fourier spectral method and compute unified device architecture(CUDA) parallelization.

Phase-field method is effective simulation technique for modeling the coherent microstructure evolution in elastically anisotropic systems. For binary system, the influence of the elastic effect is mainly seen on the interface. For the mechanical properties of interface, the interpolation of mechanical properties for two phases is required. Therefore the interfacial energy is affected by elastic energy differently according to the interpolation schemes. In this study, comparison of three existing interpolation schemes including Khachaturyan's strain interpolation scheme (KHS), Voight-Taylor's elastic energy interpolation scheme (VTS) and steinbach-apel's interpolation scheme (SAS) is discussed. We represent the elastic energy density and stress and strain for the binary system using three interpolation schemes and discuss which is suitable scheme for morphological evolution. Three models coupled with different interpolation schemes are implemented in MOOSE(Multiphysics Object Oriented Simulation Environment) framework.

Silicon-based polymer derived ceramics (PDCs) are attractive candidates for the next generation of ultra-high temperature materials (UHTMs). Within the contemporary space race and restrict safety and environmental regulations, the interest in lighter materials that withstand extreme operational conditions in hypersonic flights has emerged. The PDC route can probe complex-shaped ceramic parts (e.g., 3D printed turbine blade) with tunable chemical composition, including transition metals-containing composites, by controlling precursor architecture and pyrolysis conditions with potential for optimizing properties using concepts already developed in polymer chemistry/engineering. However, when thermally treated, preceramic polymers tend to form complex phases, leading to cracks and pores that compromise the operational safety of components. Moreover, the unknown atomic structural evolution of these materials during pyrolysis limit rational design of polymers for targeted properties, leading to largely Edisonian outcomes. To date, limited studies have been devoted to the thermal evolution of preceramic polymers from amorphous to ceramic due to long-range order requirements of traditional characterization tools.
In this contribution, we report on our efforts to better understand atomic scale structure of PDCs using synchrotron characterization methodologies. Our particular focus is the application of atomic pair distribution function (PDF) analysis coupled to reverse Monte Carlo (RMC) simulations to reveal changes in atomic-scale structure of these materials, even if they are largely amorphous. Such method advantages of using structural functions, such as coordination number histograms and bond angle distributions, extracted from RMC-generated simulations, to correlate the PDCs changes at the atomic scale. To construct realistic structural representations, the RMC-structure were constrained based on a combination of spectroscopies experiments, NEXAFS and Raman, along with TEM and Nuclear Solid-State NMR. “Mesoscopic” RMC further provides quantitatively contributions from different structural compositions (e.g., amorphous phase, graphite-like structure, nanoscale SiC, and β-SiC) during the polymer-to-ceramic conversion of commercial preceramic precursors upon a complete range of processing temperatures. The presented work will showcase the interesting structural behavior of PDCs when thermally heated, varying according to initial carbon content, branching level, and functionality. Our findings and approach will support the development of new precursors and controlled ceramics for applications not only in aero-components for possible defense needs, but also in electrical, magnetic, optical, energy-storing and catalyst fields.

Quantitative and well-targeted design of complex concentrated alloys (CCAs) is extremely challenging due to their immense compositional space. Here we show that the atomic-level pressure in single-phase face-centered cubic (fcc) CCAs consisting of 3d transition metal elements (V, Cr, Mn, Fe, Co, and Ni) (3d CCAs) originates from the charge transfer between neighboring elements by theory (quantum-mechanically derived approximation) and experiment (element-dependent lattice distortion). It allows identifying the best-suited element mix for massive solid solutions and predicting the resulting solid-solution strength using the simple electronegativity difference among the constituent elements. The method can be used to design new alloys with customized properties, such as a simple binary NiV solid solution which exceeds the yield strength of the established Cantor high entropy alloy by nearly a factor of 2, and TWIP or TRIP high entropy alloys having increased strain hardening rates while maintaining the same yield strengths.

The laser-powder bed fusion (L-PBF) method has the advantage of making complex shapes with excellent quality. Alloys made by L-PBF process share a characteristic fine cellular microstructure, which is originated by extremely fast heating and cooling rates during processing. The L-PBF also provides unique cellular microstructures, including a heterogeneity of composition when solute segregates toward the cell wall. It has been recently reported that the addition of Si into conventional CrCoNi medium-entropy alloy reduces stacking fault energy (SFE), resulting in enhancement of both strength and ductility. Here, we fabricated CrCoNiSi alloy via L-PBF method and the optimized mechanical property was obtained through precise control of processing parameters. The optimal alloy has a high yield strength of 929 MPa and a moderate elongation of 14% with sufficient strain hardening, leading to an ultimate tensile strength of 1264 MPa. High yield strength is attributed to the synergetic effect between finer cell size and nano-dispersed inclusions. Finer cell structures by extreme cooling rate successfully impede dislocation movements and hence have a significant effect on the high yield strength. Also, there are fairly large numbers of fine-dispersed nano-inclusions in the cell walls. These inclusions were found to be mostly oxides and sulfides, in diameter of less than 50nm, which gives additional strength increase based on the Orowan strengthening mechanism. As well as those conventional strengthening contributions, a strong segregation of Si and Cr toward cell wall was observed. Cr-Si rich sigma phase is stable at high temperatures over 800 celsius in this alloy system, confirmed by the phase diagram calculated by CALPHAD approach. During processing, subsequent heat is repeatedly applied to previously solidified melt pool during deposition of adjacent layers, which is similar to the formation mechanism of the heat-affected zone (HAZ) in welding. Repeated heating of solidified melt pool promotes segregation of Si and Cr to form stable sigma phase in cell wall. This sigma phase is vulnerable to cracks and provides crack propagation paths. The formation of nano-twinning was found in as-built state with a thickness of ~1.5 nm, resulted from the low stacking fault energy by the addition of Si into CrCoNi alloyAs well as the initial twins, the deformation twins from during tensile deformation, which contributes to additional strain hardening. Therefore, this work presents a new microstructural design strategy using the formation of high-temperature precipitation during additive manufacturing of MEAs, with according strategies for customizing mechanical property.

Biomedical implants require alloy with low elastic modulus, adequate strength, and high wear resistance. In this research work, the concept of heterostructure is applied by reinforcing MoNbTaTiZr high entropy alloy with different weight fractions (10, 20, 30%) to a low elastic modulus Ti-Nb-Zr alloy. Alloys were fabricated by spark plasma sintering to improve the strength and wear resistance of the alloy without scarifying the ductility of the system. While sintering, Mo and Ta diffused out of the reinforcement to the matrix, making a gradient interface in terms of composition. The gradient that is very critical to get the benefit of heterostructure is further tailored by annealing. All the samples have shown a linear increase in compressive strength as a function of reinforcement content, without decreasing plasticity. The wear resistance of the pristine alloy was also improved in a similar trend. The alloy with an optimized compositional gradient showed improved tensile strength while maintaining ductility. This novel design of reinforcing high entropy alloys in Ti-Nb-Zr alloy can give improved mechanical properties while keeping the low elastic modulus and high ductility.

