Symposium MS04—High-Entropy Alloys and Other Novel High-Temperature Structural Alloys

Most alloys are thermodynamically metastable during some stages of synthesis and service. Here we discuss scenarios where metastable phases are not coincidentally inherited from processing, but rather are engineered. Specifically, we present approaches to compositional (partitioning), thermal (kinetics), and microstructure (size effects and confinement) tuning of metastable phases so that they form stacking faults together with planar slip and can trigger athermal transformation effects such as TWIP and TRIP when mechanically or thermally loaded. Such concepts work both at the bulk scale and also at a spatially confined microstructure scale, such as at decorated lattice defects. In the latter case, local stability tuning works primarily through elemental partitioning to dislocation cores, stacking faults, interfaces, and precipitates. Depending on stability, spatial confinement, misfit, and dispersion, both bulk and local load-driven athermal transformations can equip metastable medium- and high-entropy alloys with substantial gain in strength, ductility, and damage tolerance. We explore and demonstrate these effects along three specific thermodynamic questions: First we study the roles of magnetic and vibrational entropy contributions for understanding bulk metastability. Second, grand-canonical effects arising from spatial confinement will be discussed. Third, we present alloy cases with extreme metastability, forming self-organized two-phase nanolaminates.

Understanding the thermodynamics of multi-principal-element alloys (MPEA), also known as high entropy alloys (HEA) is crucial to addressing their synthesis, properties, stability, materials compatibility and technological applications. While the thermodynamics of binary and ternary alloys have been relatively well investigated both experimentally and theoretically over the last five decades, this is not the case for multicomponent (>4 element) alloys. The classical calorimetric methods (e.g., liquid metal bath, direct reaction, and flux calorimeter) used for binary and ternary alloys will often encounter difficulties for alloys with more than three elements. Thus, a more flexible and general calorimetric approach is needed.
The present work introduces a new methodology for measuring the enthalpy of formation and mixing of metals based on high temperature oxidative calorimetry. This technique is based on the well-developed oxide melt solution calorimetry (OMSC). Using this method, the heats of formation of four binary (AlNi, AlFe, AlCo, and AlFe₃), one quaternary CrFeCoNi and one quinary Al_0.14Ti_0.18V_0.18Nb_0.26Ta_0.23 alloys were obtained. The values in kJ/(mol atom) are: AlNi, -60.11 ± 3.94; AlFe, -23.46 ± 2.39; AlCo, -51.80 ± 1.89; AlFe, -13.72 ± 2.39; CrFeCoNi, -3.24 ± 2.39; Al_0.14Ti_0.18V_0.18Nb_0.26Ta_0.23 -11.93 ± 4.80. The results are compared with predicted values from the Miedema model and with available experimental data. The methodology is thus established to be generally useful for determining the thermodynamic properties of MPEA.

While rhenium has proven to be an ideal material in fast-cycling high-temperature applications such as rocket nozzles, its prohibitive cost limits its continued use and motivates a search for viable cost-effective substitutes. We show that a simple design principle that trades off average valence electron count and cost considerations proves helpful in identifying a promising pool of candidate substitute alloys: The Mo-Ru-Ta-W quaternary system. We demonstrate how this picture can be combined with a computational thermodynamics model of phase stability, based on high-throughput ab initio calculations, to further narrow down the search and deliver alloys that maintain rhenium’s desirable hcp crystal structure. We also demonstrate how large-scale automated first-principles calculations can be used to further screen these alloys based on their oxidation resistance and melting points. Our thermodynamic model is validated with comparisons to known binary phase diagrams sections and corroborated by experimental synthesis and structural characterization demonstrating multi-principle-element hcp solid-solution samples selected from a promising composition range.

Al_xCoCrFeNi high entropy alloys received excessive scientific attention due to their promising mechanical and corrosion resistance properties. These alloys may contain a mixture of FCC and BCC phases were the latest has an important role in its’ hardening. It was demonstrated that increasing the Al fraction in Al_xCoCrFeNi alloy will lead to an increase in the volume fraction of the BCC phase in the alloy. This BCC phase is usually a mixture of a disordered BCC Fe-Cr rich (A2) and an ordered BCC (primitive cubic, B2) Al-Ni rich phases. Moreover, heat treatment history has a major impact on the microstructure and properties as well.
In the present talk, the metallurgy behavior of Al_2.75CoCrFeNi will be discussed. The experimental results suggested that sluggish diffusion and high activation energy for precipitation of A2 phase (disordered BCC) from B2 (ordered simple cubic) phase play major rule on the alloy phase composition. Moreover, the morphology of A2 phase found to be sensitive to cooling rate and the soaking temperate during heat treatment. It was concluded that the lattice distortion dominates microstructure and composition of phases.

The plastic deformation behavior of single crystals of the FCC equatomic CrMnFeCoNi high-entropy alloy and its derivative quaternary (CrFeCoNi) and ternary (CrCoNi) medium-entropy alloys has been investigated in a temperature range of 10-1273 K, paying special attention to possible variations in mechanical properties with heat-treatment that may cause different short-range ordering of the constituent elements. Deformation occurs via slip of the {111}<110> system exclusively in the whole temperature range for all alloys investigated. The CRSS values increase with decreasing temperature, especially below room temperature, so that the concept of ‘stress equivalence’ is obeyed for all alloys investigated. This is a clear indication that the strength of these alloys should be described by a mechanism based on solid-solution hardening. Dislocations are smoothly curved in the slip plane without any preferred line orientation, indicating no significant anisotropy in the mobility of edge and screw segments. Planar ½<110>{111} dislocations dissociate widely into Shockley partials for all alloys investigated, indicating their low stacking fault energies; 30 ± 5 and 11 ± 3 mJ/m² for CrMnFeCoNi and CrCoNi, respectively. The CRSS values extrapolated to 0 K for polycrystals of equiatomic quinary, quaternary and ternary alloys are reported to be well scaled with the mean-square atomic displacement from the regular FCC lattice points (calculated based on density-functional theory). This seems also the case for the CRSS values at 10 K for single crystals of the present three alloys, although some modifications are definitely needed. Deformation twinning occurs on the conjugate system in the form of the Lüders type deformation in the later stage of deformation at low temperatures in all of the three alloys. The variation of the CRSS values, twinning stress and the stacking-fault energy upon heat-treatment and its inter-correlation will be discussed.

High entropy alloys (HEA) have attracted a lot of attention in the last decades mainly due to their good mechanical, and thermal properties. HEA includes five or more alloying elements forming random solid solution phases. Based on the existing phase diagrams, multicomponent alloys would expect to contain many different phases. However, it is assumed the high value of the configurational entropy promotes the formation of simple solid solution phases inside the multicomponent alloys. It is crucial to understand why some multicomponent alloys can be formed as a single phase whereas some others cannot. In order to answer this question, several methods using atomic descriptors, empirical rules, and thermodynamic modeling have been proposed. These approaches suffer however from insufficient experimental data. Therefore, the prediction of alloys that can form random solid solution phases remains a challenge hindering the discovery of novel HEA and the understanding of what is driving their phase stability. In this work, we use DFT calculations to shed light in the relative thermodynamic stability of BCC and FCC random solid solutions. Using a regular solution model fitted on DFT computation, we created a database of almost 80,000 FCC and BCC alloys up to the quinary. We will report on the technical challenges in building this database and on the understanding of what is driving phase stability in HEA that one can extract from such a large data set. A special focus will be given to how entropy and enthalpy competition behave as the number of components increase in alloys.

High entropy alloys are a new class of alloys which consists of five or more major element in the range of 5-35 at.%. The ever increasing demands for new structural applications, high entropy alloys (HEA) are transpire as a new paradigm shift in material society. Nano-eutectic HEAs exhibit superior mechanical properties over the single-phase high entropy alloys. The optimum combination of strength and ductility are the key features of eutectic HEAs. In this work, the novel CoCrFeNiTa_x (0.2 ≤ x ≤ 0.5) HEAs with eutectic microstructure were designed and cast by arc melting. The phase composition, mechanical properties and stability of these alloys were investigated. The evolved microstructure in all the HEAs consist of eutectic lamellae with alternating α (γ-Ni)-FCC phase and β (Co₂Ta)-type Laves phase. It was observed that x= 0.2, and 0.5 alloys contains eutectic microstructure along with proeutectic phase and at x= 0.4 homogenous eutectic microstructure was obtained. The CoCrFeNiTa_0.4 HEA with the eutectic microstructure exhibited a relatively high combination of yield strength (~1820 MPa) and ductility (~21 %). Further, analysis regarding the eutectic phase stability in terms of different thermodynamic and electronic parameters considering the synergistic effect atomic size difference (δ_r), mixing enthalpy (ΔH_mix) and valence electron concentration (VEC) were carried out. Also, from literature EHEAs comprising of FCC /BCC solid solution phases and intermetallic compound/topologically closed packed (TCP) phases have been surveyed thoroughly. The assessment predict the existence of stable eutectic/near eutectic phases in all surveyed and present EHEAs when −18 ≤ ΔH_mix ≤ −6, 6 ≤ VEC ≤ 8.5 and δ_r > 3%.

Microstructural metastability engineering, especially transformation-induced plasticity (TRIP) effect, is one of the most effective strategies to enhance strain hardenability in exploring advanced high-entropy alloys (HEAs). While appreciable effort including both compositional and microstructural optimization has been accomplished to modulate the TRIP effect, the resultant transformation product, martensite, still seriously impedes the property advancement: its extensive defect density and limited hardenability inevitably lead to local embrittlement and thereby failure. Here in this presentation, we will demonstrate that by manipulating the mechanical stability of the parent FCC phase in an Fe-Mn-rich HEA, the strain induced HCP-martensite can become further transformable, activating an FCC→HCP→FCC dual-TRIP effect. With the aid of in-situ synchrotron X-ray diffraction, SEM/EBSD, and microstructural-based strain mapping techniques we reveal the corresponding transformation kinetic, kinematic, and global-local strain evolution characteristics. We will show that this sort of dual-TRIP mechanism exhibits a desirable potential to overcome the inherent property improvement threshold of classical TRIP effect.

The origin of antiferromagnetism in the entropy-stabilized oxide Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2O has been investigated using first principles electronic structure calculations within the DFT. To characterize the magnetic structure of the entropic oxide, the total energy in the ferromagnetic (FM) as well as in the type-I and type-II antiferromagnetic (AFM) configurations are calculated. In agreement with the experimental findings, the most stable configuration corresponds to the type-II AFM order, where the ferromagnetically coupled (111) planes are stacked antiferromagnetially along the [111] direction. To characterize the magnetic interactions between the nearest neighbor (NN) and next nearest neighbor (NNN) magnetic ions (Co, Ni, and Cu), the calculated total energies are mapped onto the Heisenberg spin hamiltonian and the exchange coupling constants, J₁ and J₂, are evaluated. The strong antiferromagnetic NNN coupling and the weaker ferromagnetic NN coupling is consistent with the type-II AFM magnetic configuration. The relative strengths and the signs of the exchange coupling constants are analyzed in terms of the calculated electronic structure. The results are compared to recent experimental investigations of long-range magnetic ordering in high entropy oxides.
This work is supported by the U.S. Office of Naval Research MURI program (grant No. N00014-15- 1-2863).