The nitride films have been studied with Si addition due to improvement of hardness and wear resistance. High hardness of Si-containing nitride films such as TiSiN, NbSiN, and CrAlSiN has been achieved by the formation of nanocomposite structure due to the strong thermodynamic incompatibility between nitride metal and SiN_x. Some studies of nitride HEA film with Si addition to improve their hardness announce that no nanocomposite structure was observed by high mixing entropy effect which enhances the solubility of constituent element. In some Si-containing nitride HEA films showing high hardness, the amorphous regions were observed in the nanocrystalline state. Nevertheless, the formation of nanocomposites containing amorphous has not been revealed because it is difficult to verify phase segregation within the films due to the lack of component distribution differences. The CrCoNiSi alloy, which simultaneously improves strength and elongation through the Si addition, is expected to form an amorphous structure instead of Si compound in Si-containing HEA film by forming a single crystal structure up to Si 9.1 at%. In the literature, it was confirmed that the CrCoNi alloy, which is the basis of the CrCoNiSi alloy, has high hardness and excellent corrosion resistance through the hierarchical nanostructure in the thin film. In this study, we investigated the formation mechanism of amorphous and the resulting increase in hardness for Si-added the CrCoNi thin film. Here, CrCoNiSi thin films are used to study crystal structure deposited on Al alloy substrates by magnetron sputtering from the target fabricated via hot isostatic pressing. In the crystal structure of the deposited film, the amorphous appears with increasing Si content and the crystalline disappears in high Si content. As the amorphous was formed, the cell size of the thin film increased, and the surface porosity decreased by more than half on micro-scale, and the hardness increased greatly from 4.0 GPa to a maximum of 8.8 GPa. In a reciprocal sliding test of the CrCoNiSi film, enhancing hardness with increasing Si content occurs a reduction of friction coefficient and wear amount. The amorphous formation ability according to the alloy component causes a difference in amorphous formation, which affects the microstructure and the improvement of mechanical and tribological properties.

As the world declares carbon neutrality by 2050, a "hydrogen economy society" that uses eco-friendly hydrogen energy is attracting attention. In the hydrogen economic society, structural materials that can withstand extreme environments such as high pressure, cryogenic temperature, and hydrogen embrittlement are required for safety. CrCoNi and VCoNi, representative FCC single-phase medium-entropy alloys, are known to have high strength and ductility at both room temperature and cryogenic temperature and also have excellent hydrogen-induced cracking resistance. Although the recent studies reported the VCoNi alloy can possess a higher hydrogen content and yield strength than the CrCoNi alloy, they have similar levels in the resistance of hydrogen embrittlement. However, no exact mechanism has been identified. In this study, four kinds of FCC single-phase medium-entropy alloys were fabricated with different V content to CrCoNi from VCoNi, and more V content was intended to cause an increase of solid-solution strengthening effect and stacking fault energy. The resistance of hydrogen embrittlement of each alloy was investigated after hydrogen was charged by the electrochemical method under 0.1M H₂SO₄ solution. The resistance was quantified by the hydrogen-embrittlement index (HEI). As a result, both HEI versus hydrogen high contents and HEI versus yield strength in three alloys that have V element have the same or lower values than CrCoNi. This is because as SFE increases a planar slip is promoted, and the slip-band refinement prevents hydrogen from moving to the grain boundary. Interestingly, the reduction of ductility was not observed in the V-rich VCrCoNi alloy, showing the highest hydrogen resistance among them. We investigated the mechanism with respect to the fraction of special boundary, which increases in the initial microstructure produced by heat treatment after cold rolling. Therefore, to design alloys with excellent hydrogen resistance, two hydrogen-resistance mechanisms are proposed: (1) Producing a planar slip as a deformation behavior by a delicate control of SFE, and (2) Increase of special boundary fraction through regulation of both solid-solution strengthening and SFE.

Polycrystalline Ni-based superalloys are vital materials for disks in the hot section of aerospace and land-based turbine engines due to their exceptional microstructural stability and strength at high temperatures. In the drive to increase operating temperatures and hold times in these engines, hence increasing engine efficiency and reduction of carbon emissions, creep properties of these alloys becomes increasingly important. At these higher temperatures, new deformation modes become active. Several alloy compositions and microstructure variants of commercial disk alloys are being explored, including g’ strengthened alloys as well as compositions promoting g’ and g’’ co-precipitation. Microtwinning and stacking fault shearing are important operative mechanisms in the critical 600-800°C temperature range. Advanced electron-microscopy-based techniques have been used to gain new insights into these mechanisms and the rate-limiting processes during high temperature deformation. Atomic-scale chemical and structural changes associated with stacking fault and microtwin interfaces within g’ precipitates have been identified and indicate that local phase transformations (LPT) occur commonly during creep of superalloys. Furthermore, the important deformation modes can be modulated by LPT formation, enabling a new path for improving high temperature properties. This work is part of a GOALI-DMREF program funded by the National Science Foundation in which collaborators are exploring the thermodynamic driving force for LPT formation using DFT modeling, and phase field dislocation dynamics modeling is being used to explore the interaction of dislocations with g’ microstructures under the cooperative shearing and local compositional changes associated with the LPT mechanism.

Ceramic materials provide outstanding chemical and structural stability at high temperatures and in hostile environments but are susceptible to catastrophic fracture that severely limits their applicability. Traditional approaches to partially overcome this limitation rely on activating toughening mechanisms during crack growth to postpone fracture. Here, we experimentally demonstrate a more potent toughening mechanism that involves an intriguing possibility of healing the cracks as they form, even at room temperature, in an atomically layered ternary carbide. The experiments are carried out using an in-house designed and build fixture for in situ SEM mechanical testing of carefully grown single crystals of these atomically layered ternary carbides. Crystals of this class of ceramic materials readily fracture along weakly bonded crystallographic planes. However, the onset of an abstruse mode of deformation, referred to as kinking in these materials, induces large crystallographic rotations and plastic deformation that physically heal the cracks. This implies that the toughness of numerous other layered ceramic materials, whose broader applications have been limited by their susceptibility to catastrophic fracture, can also be enhanced by microstructural engineering to promote kinking and crack-healing.

Amorphous alloys have been widely recognized as potential engineering structural materials due to their near-theoretical mechanical strength and enhanced wear and corrosion resistance, which is derived from an atomic arrangement lacking long range symmetry. However, the lack of intrinsic strain hardening mechanisms upon yielding results in strain localizing into shear bands, leading to brittle fracture in unconstrained modes of loading. Exploiting free volume modulations to promote a more homogeneous plastic response has been a central theme in the development of bulk metallic glasses, employing both intrinsic and extrinsic approaches to introduce structural heterogeneities within the matrix and have led to the development of novel interface-engineered amorphous alloys. Here, we employ synchrotron scattering techniques to quantify the short-to-medium range amorphous atomic network of an Fe-based bulk metallic glass synthesized through additive spray deposition and annealed up to the glass transition temperature. Observed variations in atomic nearest neighbor bonding are then connected to atomistic simulations of alloy nanoglasses to elucidate the effects of amorphous interfaces imparted through the spray deposition process on structural relaxation. The results show that annealing up to the glass transition temperature is accompanied by a global reduction in excess free volume in all samples, as expected. However, contrary to the atomized powders, the spray deposits retain excess free volume at interfacial regions between the amorphous matrix regions, which are shown to expand through free volume redistribution into adjacent particle cores during glass homogenization.

Welding of materials is an important manufacturing process by which two or more similar or dissimilar materials are joined. The result of the process, i.e. the consolidation of two parts into a single one, has been known since iron-age. The most rudimentary and very well-known method, the forge welding, have been mastered by the blacksmith for millennia. Forge welding is a solid-state welding process in which iron and steel are welded by heating and hammering.
Today, the type of joining process can be classified according to the presence or absence of a liquid phase. During the so called fusion welding, there is a state change, from solid to liquid and once again from liquid to solid. On the other hand, during the solid state welding process there is no state change and welding is performed without fusion of the materials. The later process suppresses the solidification-related defects and leads to low internal stresses compared to conventional welding. The Friction Stir Welding (FSW) is a solid-state welding process, it was initially conceived to join metals and alloys and today is also used to produce nanostructured materials.
In this work, FSW was used to join stainless steel 316L and aluminum alloy 5083. During the process, the materials undergo intense plastic deformation at elevated temperatures. The structure of the material at the welded region is strongly affected: the microstructure changes and new phases may form to join the pieces. The conditions of the welding are severe, diffusion processes are fast, leading sometimes to the formation of intermetallic compounds (IMC), which can exist on the nanometer scale. The formation of IMC at the interface and in the welded region is an important factor, affecting physical and chemical properties of the piece. In this work, the combined analysis by scanning and transmission electron microscopy shows that the intermetallic compounds have different size and composition, depending on the region in which they are located with respect to the interface. The size varies from few microns to submicrometric size, the composition is variable, beside Fe and Al some alloy elements like Cr, Mn, Mg and/or Ni can be present. The identification of such compounds would be difficult without using electron microscopy techniques, in particular TEM. To the best knowledge of the authors, only the formation of binary IMC, appearing in the Fe-Al phase diagram, have been reported for similar systems. We show here that the presence of more alloying elements in the new compound, besides Fe and Al, has to be carefully taken into account in order to identify correctly the phase responsible for the integrity of the welded material. In the studied case, some ternary intermetallics compounds, which has no equivalent in the binary Fe-Al system, were identified by TEM.