CrCoNi-based medium/high-entropy alloys can display exceptional combinations of strength (~1 GPa), ductility (~60-90%) and toughness (>200 MPa√m), properties which are further enhanced at cryogenic temperatures. In situ TEM observations identify multiple deformation mechanisms, associated with their high friction stress yet low stacking-fault energy, that act synergistically to generate damage-tolerance. For example, studies on CrCoNi reveal a hierarchical twin network that generates substantial 3-D barriers to dislocation motion with marked strain hardening, while simultaneously providing pathways for easy dislocation motion. Here we focus on the effect of local chemical ordering which appears to be an important factor governing the exceptional mechanical properties of these alloys. Our DFT-based Monte-Carlo simulations on CrCoNi show that variations in local chemical order have a profound effect on the stacking-fault and twin-boundary energies, the TRIP effect, and the formation energy of point defects, all features that influence mechanical properties; these predictions have recently been further confirmed by MD calculations. Despite the clear evidence from atomistic simulation, the existence of such short-range order in medium/high-entropy alloys has lacked direct experimental confirmation, having only been indirectly detected using x-rays. Here using energy-filtered TEM techniques, we provide direct experimental evidence for the presence of short-range order in the CrCoNi MEA, and comment on its significant effect on elastic and plastic deformation properties. These results highlight the possibility of “tuning order in disorder” in the form of a science-based atomistic tailoring of multiple-principal-element alloys to achieve specifically desired macroscale mechanical performance.

The material-design strategy of combining multiple elements in near-equimolar ratios has spearheaded the emergence of high-entropy alloys (HEAs), an exciting class of materials with exceptional engineering properties. While random mixing has been widely assumed in multi-principal element solid solutions, both experimental and computational evidence suggests short-range ordering (SRO) exists in many solid-solution HEAs. We employed an integrated first-principles and experimental approach to understanding the thermodynamic effects of SRO in the refractory NbTaTiV and NbTaTiVZr HEA systems. Results are compared with predictions from Special Quasi-random Structures (SQS). The existence of SRO produces distinct lattice distortion features in these HEAs and affects their mechanical properties.

The equiatomic ternary CrCoNi medium entropy alloy (MEA) exhibits an exceptional combination of high strength, ductility as well as fracture toughness. Although usually considered as a random solid solution, it has been suspected that short-range order (SRO), i.e. atomic scale correlation of elemental distribution, exists in this alloy, which may significantly affect the mechanical behavior. However, up to date, the direct observation of SRO by electron microscopy has not been possible due to their intrinsically small scale and lack of contrast. In this investigation, we confirm the existence of SRO in a thermal-mechanically treated MEA by deploying an in-column, energy-filtered transmission electron microscope. The inelastically scattered electrons can be effectively eliminated by the energy filter, dramatically increasing the signal-to-noise ratio, enabling the detection of SRO diffraction pattern. Using energy filtered dark field imaging, we show that these SRO domains are as small as several angstroms and distribute uniformly across the sample. The influence of the SRO on the mechanical behaviors of the MEA are evaluated by nanoindentation, bulk tensile testing as well as pulse-echo ultrasonic measurement. The deformation microstructure, especially the dislocation configuration is examined by diffraction-contrast STEM imaging. It was shown that SRO promotes the planarity of the dislocation slip and increases the stacking fault energy of the MEA; It also slightly improves both nanoindentation hardness and yield strength without sacrificing the ductility. These discoveries corroborate with DFT-based Monte-Carlo computation, suggesting that the degree of SRO is an intrinsic materials parameter that can be tuned to tailor the structure-property relationship. Strategies of obtaining different degrees of SRO by thermal-mechanical treatment will also be touched base in this talk.

High entropy alloys (HEA) exhibit various combinations of interesting physical properties due to the formation of solid solution stabilized by a high configurational entropy. HEAs with high melting point are promising candidates for low density refractory metals, e.g. HfNbTaTiZr alloy. Since conventional preparation by arc melting results in a dendritic structure of bcc phases, long term and high temperature annealing is necessary for homogenization of the structure [1]. On the contrary, powder metallurgy is a fast way of producing fine grained alloys. The HfNbTaTiZr powder prepared by gas atomization was processed by spark plasma sintering (SPS) at various sintering temperatures. By characterization of its microstructure, lattice defects and mechanical properties [2] the sintering conditions were optimized in order to obtain a fully dense single-phase material.
Defects formed during the process of SPS were characterized by positron lifetime spectroscopy (LT). LT is a well established non-destructive experimental technique with high sensitivity to open-volume defects (e.g. vacancies and vacancy clusters, dislocations, grain and phase boundaries). Moreover, the experimental LT data can be directly compared with ab-initio theoretical calculations. Analysis of measured LT spectra enables both identification of the type of defects in the material as well as evaluation of their concentration. Since the concept of HEAs is inherently related to severe lattice distortions which affect also open volumes, any information on the defects in the bulk solid solution phases is invaluable. LT may be used also for monitoring of the remaining porosity at the very atomic level in alloys prepared by SPS.
[1] O. N. Senkov, et al., J. Alloys Compd. 509 (2011), 6043.
[2] F. Lukac, et al., J. Mater. Res. (2018), 1-11.

It is well known that addition of molybdenum to steels and nickel-base alloys can increase the mechanical strength and the resistance to pitting corrosion. However, it is less known how Mo addition may impact the atomic structure and hence the intrinsic physical and mechanical properties of the alloys as a function of temperature and composition. In present work, the evolution of the atomic structure (including short range order, atomic occupations, or local lattice distortion) of Co-Cr-Fe-Ni-Mo high entropy alloys in liquid, in solid, and during solidification is first studied using density functional theory (DFT) methods, ab initio molecular dynamics, and Monte Carlo simulations. This information gives implications for the mechanical properties from a thermodynamic perspective. As the next step, the elastic constants and stacking fault energies of the model alloys are calculated using DFT, which further improve our understanding of the impact of Mo on the yield stresses.