Highly heterogenous and non-equilibrium microstructures require rigorous characterization of mechanical performance with good spatial resolution. Although such materials systems are complex, the ability to engineer unique combinations of properties make them of general interest to the materials science community. Here, nano/micromechanics are used to explore microstructure – microchemistry relationships in industrially relevant material systems, including grade 2 titanium welds. Not only is this system heterogeneous, but the rapid cooling gives rise to non-equilibrium microstructures. High throughput nanoindentation is used to determine local mechanical behavior, and the results are correlated with electron backscatter diffraction and energy dispersive spectroscopy data to provide a detailed map of chemistry/structure/performance. Unsupervised machine learning is used to cluster the data, accelerating the analysis of, and correlation between, regions of the material from which each of their statistics can then be extracted. To characterize the local toughness, micro-cantilever beams with accompanying local chemical and structural analysis were also fabricated and tested.

Single crystal Ni superalloys are typically are used in power generation and aviation applications due to their unique properties. Recently, incidents of failure due increased temperature around root blade regions has caused Type II hot corrosion leading to cracking in blade roots resulting in catastrophic failure [1]. Understanding the failure mechanism and crack characterisation is vital in solving this industrial issue.
Here we demonstrate a unique workflow of characterization using X-ray microscopy aided with deep-learning based algorithms for data reconstruction and segmentation, combined with FIB-SEM and electron microscopy in order to characterize cracks and crack tips developed during stress corrosion cracking.
By extracting the fracture tip, both crystal plasticity and crystal deformity can be studied in detail resulting in orientation tomography of the corroded region of stress. Combining this correlative workflow we are able to demonstrate a unique technique in C-ring analysis and identifying structural defects not visible using typical microscopy techniques.
[1] : L. Brooking, J. Sumner, S. Gray & N. J. Simms (2018) Stress corrosion of Ni-based superalloys, Materials at High Temperatures, 35:1-3, 120-129, DOI: 10.1080/09603409.2017.1392414

The cyclic stress-strain response of a material provides insight into fatigue properties that may limit the performance of fracture-critical rotating components in gas turbine engines. Historically, conventional cyclic / fatigue is a costly property to assess because of the volume of material required for test coupons and also the time to run each test. Therefore, a critical need exists for the development of novel experimental approaches that can rapidly evaluate the relative changes in cyclic response as a function of alloy chemistry and thermo-mechanical processing history. This becomes especially critical in the materials development efforts, which require systematic exploration of a large materials space. In this work, we present a novel approach using spherical microindentation to deduce stress-strain responses and a cyclic stress-strain curve for Ti-6Al-4V. The results show promise for obtaining reliable, high-throughput, quantitative assessments of the cyclic response.

Solid Oxide Fuel Cell (SOFC) attracts great attention among various types of fuel cells because of the high efficiency and the flexibility of fuel selection. The high operating temperature above 1000 °C was an obstacle to the commercialization of SOFC, but with the advent of the new conductive electrolytes (e.g., doped lanthanum gallate), the operating temperature was reduced to below 800 °C. These developments made metallic interconnects applicable to SOFC stacks instead of expensive ceramic interconnects.
The development of metallic interconnects has been one of the main interests of material scientists over the past two decades. For improving oxidation resistance, 22 wt.% Cr containing ferritic steels were introduced to make the protective Cr₂O₃ layer on the surface. However, this Cr₂O₃ layer produces the volatile Cr species when reacted with the humid air in the high temperature, and the reaction between these volatile Cr species and electrolytes caused the degradation of SOFC stack performance. Therefore, controlling the oxidation resistance and the Cr volatilization becomes the crucial point to the development of metallic interconnects.
In this study, the influence of alloy grain size of the Ferritic stainless steel interconnects was evaluated by the weight gain and Cr evaporation amounts during long-term oxidation. Small grain-sized specimens show a higher oxidation rate, but lower Cr evaporation rate. Intensive microstructural analyses were conducted to obtain the interrelation between the overall oxidation behavior and the grain size. It reveals the intergranular oxide formation at the grain boundaries is the main factor to make the differences in the oxidation behaviors of the interconnect specimens. Especially, cross-sectional 3D EBSD analyses were carried out to investigate the role of alloy grain boundaries. The different grain boundary oxide density results in different oxidation and evaporation behaviors.

In the last decade, Mg alloys containing long-period ordered phase (LPSO) have been attracting considerable attention as a structural material for engineering application and transportation (airplane and automobile) industry, particularly due to their excellent strength to weight ratio.
The present study focuses on investigation of the thermal stability of the microstructure of the rapidly solidified ribbon-consolidated Mg-Zn-RE (rare earth) alloys. In the as solidified state, the material has a very fine grain structure with an average grain size below 1 μm. Moreover, dispersive Zn- and RE-rich stacking faults segregated in basal planes can be observed in the microstructure. The alloys are characterized by a weak basal texture with a more pronounced intensity at the (10-10) pole. Such comprehensive microstructure results in superior mechanical properties, e.g. the Mg_97.94Zn_0.56Y_1.5 alloy has shown a yield strength of 360 MPa and an elongation of 18%. In order to study the thermal stability of such microstructures, isothermal annealing in a range of 300 °C - 500 °C was applied. Scanning electron microscopy (SEM), including backscatter electron images (BSE), and electron backscatter diffraction (EBSD) techniques have been used to study the microstructure changes. Furthermore, X-ray diffraction measurements were performed for reliable texture analysis. The microstructure was found to be stable with increasing annealing temperature up to 400 °C. With further temperature increase, the growth of the grain size and changes in the texture of the alloy, particularly, redistribution of the intensity at the (10-10) pole, can be related to the recrystallization process. Microstructure changes are correlated with the results of the microhardness measurements.

Steelmaking contributes to 7% of the total annual CO₂ emissions globally. One approach to decarbonize steelmaking is to shift from the conventional coal-based reduction of iron ores in blast furnaces to hydrogen-based direct reduction, changing the emissions from CO₂ to H₂O. Despite its opportunity, hydrogen-based steelmaking, has been slow to commercialize because iron ore reduction with hydrogen is slow and energy-intensive (i.e. endothermic). While extremely important, the chemistry in these reactors is often complex, as the native ore includes particles from ~7-nm to 1-mm sizes, which exhibit different kinetics and mass transfer that are poorly understood. In this work, we explore the kinetics in the H₂ reduction chemistry of magnetite (Fe₃O₄), from the industrial Fe₃O₄ to the lab-synthesized nanoscale Fe₃O₄. Using in situ synchrotron X-ray diffraction, we demonstrate the kinetics of this 2-step reaction; we identify that the initial magnetite to wustite (FeO_x) reaction occurs quickly, while the subsequent wustite to metallic iron is the rate-limiting step. A closer look at the temperature dependence reveals that the quantitative picture of these kinetics must account for the reaction’s facet-dependence. Beyond the kinetics, we also observed mesoscopic structures that evolved during the reaction. The initial 10-nm Fe₃O₄ nanoparticles appear to self-assemble and ultimately form elongated and irregular structures (i.e. “nano-worms”), which we confirm with in situ SAXS. Our findings suggest that the (220) facets preferred by the chemistry may instigate hierarchical structuring, indicating key challenges for mass-transport in this system. Our deep understanding of the direct reduction of Fe₃O₄ using hydrogen sheds light on strategies to improve the performance of direct iron reduction, further commercializing hydrogen steelmaking.