The HfNbTaTiZr refractory alloy is intended for high temperature applications and crystallizes in the bcc structure at high temperatures. The microstructure of this alloy is studied by means of the density functional theory using the supercell approach. It is first explained why HfNbTaTiZr is not a true high entropy alloy. The tendency of the alloy to short range ordering is then demonstrated for several partially ordered structures based on the bcc lattice. Next, the Metropolis Monte Carlo algorithm is employed in order to simulate the short range order in the HfNbTaTiZr alloy. The tendency to short range ordering increases with the decreasing temperature. This indicates that a phase separation could occur at lower temperatures, which is indeed detected experimentally. The question why the alloy could not have an hcp structure at lower temperatures is also addressed. Finally, the structure of a ∑5 grain boundary in the HfNbTaTiZr alloy is investigated. Such a grain boundary exhibits quite low grain boundary energy and the segregation of Ti is detected. The results of the simulations are compared with other experimental data available.

A key consideration in designing high-performance materials with exceptional strength and toughness is tuning the microstructure. Dealloying is a processing method for producing metallic materials with a unique bicontinuous microstructure, which has been shown to exhibit size-dependent improvements in mechanical properties. Previously, dealloying was limited to electrochemical systems, but the emergence of liquid metal dealloying and solid state dealloying has expanded the capabilities of this technique to include refractory alloys.
Powder-based dealloying is a novel approach to creating bulk quantities of refractory metal composites with a complex, interconnected network of ligaments. By mixing and pressing together powders of the parent alloy with powders of the metallic solvent into a bulk sample, we can create a material in which dealloying occurs uniformly throughout the bulk of the material, not just at the geometric surface. These experiments offer insight into the microstructural evolution of dealloyed refractory metal composites as a function of time, temperature, and composition. Here we present an assessment of the mechanical properties of these materials, in addition to a study of the dealloying kinetics between pressed powders. Studying the fundamental kinetics of powder-based processing is the first step to developing the high-temperature processing techniques necessary to apply dealloying to additive manufacturing of refractory alloy composites.

The yield strengths of High Entropy Alloys (HEA) have been shown to correlate with picometer-scale lattice distortions. Therefore, distortion measurements could be used to estimate strengths. Alternatively, a predictive model of HEA strength based on solute misfit volumes is available. Here, the connection between lattice distortions and misfit volumes is examined for two model ternary alloy families, fcc Cr-Fe-Ni and bcc Nb-Mo-V. The root mean square lattice distortion is nearly proportional to the misfit volume parameter over a wide composition range. The reported correlation of yield strength with lattice distortion is thus a consequence of the correlation between lattice distortion and misfit parameter and the derived dependence of yield strength on the misfit parameter.

The study of HEA in the form of thin films aims at providing missing data on HEA phase stability. Indeed, the small bulk volume of a film, combined with the presence of grain boundaries and interfaces, helps in achieving thermodynamic equilibration of these complex alloys. In addition, polycrystalline films can be used to test the role of grain boundaries on the mechanical properties of HEAs.
Here we present a study of the stability of polycrystalline CoCrFeNi thin films, grown on (0001) α-Al₂O₃ (c-sapphire), with respect to annealing treatments. It has been found that extremely large grains can grow in the film depending on their orientation relationship to the c-sapphire substrate. Despite their complex chemistry, the grains in the 200 - 670 nm thick CoCrFeNi FCC films investigated, adopt the same orientation relationships as those of pure FCC metal films on c-sapphire. However, the grains in CoCrFeNi grow much larger at homologous annealing temperatures.
The films have been synthesized at room temperature, using magnetron sputtering from four pure element targets in a chamber with UHV base vacuum. They consist of 30-100 nm wide columnar FCC grains with a <111> texture. Upon annealing for 1 hour in the range 973 K to 1423 K under an Ar-H₂ atmosphere, grain growth and grain boundary grooving compete to either stabilize or break-up the film. The microstructure evolves into larger grains and/or islands, of four different orientation relationships: OR1 (Me(111)[1-10]//α-Al₂O₃(0001)[1-100] and OR2 (Me(111)[1-10]//α-Al₂O₃(0001)[11-20] and their twins (OR1t and OR2t).
Thinner films and higher temperatures favor the dewetting of the film to form single-crystalline islands. Dewetting initiates from holes which nucleate at grain boundary triple junction grooves, and which have deepened sufficiently to reach the substrate. At the highest temperatures, the OR2 and OR2t grains grow to sizes exceeding 1000 times the film thickness. The migration of the grain boundaries of these grains is fast enough to overcome both grooving and the nucleation of dewetting holes.

An alternative route toward production of high entropy alloy nanoparticles (HEA-NPs) is demonstrated using metal carburization and subsequent precipitation of graphite inside carbon nanotubes. Nanoparticles exhibit barely seen structures, including coexistence of different crystal structures and multiple phases. Magnetic hardening is also observed with saturation magnetization much greater than ferromagnetic particles consisting of single and/or binary components. Calculations support experiments and further reveal carbide mediated HEA-NP formation to be thermodynamic favorable.

High entropy alloy (HEA) thin films are immensely studied in recent times due to their excellent mechanical and physical properties having high hardness, superior corrosion resistance, and high electrical resistivity. This study reports the findings of radio frequency (RF) magnetron sputtered HEA thin films of AlCoCrCu_0.5FeNi from a single stoichiometric target on Si (100) substrate in presence of argon with a substrate heating of 250 ^οC at three different powers of 200 W, 250 W, and 300 W. The argon flowrate was maintained at 15 sccm with a deposition pressure of 10 mTorr for all the films grown at three different powers. The characterization results reveal that the increase in deposition power from 200 to 300 W increases the average grain size roughly from 27 to 80 nm as well as the surface roughness of the films from 2.64 to 25.2 nm observed from the Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) micrographs, respectively. X-ray diffraction (XRD) indicates that all the films are polycrystalline in nature with a mixture of FCC and BCC having a lattice parameter of 3.61 Å and 2.87 Å, respectively. The surface chemistry analysis from the X-ray Photoelectron Spectroscopy (XPS) confirms the presence of Al₂O₃ and Cr₂O₃ on the surface of all the HEA thin films grown at different deposition powers, acting as surface protective layers for superior oxidation and corrosion resistance. Energy Dispersive Spectroscopy (EDS) mapping confirms the homogeneous distribution of all the elements throughout the films grown at three different powers. The hardness data measured from the nanoindentation test shows a significant decrease in hardness from 15.95 GPa (200 W) to 4.84 GPa (300 W) with the increase in deposition power. The HEA films grown at various deposition powers were quite resistive and the highest resistivity of 945.13 µΩ-cm was obtained for the film grown at 250 W measured using the four-point probe method. The above characterization results show that the variation in deposition power is an important parameter to tune the microstructural properties of AlCoCrCu_0.5FeNi HEA thin films during the sputtering process which intern governs the chemical and physical properties of the sputtered HEA thin films having potential applications as hard and protective coatings suitable for the energy and aerospace industry.