Copper and aluminum alloys in a lap-joint configuration are successfully joined at solid-state by electrically assisted pressure joining (EAPJ). For joining, electric current with different current densities is directly applied to the specimen assembly during continuous axial plastic deformation. All experiment cases are carried out at nearly the same temperature with constant displacement rate and joining time. Microstructural analysis confirms that the joint is completely generated without melting for the selected materials. In addition, the mechanical properties of the joints are evaluated by tensile experiment, which suggests that joint properties are strongly affected by the electric current density. The present study demonstrates that the concept of EAPJ is promising and applicable for the fabrication of the joint of dissimilar material combinations.

Thanks to the excellent strength-to-weight ratio Mg alloys have been attracting considerable attention as a structural material for engineering applications and the transportation (airplane and automobile) industry. Moreover, the biocompatibility of Mg motivates their further development as a bio-implant material, e.g., for orthopedic implants, screws or/and cardiovascular stents. Therefore, there is a need for the development of Mg alloys with enhanced properties with respect to a particular use.
The Mg-Zn-RE (rare earth) alloys prepared by the consolidation of rapidly solidified (RS) ribbons are characterized by complex microstructure with elongated worked and dynamically recrystallized grains with an overall average grain size of about 800 nm and weak basal texture. Using a low amount of alloying elements (up to 2at.%) results in the formation of solute segregated Zn- and Y-rich stacking faults (SFs). In order to optimize the performance of alloys, the influence of both amount of the alloying elements (between 1-2 at.% of Zn and Y) and the metal flow rate during the consolidation of RS ribbons (via extrusion) on the microstructure and resulting mechanical properties has been elucidated. The alloy with optimal values of tensile yield strength and elongation of 362 MPa and 18.9 %, respectively, has been subjected to corrosion performance testing. Corrosion behavior of RS ribbons-consolidated alloy is compared with one of an extruded alloy of the same composition having a larger grain size of α-Mg (9 µm) and wavy fractures of long period stacking ordered (LPSO) phase (10 µm). For evaluation of corrosion resistance, besides common electrochemical methods, such as hydrogen release, electrochemical impedance spectroscopy, cathodic polarization, an in-situ acoustic emission technique was employed during immersion of the samples in NaCl solution. The tensile properties and corrosion performance were correlated to the microstructure features, such as dispersion of SFs and LPSO phase, grain size, distribution of internal strain via KAM analysis. Inhomogeneous distribution of internal strain in the complex microstructure was found to be of key importance for the performance of dilute RS ribbons-consolidated alloys.

Toughness, stiffness, and strength are major characteristics of biological hierarchical composites, and their combination is a long-sought objective for engineering design. Nature achieves the balance of such properties through a combination of multiscale key features. Yet, emulating all these features and achieving multifunctionality into synthetic de novo materials is rather challenging. Here, we fine-tune manual lamination, to implement a newly designed bone-inspired structure into fiber-reinforced composites, also providing repairing ability. We draw inspiration from the microstructure of cortical bone and we implement the characteristic features (i.e., osteons, Haversian canals, and lamellae) into a fiber-reinforced composite at a larger scale. We adopt a comprehensive approach, which includes design via numerical simulations, ad hoc manufacturing techniques, and experimental testing to achieve a novel composite design with an optimal tradeoff of fracture toughness, stiffness, strength, and lightweight.
The proposed solution represents an optimal lightweight material with better performance than conventional materials, such as metals and alloys, and additional functionalities. The results also show how the new design significantly boosts the multifunctionality compared to a classic laminated composite, made of the same building blocks, also offering an optimal tradeoff with stiffness and strength. The major observed fracture mechanism is the continuous deviation of the crack from a straight path, promoting large energy dissipation and preventing a catastrophic failure. The hollow canals, inspired by the bone Haversian c., besides contributing to a further weight reduction, also provide a route to add self-healing capability. The new insights resulting from this study can guide the design of de novo fiber-reinforced composites toward better mechanical performance and multiple functionalities with the final goal of consolidating multiple parts and functions into a single one, reaching the level of synergy and multifunctionality of their natural counterparts.

Strong, ductile, thermally stable, and irradiation tolerant materials are in urgent demand for improving the safety and efficiency of advanced nuclear reactors. Materials that employ microstructure features to manage radiation damage and maintain high-temperature mechanical properties are especially desirable. We design amorphous ceramic and metallic composites with two distinctive microstructures – amorphous ceramics composites (ACCs) containing nanoscale metal-rich heterogeneities in amorphous ceramics matrix and amorphous ceramic reinforced metals (ACMs) containing fine-scale, thermally-stable, amorphous ceramic particles in metal matrix - through tailoring the composition and microstructure of amorphous ceramics and metal phases. Both ACCs and ACMs exhibit improved mechanical properties, helium management and irradiation resistance in high temperature and irradiation environments. More importantly, we will develop the processes-microstructures-properties relations in order to accelerate the discovery and development of amorphous ceramic and metallic composites for applications under extreme conditions.

The properties of nanostructured materials can differ from their equivalent bulk material by an order of magnitude. At the nanometer scale structural characteristics start to play an important role, and size effects can become the predominant factor on the final property. Using a novel polymer templating method followed by heat treatment, we fabricated low density (sub-10%) nanofoams of Cu, Cu-Ni and Cu-Zn alloys with alloy concentration from 10at% to 25at%, ligament size and relative density. The microstructural characteristics were observed under electronic microscopy (SEM) and X-ray diffraction (XRD). The modulus and strength were determined using instrumented indentation tests to find the modulus and hardness of these materials. The linear association between parameters studied and the dependent variables (Modulus and hardness) was found using Pearson Correlation Coefficient (PCC). Relative density was found as one of the dominant parameters on the mechanical properties, showing significant decreases in strength and modulus around 6% relative density. The mechanical properties results are not only a consequence of the relative density; however, the combination of ligament size and strengthening mechanisms do significantly influence the result. The structure of nanofoams can be tailored to have desired outcomes by controlling the combination of foam chemistry, architecture, and density.

The development of carbon fiber-reinforced polymer composites has been attracted among scientific community due to their lightweight, exceptional mechanical performance, robustness, and long service time. The performance of composite highly depends on several factors including the type of reinforcement, polymer matrix, and the interface layer. Aside from the durability and long-lasting properties of carbon fiber, the carbon-fiber-reinforced polymer composites usually have limited long-term mechanical performance due to their poor compatibility and weak interactions between fiber and polymer matrix. Herein, we developed high-performance carbon fiber epoxy composites with improved interfacial interactions between fiber and matrix via surface modification of carbon fiber that determines the effectiveness of load transfer across the filler-matrix interface. The chemical functionalization of carbon fibers using silane treatment delivered amino-functionalized carbon fibers that actively interacted with epoxy matrix leading to enhanced mechanical properties. Raman mapping results clearly demonstrated the formation of the distinguished interphase region between amino-functionalized carbon fiber and epoxy polymer confirming the presence of strong interfacial interaction between matrix and carbon filler. Effective covalent bonding between amino-functionalized carbon fiber and epoxy matrix afford uniform fiber distribution in epoxy matrix and eliminated the phase separation during the thermal cycling process. Moreover, the enhancement of interfacial interaction between fiber and epoxy matrix noticeably improves tensile strength (from 62 MPa to 73MPa) with 5% wt. carbon fiber loading. Such improvements in the mechanical performance of carbon fiber epoxy composites suggest the great potential of surface modification of carbon fiber in the development of high-performance carbon fiber reinforced composites for a wide range of applications.