Recently, High Entropy Carbides (HECs) have been predicted the selection of candidate compositions with the phase stability from an entropy-forming-ability (EFA) descriptor from first principle. The predicted compositions are applied to disordered refractory five metal carbides, experimentally synthesized as rocksalt structure and measured their mechanical properties. We report in this work the results of DFT calculations that were carried out to explore the degree to which properties of high entropy carbides containing different five-atom combinations from the elements Nb, Ta, Mo, W, Ti, Zr, Hf and V can be related to the properties of their respective binary compounds. Cohesive energy, bulk modulus and lattice constants from the Density Functional Theory (DFT) calculations were studied and compared between HECs composition and averaging the values from the binaries. These properties from the binary averages are either matched or slightly under-over estimated the DFT results. For the carbon vacancy formation energy (CVFE), the majority of HECs are distributed roughly within defined upper and lower limit CVFEs of the constituent binary carbides. Mixing multiple metal compositions in HECs still introduces the similar properties from the respective binaries and indicates more energetically stable in structures.
We also investigate the effect of carbon stoichiometry on thermal conductivity of high entropy carbide and its influence on the electrical contribution to thermal conductivity using DFT calculations together with semi-classical Boltzmann transport theory. The energy and electrical conductivity of the fully stoichiometric system, as well as systems containing carbon vacancies and interstitials were calculated. Both the vacancy and interstitial formation energies are positive, however, the energy for creating interstitial carbon is considerably larger than that for creating a vacancy such that the formation of a second carbon phase is highly favorable when excess carbon is introduced into the system. These results are consistent with experiment, where carbon indicative of a second phase is measured for high methane flow rates.

A robust ultrathin diffusion barrier layer is demanded for the Cu interconnects of integrated circuits to inhibit the rapid diffusion of Cu atoms and to prevent Si devices from failure. In recent years, based on the large positive mixing enthalpy between Cu and refractory metals and the thermodynamically-driven solute segregation, a “self-forming” technique was developed to generate a thin layer of refractory metal from a Cu alloy film doped with refractory solutes, for use as a diffusion barrier. Accordingly and additionally attributed to the high mixing entropy of multiple components, for the first time in our recent study, an ultrathin solid-solution layer of quinary V-Nb-Mo-Ta-W refractory alloy was self-formed from a Cu(V,Nb,Mo,Ta,W) alloy film. As confirmed by high-resolution TEM observations and elemental mappings, the doped solutes segregated to the Cu/Si interface and formed a continuous quinary alloy layer of only 1.5 nm thick. Because of the sluggish diffusion in high-entropy alloys, the quinary alloy layer was found to resist the interdiffusion of Cu and Si at 700 degree C, higher than the resisting temperature (< 600 degree C) of unitary metal layer. In-situ TEM characterization of the segregation of the quinary solutes V-Nb-Mo-Ta-W in a Cu(V,Nb,Mo,Ta,W) alloy foil was further conducted, and the diffusion kinetics of the solute elements in Cu were estimated. Elemental mappings indicated the entropy-stabilized incorporative diffusion of the five solute elements at an average rate of about 50 nm/h at 400 degree C. Mo and W segregated at a higher rate owing to the higher mixing enthalpies with Cu for 19 and 22 kJ/mol, respectively, while V, Nb and Ta segregated at a lower rate due to the lower enthalpies for only 2 to 5 kJ/mol.

High-entropy alloys (HEAs) are at the hot topics of metallic materials research because of their outstanding mechanical properties, e.g. high fracture toughness in extreme temperature conditions. Currently, most researches focus on examining their plasticity instead of the elasticity. Compared with conventional alloys, HEAs may perform superior elastic properties because of the uneven local lattice strain. It is difficult for experimentalists to clarify the effect of severe lattice distortion on their mechanical properties. In order to investigate the elastic anisotropy, we conducted molecular dynamic simulations, composed of 1 ~ 5 atomic types containing Ni, Ni₉₈W₂, Ni₉₆W₄, FeCrNi, and CoCrFeMnNi, under the Lennard-Jones potential, embedded-atom method (EAM) potential, and modified EAM potential. We analyzed Young's modulus E (h k l) and Poisson's ratio v (h k l,θ) along [100], [110], and [111] loading direction for FCC crystals, and explored the performance of the elastic properties for each element type in HEAs. Our results suggest that electron density inconsistency among atomic elements dominates the elastic anisotropy of HEAs rather than the atomic radius difference.

Laser shock peening process is a well-developed surface enhancement method which enhance fatigue life and damage tolerance of metallic materials by introducing a beneficial residual compressive stress. In this study, the laser shock peening effect on microstructure and mechanical behavior in face centered cubic CrMnFeCoNi high entropy alloy, known as Cantor alloy, was investigated. Three specimens with different thermomechanical condition, or recrystallized, cold rolled and laser shock peened after recrystallization, were subjected to hardness and sliding wear test to quantify the laser peening effect. Clear improvement of hardness and wear resistance was confirmed with comparable level to the rolled condition as well as residual stress reached several hundred MPa near the surface.