Uniformity has served as a primary scheme for the microstructure control of ferrous alloys, which has been successful, pervading all aspects of steel products. Nevertheless, ever increasing demands for the safety and environment issues requires unprecedented properties that is difficult to be obtained by the standard practice. As a new pathway for the microstructure control, we consider an active utilization of chemical heterogeneity. In general, the heterogeneity in microstructure is supposed to be suppressed to avoid any possible degradation in the structural properties. However, a sophisticated application of heterogeneity enables a precise control of microstructure in various scales that provides more enhanced properties compared to the conventional ones. As a case study, we demonstrate the influence of chemical heterogeneity on microstructure and tensile properties of ultra-high strength steels mostly covered with martensite. Chemical heterogeneity was produced by intercritical annealing prior to austenitization. It generated micro-scale and nano-scale Mn enriched domains, which originated from the austenite and cementite evolving during intercritical annealing. Chemical heterogeneity in both Mn-enriched domains were almost maintained after austenitization, eventually contributing to simultaneous improvement in strength and ductility. Micro-scale Mn enriched domain generated strain partitioning between Mn-depleted and Mn-enriched domains during tensile deformation due to dissimilar solid solution strengthening. The strain partitioning resulted in higher back stress, which contributed to the improvement of strength. Meanwhile, nano-scale Mn enriched domain evolved into exceptional stable nano-sized austenite, which enhanced the ductility by exhibiting a persistent TRIP effect.

Research for improving mechanical properties and durability of molds, dies and tools has been continuously carried on. Improved mechanical properties and durability of such tools bring advantages to relevant industries, in a way of reduction in cost and replacement-cycles, and improvement in product quality and productivity. Recently, application of direct energy deposition (DED), one of additive manufacturing techniques, on hard-facing for dies and tools. DED allows 3D stacking on arbitrary surface shape with desired thickness, enabling uniform and rapid hard-facing on complex dies and tools. In this study, M4 powder was deposited on D2 substrate to investigate microstructure and mechanical properties of the deposited layers. The deposited surface layers rapidly solidified showed chilled zone at the interface with substrate and predominant columnar dendrite structure toward the upper surface. Unlike the bulk counterpart of M4, no coarse carbides were formed, and tens of nanometer-sized fine carbides were located in the interdendritic region. The M4 powder subjected to standard normalizing and quenching treatment, generally shows Ms point around 200 °C. Thus, the deposited layers was expected to show abundant martensite phase as deposited, but rather showed more than 40% of retained austenite surprisingly, which undoubtedly degrades hardness of the hard-faced surface. It can be thus inferred that powder for hard-facing should not be selected based on the classical estimation of Ms point, which is not considering rapid solidification process. Thus, we here propose a new approach to power alloy design which is necessary for hard-facing of steel-based dies and molds using DED method.

Advanced multi-component alloys, such as bulk metallic glasses and high entropy alloys, exhibit outstanding mechanical properties. Phase formation and physical properties of such material systems depend largely on their chemical compositions. Vast composition ranges, therefore, need to be investigated to discover novel multi-component alloys with enhanced mechanical properties. In this work, we propose an experimental technique that combines combinatorial synthesis and a high-throughput mechanical testing method to efficiently carry out tensile testing for a broad range of metals and alloys. Si-based microfabrication processes, as well as the combinatorial magnetron sputter deposition process, were used to fabricate freestanding tensile specimens of thin-film alloys with composition spreads. The membrane deflection experiment (MDE) has been used to determine the thin-film alloys' stress-strain curves. A 3-axis indentation system with reasonable force and displacement resolutions (5 nm, 60 nN for displacement and force) has been built and utilized for the MDE. A vision-based control scheme was integrated into the system to automatically align the indenter tip to a specific position of the sample. Details of the experimental technique and some results on combinatorial thin-film alloys will be shared.

Copper (Cu), a ductile and conductive metal, has become an essential component in engineering applications and its 3D architecture is also attracting attention as new structural material. Unfortunately, the fabrication of copper using additive techniques, specifically with the high-resolution laser powder bed fusion (LPBF) technique remains challenging. This is mainly due to the high reflectivity of pure Cu to infra-red laser (λ=1030-1070 nm) which leads to poor melt pool, rough surface, and internal fusion defects. Therefore, the additive manufacturing of pure Cu 3D architectures still remains unrealized and its mechanical properties still need to be investigated.
This presentation, for the first time, will report pure Cu 3D architectures, specifically 3D truss-lattice structures, additively manufactured by using a more conducive green laser (λ=515 nm) based LPBF and their systematic structural, microstructural, and mechanical characterization. We selected an octet-truss structure and a cuboctahedron structure as our model architectures because they deform via different mechanisms (octet-truss shows stretch-dominated and cuboctahedron shows bending-dominated deformation). Also, by fabricating the same 3D copper architectures using both the typical infra-red laser and the green laser, we could establish a laser-wavelength dependent comparison of their structural and microstructural features using a combination of optical microscopy and electron backscattered diffraction (EBSD).
Subsequently, we conducted chemical etching of the Cu architectures in a solution of ferric chloride to reduce their surface quality. From sequential SEM images of Cu architectures chemically etched for 0-900 sec, we were able to establish an optimized chemical etching protocol to remove the unmelted particles and improve the surface roughness of the Cu architectures. The surface roughness, internal structural features, and manufacturing defects such as voids of the etched Cu architectures were then ascertained using Micro-computed tomography (MicroCT) based 3D reconstruction.
Finally, we evaluated the mechanical properties of the Cu lattice structures under various strain rates from ~0.001/s to ~1000/s using a combination of dilatometer and split-Hopkinson pressure bar. Specifically, the dilatometer allowed us to investigate the mechanical properties of the lattices at the elusive intermediate strain rates of ~1-10/s. From the stress-strain curves, we were able to extract the mechanical properties of the Cu architectures including stiffness, strength, and shock absorption efficiency. With the MicroCT of pure Cu architectures before and after the deformation, we could capture the rate-dependent internal structure of the two architectures under compression. Combined with the finite element analysis conducted using an accurate model of the lattices from MicroCT based 3D reconstruction, we could also simulate the structural evolution of two the different Cu lattices at a wide range of strain rates.

The discontinuous precipitation (DP) was known to mechanically detrimental mechanism compared to normal precipitation (continuous precipitation, CP), while the DP-structure has higher conductivity than the CP-structure. To get the optimum balance of strength and conductivity, we controlled the fraction of DP-structured area in the Cu matrix of Cu-4Ni-1Si alloy by changing aging time. After successive cold-working, the alloy showed considerably wide ranges of strength and electrical conductivity, from 500 to 1,170 MPa, and from 23 to 50% IACS, respectively. The accelerated strengthening of the alloy with DP is caused by uni-directional alignment and enhanced plastic deformation of DPs in alloy after cold-working. The DP-structured area was more severely deformed than CP counterpart region during cold-working when the alloy has a mixed form of CP and DP. The alloy with CP- and DP-structures leads a drastic increase of strength without sacrifice of conductivity.

Ti-6Al-4V is widely used in the aerospace industry because of its high specific strength, excellent heat resistance and corrosion resistance. On the other hand, the manufacturing cost is higher than other alloys due to its high cutting and material cost. AM (Additive Manufacturing), which can provide them with enhanced functionality through high flexibility of design and improvement of material yield by near-net shape, is expected to be a solution to the above problem. In particular, the Laser-PBF (Powder Bed Fusion) method enables high-precision building of complex shapes, so this method is being applied to products.

However, since process of AM is a new technology, there are many unclear points regarding the relationship between manufacturing conditions, quality, and mechanical properties, and it is important to clarify the fundamental phenomena in order to obtain stable material properties. The purpose of this study is to control material properties of additive manufactured Ti-6Al-4V. The test specimens were built by L-PBF method under several conditions of process parameters (power, scan speed, hatch spacing, layer thickness). Also, we clarified the thermal history during building through the analysis and estimated the effect of the building condition on the As-built microstructure. Then, we clarified the influence of the process parameter on the As-built microstructure from the comparison of the experimental and analytical results.

The prior-β grain size, which is dominant in the As-built microstructure, largely depends on the spacing of the laser scan paths. In addition, it was indicated that the proportion of α’ martensite was related to the thermal history during building.

Plasma-facing materials (PFMs) in a nuclear fusion reactor are exposed to high heat flux and energetic particles, so radiation damage-tolerant materials need to be systematically investigated and developed. Although tungsten-based alloys with nanocrystalline structures have been reported to mitigate radiation-induced damages effectively, vast combinations of microstructures and compositions need to be investigated to discover novel PFMs with the ideal set of properties. Trial-and-error-based traditional irradiation experiments can be inefficient to explorer the huge design space. In this study, we utilize a combinatorial approach that allows efficient characterizations of composition-microstructure-dependent irradiation damage behaviors of nanocrystalline W-based alloys. Structural, thermal, and mechanical properties were analyzed before and after He-ion irradiation experiments for W-Re-Ta thin-film alloys with composition spreads. We find that the distribution of He-based defects can be significantly altered by the solute concentrations Ta and Re in W, which critically affects the property changes upon irradiation.

The ZrB₂ and HfB₂ materials are promising for application in hypersonic aerospace, cutting tools, metallurgy, microelectronics and refractory industries. The structure and properties of sintered under high pressure (4 GPa) - high temperature (1800 ^oC) or HP-HT conditions ZrB₂, HfB_2, ZrB₂+30%TiB₂ and ZrB₂-20% SiC refractory materials are under consideration. HP-HT sintered HfB₂ (a=0.3141, c=0.3473 nm γ=10.42 g/cm³) demonstrated hardness H_V(9.8 N)=21.27±0.84 GPa, H_V(49 N)=19.29±1.34 GPa, and H_V(98 N)=19.17±0.5 GPa and fracture toughness K_1C(9.8 N)=6.47 MN×m^0.5. High pressure sintered ZrB₂ (a=0.3167 , c=0.3528 nm, γ=6.1 g/cm³) demonstrated H_V(9.8N)= 17.66±0.60 GPa, H_V(49 N)= 15.25±1.22 GPa, and H_V(98 N)= 15.32±0.36 GPa and K_1C(9.8 N)=3.64 MN×m^0.5. Addition of 30 wt.% of TiB₂ to ZrB₂ did not allow to increase hardness of the material essentially (H_V(9.8 N)=17.75±2.36 GPa, γ=5.29 g/cm³ ). Addition of 20 wt.% of SiC to ZrB₂ and sintering under high pressure allowed essential increase of hardness to H_V(9.8 N)=24.18±0.7 GPa, H_V(49 N)=16.68±0.5 GPa, and H_V(98 N)=17.59±0.4 GPa and fracture toughness (K_1C(9.8 N)=6.49 ± 0.25 MN×m^0.5, K_1C(49 N)=7.06± 1.55 MN×m^0.5 , K_1C(98 N)=6.18± 1.24 MN×m^0.5) of composite ZrB₂- SiC material (γ=5.03 g/cm³). The work is supported by Projects NATO SPS G5773; 03-03-20, ISM-29/20, III-3-20 (0779), of NASU.

Vermiculite, layered ionic material, belongs to the family of layered silicates. It is very difficult to exfoliate vermiculite into the single sheets, compared to other layered clays with low surface charge, as the surface charge of vermiculite is too high, reaching values between 0.7 – 1.
We present here efficient method of exfoliation of vermiculite crystals into the single sheets, and their physical properties. Vermiculite 1 nm thin single sheets has been examined regarding their composition, conductive, piezoelectric, mechanical and magnetic properties, together with the determination of the band gap value, using XPS, SPM, XRD, magnetic property measurements and EELS.
Determined properties showed that vermiculite is very interesting 2D material for fabrication of 2D heterostructures, promising for combining it with the graphene and other (semi)conductive materials. Moreover, large scale exfoliation process allows large-scale fabrication of the heterostructures by the self-assembly.

The electrochemical behaviors resulting from the microstructural modifications from several joining and processing techniques are assessed and compared to that of the parent materials in a series of studies. Experimental and numerical methods are implemented to explore the electrochemical behaviors such as galvanic corrosion, passivity, and pitting corrosion of the modified microstructures in detail. It is observed that significant microstructural changes such as grain refinement, increased dislocation densities, and high local strain greatly impact the electrochemical behaviors of the resulting microstructures. Numerical models are built using electrochemical parameters extracted from the experimental approaches to perform additional studies on the electrochemical behaviors. Afterward, the numerical results are further validated by comparing them to the experimental results. Additionally, the models in these studies are capable of tracking the moving anodic boundaries and also performing additional studies which are difficult to do experimentally.

The interactions between lattice defects on a nanoscale have significant effects on the macroscopic mechanical properties of crystalline metals. Further understanding of the effects of lattice defects on fundamental plastic deformation units such as dislocation is key to develop structural materials with excellent mechanical properties. The advances in computer power and simulation methods enable us to evaluate the interaction between dislocation and other lattice defects from various approaches. We here focused on the dislocation-solute interactions and dislocation-grain boundary interactions in body-centered cubic iron. We investigated the interaction details and effects of lattice defects on the mechanical properties based on atomistic simulations. We evaluated the screw dislocation-solute interaction for various solute elements and solute positions using large-scale first-principles calculation. The interaction energy is affected by the relative position between dislocation and solute atoms, dislocation core types, and solute elements. The obtained interaction energies were utilized in dislocation modeling in larger time and space scales or in theoretical equations in order to predict the effects of solute atoms on dislocation dynamics and macroscopic yield strength. We also evaluated the dislocation-grain boundary interactions using atomistic simulations based on the empirical model of the interatomic interaction. The grain boundary is a planar fault and acts as an important role in grain boundary strengthening of structural materials. The dislocation-grain boundary interaction in iron was evaluated for different grain boundary types using nanomechanical simulations such as nanoindentation. When the indentation is conducted near the grain boundary, dislocation nucleation and propagation behaviors beneath the indenter are affected by the grain boundary. The simulations reveal the stress field beneath the indenter, activated slip directions, and dislocation components mainly interacting with the grain boundary. The dislocation behaviors are affected by the crystallographic orientation in the inner grain as well as the grain boundary. In experimental nanoindentation, the dislocation behaviors across the grain boundary depend on the grain boundary type. The atomistic simulation results provide factors affecting the dislocation behaviors near the grain boundary and a deeper understanding of experimental results.