While first-principles methods can compute activated state energies using the nudged-elastic band method, upscaling to mesoscale mobilities requires the solution of the master equation. The general solution for mass transport coefficients involves the inversion of rate matrix: the Green function. However, while the rate matrix is typically sparse, its inverse is not; moreover, numerical algorithms for the Green function are currently only available for cases of dilute defect concentration. By recasting the calculation of transport coefficients as a variational problem, we can compute transport coefficients from thermal average quantities instead of trajectory-based calculations. The variational principle also ensures that our diffusivity calculations are upper bounds of the true diffusivities. Replacing kinetic Monte Carlo calculations with Monte Carlo averages, or approximations for thermal averages, also increases the efficiency of mass transport calculations. We showcase this approach for random multicomponent systems.

High-entropy alloy (HEAs) are a class of multicomponent alloy made with equal or near equal quantities of five or more principal elements. In this study, the problem of interdiffusion in HEAs is addressed. Detailed studying of the diffusion composition profiles in several interdiffusion couples in CoCrFeMnNi is performed. A constant as well as a composition dependent interdiffusion matrix was used for investigating the diffusion behaviour in CoCrFeMnNi HEAs. These composition dependent interdiffusion matrices are calculated according to the Darken and the Manning formalisms. MATLAB programing language is used as the main tool for generating composition profiles using an explicit finite difference method (EFDM). The simulated composition profiles are found to be in a very good agreement with the available experimental results [1, 2]. The ultimate purpose of this modelling is to systematically investigate interdiffusion in CoCrFeMnNi alloys in diffusion couples with substantial changes in composition.

Keywords: Interdiffusion, High entropy alloy (HEAs), Manning approach, Darken approach

[1] K.-Y. Tsai, et al., Acta Materialia, 61 (2013), 4887.
[2] J. Dabrowa, et al., J. Alloys Compounds, 674 (2016), 455.

High-entropy alloys, or complex concentrated alloys, are currently of significant attention in materials science owing to their exceptional mechanical properties that include the considerable cryogenic ductility of FCC-type alloys and the superior elevated-temperature strength of BCC-type alloys. It has been proposed that, in single-phase high-entropy alloys, the intrinsic atomic-level complexity causes a severe lattice distortion and an atomic-level pressure variation, which further affects the dynamic behavior of crystalline defects. However, the atomic-level formation mechanism of crystalline defects and the interaction of these defects with the intrinsic distorted and heterogeneous environment in high-entropy alloys remain unclear. In our previous in-situ microscopic experiments of micro- and nano-pillar compression in several crystalline systems of high-entropy alloys, the gum-like uniform barrel-deformation accompanied with the abundant nucleation sites of plastic regions was clearly observed. With the aid of atomistic simulations, the soft spots identified by the spatial distributions of low-frequency vibration modes have been further found to serve as the precursors of atomic-level rearrangement in solid-solution alloys. In high-entropy alloys, the successive defect growth exhibits a chain-like manner which relies on the coupling of local shear transformations and vortex-like atomic motion. This formation and growth mechanism resembles the reported nucleation mechanism of shear transformation zone in metallic glasses, which is different from the dislocation slip-induced planer fault growth in dilute solution alloys. In addition, the extremely short correlation length of mechanical heterogeneity in high-entropy alloys suppresses the synchronized motions of dislocation line, which impedes the long-range dislocation slip and contributes toward a strong strain hardenability. The critical defect formation mechanism at the atomic level provides the theoretical description of various-signature mechanical properties of high-entropy alloys.

High entropy alloy (HEA, also known as compositionally complex alloy) refers to simple-phase solid solution alloy that contains multiple principal components in equimolar or near-equimolar ratios. To computationally address the complexities of this type of high-order alloy systems, we have performed atomistic simulations to predicte the solid-solution stability as well as the short-range ordering in CoCrFeNi and AlCoCrFeNi bulk alloys and CoNiRuRh nanoparticles. In our simulations, the interatomic interactions were described using a set of modified embedded atom method (MEAM) interatomic potentials for these alloy systems. First, we used atomistic simulation methods to examine solid-solution phase formation rules for CoCrFeNi high entropy alloy. Using the Monte Carlo (MC) simulations based on the developed MEAM potentials, we sampled the thermodynamically equilibrium structures of the CoCrFeNi alloy and further predicted that the CoCrFeNi alloy could form a solid solution phase with high configurational entropy of 1.329R at 1373 K. Then, we examined the stability of this solid solution phase of the CoCrFeNi alloy against the well-recognized solid-solution phase formation rules by varying the MEAM potentials and thus tuning the atom size and mixing enthalpy in the alloy. Our simulation results revealed that it required atom size difference effect and mixing enthalpy effect -10 kJ/mol < ΔH » 0 kJ/mol for the modeled CoCrFeNi alloy to remain a single solid solution phase. Furthermore, we studied the stability of solid-solution phase of AlxCoCrFeNi HEAs using the developed MEAM potentials and the atomistic MC simulation method. In our MC simulations, different constituent elements were allowed to exchange their positions and thus the modelled HEAs were relaxed to their thermodynamic equilibrium states after several millions MC steps at 1300 K. The mixing Gibbs free energy of the HEAs was calculated using adiabatic switching thermodynamic integration method. We predicted that the AlxCoCrFeNi HEAs would form a single fcc solid-solution phase when x<0. 21, a single bcc solid solution phase when x>1.08, whereas a mixture of fcc and bcc phases when 0. 21<x<1.08. Our theoretical results are quite consistent with experimental observation. Moreover, we investigated the formation of solid solution phase in Co_0.12Ni_0.14Ru_0.43Rh_0.30 nanoparticles with size ranging from 2 to 5 nm through a combined molecular dynamics (MD) and MC approach. Our simulation results indicated that the local severe lattice distortion could block the diffusion of atoms and hence lead to a stable solid solution phase during a carbothermal shock synthesis procedure. Consequently, we have demonstrated that atomistic simulation techniques as useful methods for understanding the composition-structure-property relation of novel high entropy alloys.