It is becoming increasingly important to use data on materials in order to accelerate materials development, particularly data on the microstructure of materials because they are associated with both processes and characteristics. The data typically acquired for microstructures are microstructural images; however, it is inefficient to use them directly for materials development because the microstructural images are two-dimensional unstructured data. Therefore, it is necessary to apply image analysis, but this also presents some challenges.
The first issue relates to the workload for image analysis. To analyzes the microstructural images, it is necessary to determine the region of interest (ROI) which is time-consuming and requires a highly skilled analyst. The second issue is the feature extraction from the microstructural images. Although various extraction methods have been proposed, extracting the features of the morphology of the ROI is still considerably challenging.
In this study, we developed a workflow that consists of deep neural network (DNN) based segmentation method [1] and persistent homology analysis [2] to solve the above problems. The DNN-based segmentation method was utilized to solve the workload problem by automatically converting the microstructural images to segmented images. The persistent homology analysis was utilized for feature extraction by expressing the geometric shape and intermingling characteristics of ROI as a persistence diagram. The persistence diagram can extract the morphological characteristics of both connected and isolated regions.
The workflow was applied to a Cr-based alloy with a complex structure. Sample of the alloy were prepared by cutting from a cylindrical ingot that was formed by pouring and cooling molten metal in a mold. The exterior of the ingot cooled rapidly while the center cooled gradually. The microstructure at three positions (center, edge, midpoint of the center and the edge) in the ingot were observed by using a scanning electron microscope and obtaining 50 field of view backscattered electron microscopic images. The images were automatically converted to segmented images by applying the DNN-based segmentation method. Then the persistent homology analysis was carried out to extract the features of the ROI as a distribution of plots in the persistence diagram. The persistence diagram from the 50 field of view images was integrated to reduce the cross-field variation. We defined new feature points in the persistence diagram distribution to investigate the relation between the cooling rate and the thickness of the ROI on the microstructures. The new feature points revealed that the difference in the microstructures of the sampling points were caused by differences in the cooling rate.

[1] V. Badrinarayanan, A. Kendall, and R. Cipolla, IEEE Trans. Pattern Anal. Mach. Intell. 39, 2481-2495 (2017).
[2] Edelsbrunner et al., Discrete Comput. Geom. 28, 511–533, (2002).

Nanostructured metal/ceramic multilayers via tailoring layer thickness opens a pathway to design high strength and toughness composite materials. In this work, we employed a combination of Berkovich and Spherical nanoindentation to study the in-depth mechanical properties and strain hardening/softening mechanism of highly textured Cu/c-BN multilayers. Berkovich nanoindentation is able to measure the hardness, which is proportional to flow stress (σ_f), while Spherical nanoindentation successfully captures the yield stress (σ_y) and post-yield extrusion behavior. Both yield stress and flow stress increase with decreasing layer thickness (h), and reach their maximum (σ_y = 15 GPa; σ_f = 21 GPa) when h = 5 nm. The multilayers show significant softening when h is less than 5 nm. Microscopic characterization confirms that the swap of deformation mechanisms is responsible for the size-dependent deformation behaviors. Plastic extrusion of Cu layers and brittle fracture of c-BN layers accompanied by dislocation pile-up are dominant when h > 5 nm, while Cu and c-BN layers are co-deformed when h < 5 nm. The findings provide insights into the design of advanced metal/ceramic nanolayered composites with excellent mechanical properties.

It is known that shock loaded metals exhibit shear stress relaxation at the plastic wave front, i.e., the stress state of the material deviates substantially from the yield surface when longitudinal stress or pressure reaches the peak value, but no consensus has ever been reached on its origin and mechanisms due to the multiscale nature of plasticity. To this end, the current study takes a top-down approach by conducting a theoretical and numerical investigation on the macroscale (continuum scale) and simulations on the mesoscale with crystal plasticity (CP), followed by an analysis of the microscale factors based on molecular dynamics (MD) investigations from the literature. On the macroscale, theoretical derivation uncovers that the total strain rate of less than 1.5 times the equivalent plastic strain rate is the critical condition for shear stress relaxation in the isotropic elasticity and von Mises plasticity framework. Evolutions of stress and strain during dynamic deformation of aluminum under shock loading is then examined via continuum finite element simulations, assuming Johnson-Cook (JC) constitutive relation, and considering the nonlinear elastic behavior and equation of state (EOS) at high pressures. The results suggest that strain rate hardening is responsible for the appearance of shear relaxation, and verify that relaxation begins exactly as the theoretical condition predicts. Other factors play subsidiary roles; thermal effect is negligible under low pressure and its promotion of relaxation becomes remarkable when approaching melting, and strain hardening slightly suppresses relaxation. These findings are all echoed in meso-scale simulations with the CP model and hyperelastic constitutive equation. The role and contributions of strain rate hardening, strain hardening and thermal softening to shear relaxation agree well with macroscopic observations. In addition, contribution of deformation heterogeneity is investigated on the mesoscale, and the simulation results show that the additional dissipation induced by grain boundaries' interference with dislocation slip has a subordinate stimulative effect. Results of both macro- and meso-scopic analysis indicate that mechanisms related to strain rate effect should also be the dominant factor for relaxation on the microscale, and the contribution or connection to strain rate hardening should be the guideline for evaluating the relevance and importance of each microscopic mechanism. Consequently, nucleation and multiplication of dislocations are identified as the main cause of shear relaxation on the microscale, based on existing MD simulations and analysis from dislocation theory. In summary, a top-down perspective of the cause and a full picture of shear relaxation during shock loading is established, which reveals that the strain rate effect plays a vital role while contributions of other effects are limited.

Deformation behavior of light weight and high strength hexagonal metals such as Mg and Ti alloys is determined by the coupled dislocation and twinning plasticity. Establishing the quantitative relations between the mechanical properties and plasticity as well as microstructure evolution can promote developing high performance hexagonal materials.
We develop a coupled crystal plasticity finite element-phase field (CPFE-PF) model to predict the mechanical properties of these materials under complex loading conditions. In this coupled CPFE-PF model [Guisen Liu, Hanxuan Mo et al, Acta Materialia 202 (2021) 399–416], the CPFE solves the elastic fields and plastic deformation, and passes the elastic fields to PF for calculating elastic driving force for twinning. PF method spatially distinguishes twin domains from parent and passes the evolution rate of twins to CPFE for calculating twinning rate. In this way, the coupled CPFE-PF model can predict evolution of 3D twins and influence of coupled dislocation-twinning activities on deformation behavior of magnesium alloy that agree well with experimental observation.
To model the evolution of physically sharp twin/parent interface during twinning deformation (instead of artificial diffuse interface due to long-range stress field) and remove the results dependence on mesh orientation, we further improve this CPFE-PF model from two aspects: 1) adopt a coarse mesh in CPFE calculation of stress and strain, and a local refined mesh in PF computation of interface evolution with numerical smoothing and fitting; 2) the twin/parent interface can only propagate when the neighboring partially twinned coarse element has fully transformed to twin phase. This improved model can obtain a relatively sharp interface (about one-element in thickness) during twinning deformation, and meanwhile retain high computation efficiency in solving elastic field. Furthermore, this local refinement-smoothing method solves the originally mesh orientation dependent interface normal and stress distribution, and can predict same twin morphology irrespective of mesh orientation.
The improved CPFE-PF model can model the evolution of physically sharp interface and meanwhile obtain numerically mesh-insensitive twin morphology as well as stress response. Thus, it has promising application in modeling twin-twin interaction to prevent the diffuse mixture of multiple domains (multi-variant and parent), and can be further generalized to model martensitic transformation which requires both physically sharp interface and numerically mesh-insensitive interface orientations. This model can be further developed by incorporating more atomistic twinning mechanisms such as automatic nucleation to quantitatively predict the effects of complex processing or loading conditions on the mechanical behavior of Mg and Ti alloys, where coupled dislocation-twinning plasticity is involved, providing reference for designing high performance hexagonal alloys.

Metal-metal nanolaminates made by accumulative roll bonding (ARB) is a developing new class of structural nanocomposites that possesses high strength, good ductility, and can also be produced in bulk. These nanolaminates exhibit very strong crystallographic textures and mechanical anisotropy. Yet, mechanical characterization of ARB nanolaminates is often limited to tension along rolling direction (RD). Using in situ pillar compression, we report that in-plane shear deformation mechanism in Cu(63nm)/Nb(63nm) ARB nanolaminate strongly depends on the shear direction. Plastic shear along the RD abruptly changes into crystallographic rotation and later into interface shear as the shear direction deviates away from the RD. The observed shear mechanisms are driven by competition between different slip systems in Cu layers and at the interface. We expect similar deformation anisotropy to happen in other ARB nanolaminates with sharp texture and interface orientation. Comprehensive characterization of such nanolaminates may require considering more than just the usual rolling/transverse/normal direction anisotropy.