Metallic alloy surface oxidation process at high temperature remains inadequately understood. The early stages of this complex process involve structural and chemical transformations that require to be observed and described at the atomic scale. When it comes to high temperature alloys with complex composition such as high entropy alloys, the possibility of multiple oxide product phases further complicates the oxidation mechanisms. Hence, we employed in situ transmission electron microscopy, an environmental reaction chamber attached to an atom probe tomography and synchrotron based x-ray absorption near edge spectroscopy, in order to reveal the structural, compositional and chemical state transformations occurring during early stages of metallic alloy oxidation at high temperatures. This multimodal approach was used to analyze oxidation of selected model and commercial Zirconium alloys, Titanium alloys and high entropy alloys at high temperatures. We demonstrate the atomic scale changes to the composition of oxide layers, formation of polycrystalline structure of oxides, and presence of metastable phases in the early stages of oxidation. The time dependent variation of structure, composition and chemical state of the oxide layers can be correlated to corresponding alloys mechanical properties and thus can guide the development of accurate predictive mechanistic models.

The thermal conductivity of high entropy alloys does not follow typical trends in metals. Not only are they an order of magnitude less conductive than what rule of mixtures would predict, but their conductivities are also known to increase with temperature between 300-550K. Here, transport is calculated in an analytical model as the sum of contributions from the electron and phonon subsystems. For the lattice subsystem, conductivity is treated with the Leibfried and Schlömann formulation with perturbations for both electron-phonon interactions and point defect scattering. The electron subsystem is dominated by alloy scattering, which is calculated based on a virtual crystal approximation derived from each constituent’s Lennard-Jones potential. The model predictions are then compared to time domain thermoreflectance measurements in the Al_0.5CoCrFeNiCu as well as previously published measurements of the Al_0.37CoCrFeNi alloy.

This work was supported by the Sandia National Laboratory Directed Research and Development
(LDRD) program. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-NA0003525. This work describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S.
Department of Energy or the U.S. Government.

Development of new structural materials for service under extreme conditions is slowed by the lack of high-throughput test protocols for investigating properties under relevant operating conditions. Here we demonstrate a method that integrates high-throughput nanoindentation mapping with precise temperature control under vacuum atmosphere. Al_xFeCrNiMn (x=0, 0.3, 1) high entropy alloys (HEAs), which may possess the strength and stability required for high-temperature structural materials for nuclear applications, are investigated here. These alloys have distinct microstructural morphologies, and nanoindentation mapping reveals the mechanical behavior of the distinct phases as a function of temperature up to 400^oC. The complexity of the microstructure, investigated under correlative conditions, allows the determination of specific phase properties. The FeCrNiMn consists of a FCC matrix with BCC precipitates exhibits significant softening in both phases at elevated temperature. In contrast, both the FCC phase and FCC-BCC phases present in Al_0.3FeCrNiMn show approximately 90% retention of the room temperature hardness at 400^oC, and AlFeCrNiMn with BCC and B2 structures show a similar, 85%, retention of hardness.
Artificial intelligence algorithms, e.g. clustering, allow the for the relatively fast binning of structure and property into statistical forms without losing the location information required for the reproduction of maps. Experimental techniques and a brief discusion of what is required to validate the AI clustering is presented.

Refractory high-entropy alloys (RHEAs) are an alternative type of HEAs that possess BCC matrix and superior high-temperature strength compared to their FCC predecessors, making them ideal structural materials for extreme nuclear environment. The mechanical properties of a single-phase quaternary TiVCrTa alloy were investigated after ion-irradiation, using advanced micromechanical tests and high-resolution electron microscopy (HR-EBSD, TEM). Nanoindentation results of the as-cast sample irradiated to 3.5 dpa showed no irradiation hardening and TEM confirms negligible defect accumulation. Deformation mapping around the nanoindents via HR-EBSD were also carried out to provide mechanistic understanding of the irradiation resistance in term of change in strain localisation and GND density. Future experiments will apply high-temperature nanoindentation on the irradiated samples so the mechanical performance under relevant reactor operating conditions can be measured.

High Burnups can cause major structural changes in the edge of the fuel rod and a general degradation of the thermal conductivity. In irradiated oxide fuels of UO₂ and PuO₂, several fission products (FP) are produced and they take various chemical states depending on the conditions of the fuel. This work, based on the energy of the specified metallic fuels predominantly has some passive safety features during core off-normal events. During the structural and mechanical property changes, dissolved fission gases as well as thermal expansion cause the fuel to expand and swell to the cladding, where the fuel at the interface will transform to the molten phase, reducing reactivity in the reactor and helping to prevent the core from going supercritical. Uranium in particular has a low melting point; however, by alloying to metals that are also in BCC phase at high temperatures, the melting point can be increased. Lattice parameter, Burnups and the thermal conductivity were calculated for specified UO₂ and PuO₂. This calculations relate the degradation of thermal conductivity with a number of pores and increasing temperature. Finally, the migration energy barrier and the recovery energies of the obstruction type defects were calculated by molecular dynamics and molecular statics simulation.

The present presentation reports experimental observations of crack propagation in both tough and brittle metallic glasses (MGs) via in situ high-resolution transmission electron microscopy (HRTEM) tensile tests, illustrating the cracking behavior of MGs is governed by an intrinsic cavitation mechanism ahead of the crack tip. Based on the experimental results, we proposed a model of surface diffusion induced cavitation in the strained region ahead of a crack tip and conducted numerical simulations to verify the proposed the model. The simulation results catch the main and essential features of the cavitation mediated cracking behavior observed in in situ HRTEM tensile tests. In addition, we also observed in in situ HRTEM tensile tests that introducing nanocrystals in the front of the crack can effectively blunt the crack tips and retard the crack propagation. The in situ HRTEM observations and numerical simulations validate the proposed model of surface diffusion induced cavitation and cracking in MGs, thereby offering new insights into the deformation and cracking mechanism and paving a novel way for the design of MGs with high toughness.