Machine learning based prediction models have been spotlighted as a new approach to discover alloy compositions with outstanding properties. It is however challenging to acquire a large amount of strength data for vast composition ranges for the models. Combinatorial synthesis and high-throughput experiments can provide a practical pathway to obtain various composition dependent properties of thin films libraries. In this study, magnetron sputter deposition was used to produce ternary thin film alloys with composition spreads. X-ray diffraction, scanning electron microscopy, and nanoindentation were performed to characterize composition dependent microstructural properties and hardness. Then a machine learning model that can predict hardness of the alloy thin films was constructed. We show that the model has a high accuracy and can be applied to develop novel thin films with high strength.

Many traditional metallic alloys and structural polymers exhibit dynamic embrittlement. In contrast, nanophase-segregated copolymer, polyurea PU1000, is recently found dynamically tougher than many metallic high-strength alloys. However, it has been challenging to evaluate the dynamic toughness and identify associated dynamic toughening mechanisms of the self-organized nanostructured copolymer. Here, we present the discovery of nanoscale dynamic toughening mechanisms of the segmented block copolymer that can be generalized for designing advanced nanocomposites.
To characterize the novel dynamic toughening mechanisms, firstly, we accurately measured the dynamic toughness of polyurea PU1000 by developing a framework of a big-data-generating experiment that uses a streak camera system. The toughness was measured by detecting the continuous-time history of displacement information along a line on the surface of a sample under plate-impact loading, employing a newly invented Dynamic Line-Image Shearing Interferometer (DL-ISI). Then, the dynamic cohesive-zone parameters of the running crack could be inversely extracted with a deep-learning framework based on a convolutional neural network (CNN) and a state-of-the-art conditional generative adversarial network (cGAN) model pre-trained with a corresponding FEM dataset. The dynamic fracture toughness of polyurea was measured 12100 ± 1100 J/m² under a very high crack-tip loading rate, ~10⁷ MPa√m/s. The local strength near the crack tip under a local strain rate of ~10⁸ s-1 was evaluated 294 MPa, which is nearly three times the 105 MPa spall strength at ~10⁶ s-1 strain rate.
Secondly, we revealed molecular-level dynamic toughening mechanisms of polyurea through in-situ AFM experiments and mesoscale coarse-grain molecular dynamics simulations. For the experiment, we employed a novel in-situ AFM loading device with invariant observation sight to collect high-resolution in-situ AFM tapping-mode phase images of polyurea under stress-relaxation at various fixed strains ranging from 0% to 260%. From the in-situ AFM and coarse-grain molecular dynamics (MD) simulation studies, we found that the hard-domain fragmentation and subsequent self-healing processes of bidentate hydrogen bonds promote both ductility and strength of the copolymer in a broad range of strain rates. The mesoscale coarse-grain MD simulations showed the hydrogen-bond cross-linked hard domains fragments under high-strain-rate deformation in the timescale of nanoseconds, which contributes to the high toughness of crack-growth incubation. In the in-situ AFM study, we could also observe hard domain fragmentation under moderate strain rate, followed by self-healing process initially in lateral direction then relaxation process of self-healing in the loading direction. The nanoscale self-healing processes help recover the strength of the nanocomposite, increasing dynamic running-crack toughness.
In this work, we found that nanoscale fragmentation and self-healing at the nanoscale foster the strengthening and toughening of the nanocomposite in a broad range of strain rates.
The long rod-shaped hard nanophases (MDI) embedded in a soft matrix (polytetramethylene oxide) fragment primarily due to the shear-lag mechanism of interfacial load transfer between the hard and soft phases. As the matrix viscoelastically relaxes and the fragment size becomes smaller at higher strain rates, the nanoscale fragmentation and self-healing mechanisms enhance both strength and toughness under a high loading rate. We believe these newly revealed nanoscale strengthening and toughening mechanisms can be further capitalized for designing advanced polymeric nanocomposites and a particular class of high-entropy alloy nanocomposites.

Owing to the unique tribological features, nickel alloys with rich nickel content are preferred over conventional alloys in tribological applications such as bearings and gears. Friction and wear problems are inevitable among mechanical components in various engineering applications. The design of these components are based on surface properties of materials. Therefore, it is required to understand the tribological behaviour of a structural material in order to determine a way for the optimum work efficiency of the material in the application. One way for an optimum friction and wear characteristics of materials is to analyse the anisotropic tribological response of the material and choose a better plane of contact for the application. To elucidate the best wear and friction properties, scratch experiments in combination with EBSD are performed on (100), (110) and (111) planes of bulk pure single crystalline nickel. Furthermore, the scratch test shows complex tribological responses and thus requires a deep insight on the influence of crystallographic orientation on elastic-plastic behaviour of the material during the scratch test. For this, similar scratch tests are performed using three dimensional molecular dynamics simulations, individually on the planes of single crystalline nickel with a rigid diamond indenter to understand the dislocation generation and propagation, material pile-up and surface topography of nickel during scratch. EAM potential is used for the interatomic potential among nickel atoms and Morse potential is used for interaction between nickel and carbon atoms in the diamond indenter. The indenter is made rigid and the interatomic potential among carbon atoms is ignored during the simulation. A correlation for the underlying mechanism and surface morphology during scratch is obtained for the results obtained from experiments and molecular dynamics simulations. Current work establishes an interplay between crystallography, friction and wear properties, deformation mechanisms, and surface morphology of the single crystalline nickel.

Polyimide (PI) aerogels are a novel class of porous materials, which due to their remarkable features, including lightweight, high service temperature, super thermal insulation, and low dielectric constant, have received considerable attention for industrial applications. The previously studied PI aerogels were mostly in the form of bulk monolithic, and a very limited number of studies were conducted on thin film configuration with enhanced functionality. In this study, we propose a novel multifunctional thin film PI aerogel with nanostructured assembly as an efficient thermal insulator. Investigation of physical and thermal properties of fabricated aerogels confirmed their multifunctionality. Studying the structural morphologies of aerogels revealed their interconnected nanostructured assembly with fiber diameters < 33 nm. Furthermore, such a porous structure with over 90% porosity could effectively lower the solid thermal conductivity and thus contribute to their super thermal insulation behavior. It was further demonstrated that due to the imide ring (-CO-NH-CO-) in the PI backbone, they exhibit excellent thermal stability with onset of decomposition of over 510 °C. Such a combined extraordinary features of the developed thin film PI aerogels made them an ideal candidate to be used as ultra high thermal insulator for the next generation of electronic devices.

Thermally activated deformation mechanisms play an important role in the plastic deformation of many material systems, ranging from dislocation creep at high values of T/Tm, to the brittle to ductile transition at low values of T/Tm, to grain boundary driven deformation process in nanostructured alloys at intermediate temperature. In this talk we will show, how Nanoindentation testing protocols can be tailored to analyze these effects across different temperature and time scale, focusing on a new constant contact pressure creep approach.
In the past different loading protocols have been developed to investigate the creep properties of materials using instrumented indentation testing technique. Recently, a new indentation creep method was presented, in which the contact pressure is kept constant, similar to the stress in a uniaxial creep experiment and the resulting creep strain rate is analyzed as a function of creep time. Applying this technique to nanostructured alloys, two distinctively different creep regimes have been observed, at low and high contact pressures. Here, the results of constant contact pressure creep tests are compared to uniaxial and constant load hold indentation creep experiments on different ultrafine grained Cu and Cu-X solid solutions. The limitations of the new creep method are discussed and compared to the results of uniaxial testing down to indentation strain rates of 10^-6 s^-1. Large reductions in the applied stress during uniaxial and indentation experiments result in a pronounced change in the power law exponent of the nanostructured alloys and relaxation processes seem to dominate the material response.