Shape-memory alloys (SMAs) have wide aerospace and biomedical applications due to the capability to recover from large deformations via solid-to-solid transformations from martensitic to austenitic phases. Recently, Ni-Ti-Hf SMAs have attracted tremendous research interests because their phase transformation temperature is higher than that of traditional Ni-Ti counterparts. This expands their applications in high-temperature environments. Our previous research has indicated that the presence of nano-scale precipitates could further improve the cyclic transformation response in the Ni-Ti-Hf system. However, there lacks a detailed understanding of these precipitates, including crystal structure, chemistry, size, distribution and volume fraction. Moreover, the precipitate evolution as a function of heat treatment temperature and time is also not well understood. In this work, we combine STEM with TEM tomography to construct 3D models for these nano-scale precipitates to characterize and understand why nano-scale precipitates in SMAs could lead to even more consistent actuation responses in Ni-Ti-Hf SMAs.

In order to reduce the fossil fuel consumption and greenhouse gas emissions, there is a continued demand for the new generation high temperature materials and lightweight materials for better performance. Integrated Computational Materials Engineering (ICME) has the capability to replace the time-consuming and labor-intensive traditional trial-and-error approach to expedite the design of novel materials and predict the microstructure and performance of materials produced by advanced manufacturing. However, the physics-based or data-based models in ICME highly rely on the accurate understanding of microstructure in materials during processing and service. For example, in order to produce shaped products that possess consistent and high-quality property performance using additive manufacturing, it requires the provision of solutions to problems, such as coarse columnar microstructures, porosity and residual stresses, which requires rapid and reliable microstructure determination using multi-scale ex-situ and in-situ characterization techniques. This presentation is mainly focused on the use of analytical microscopy techniques to explore the fundamental mechanisms in the development of novel metallic materials. Two studies about designing high temperature used Ni based superalloys and high strength lightweight titanium alloys for aerospace application via microstructure engineering utilizing advanced characterization will be introduced. The effect of Re on the long-term creep behavior and microstructural evolution in Ni-based single crystal superalloys have been systemically studied via multiscale electron microscopy, atom probe tomography and synchrotron XRD. The results show that Re is an effective creep strengthening element for Ni-based single crystal superalloys and long-term creep life of the alloys can be significantly improved by the addition of small amount of Re. Ultra-high strength metastable beta titanium alloys, via precipitation strengthening through the omega phase assisted alpha precipitation mechanism, have been realized by coupling the atomic resolution aberration-corrected S/TEM with phase field modeling. The pre-formed nano-scaled omega phase particles can alter the local structure and composition in the parent phase matrix and therefore provide additional driving force and nucleation sites for the formation of super-refined scale alpha phase precipitates.

The solute-induced softening and hardening effects in bcc W for twenty-one substitutional alloying elements (Al, Co, Cr, Fe, Hf, Ir, Mn, Mo, Nb, Ni, Os, Pd, Pt, Re, Rh, Ru, Ta, Tc, Ti, V and Zr) are examined to search for a similar softening effect as that observed with Re. The changes in energy barriers of dislocation motion caused by solute-dislocation interactions are directly computed via a first-principles approach with flexible boundary conditions. The effect of solutes on the critical resolved shear stress of the ½ <111> screw dislocation in bcc W at room temperature is quantitatively predicted, as a function of alloy concentration, via a mesoscopic solid-solution model using the first-principles results as input. Al and Mn are proposed to be promising substitutes for Re as these two elements introduce similar softening effects as Re in bcc W. In addition, the trends of the solute-dislocation interactions, and their correlations to the dislocation core structure geometries are discussed.

The potential of nanomultilayers (NMMs) as a route to synthesize nanostructures is explored by examining the microstructural evolution of refractory Mo-Au, Hf-Ti and Ta-Hf NMMs through differential scanning calorimetry (DSC) and heat-treatments at different temperatures. Transitions occurring between room temperature and 1000⁰C are identified using DSC scans. The microstructural evolution of these materials and the nanostructures that develop after annealing are not well understood but can provide insight of active thermal processes and stabilization mechanisms. A wide range of characterization techniques including TEM and APT were utilized to examine compositional changes and gain insight into thermally activated events such as layer roughening, solute grain boundary segregation and phase separation. Understanding the evolution of these NNMs will allow tailoring nanomaterials with enhanced thermal stability.

The improvement of mechanical properties must be achieved by designing and constructing more suitable microstructure, such as hierarchical microstructure and nano microstructure. In order to significantly enhance the creep resistance of titanium alloys, one two-level hierarchical microstructure with micro-TiB whiskers (TiBw) and nano Ti5Si3 reinforcements were constructed to form the modified composites by powder metallurgy combining with in-situ synthesis and precipitation. The micro TiBw reinfrocement were in-situ synthesized around titanium matrix particles, which formed the first scale network microstructure. The nano Ti5Si3 particle were precipitated and distributed in the beta phase around alpha phase which formed the second network microstructure. The results showed that the high temperature strength has been significantly enhanced. The tensile strength at 550oC for Ti6Al4V matrix composites is increased to 1050MPa. Moreover, the creep rate of the modified Ti6Al4V matrix composites was remarkably reduced by an order of magnitude compared with the conventional Ti6Al4V alloys at 550 oC, 600 oC, 650 oC under the stresses between 100 MPa and 350 MPa. Moreover, the rupture time of the composites increased by 20 times, compared with Ti6Al4V alloys at 550 oC/300 MPa. The superior creep resistance can be attributed to the two-level hierarchical microstructures and the two-scale reinforcements. The micro-TiBw reinforcement in the first network boundary contributed to creep resistance primarily by blocking grain boundary sliding, while the nano-Ti5Si3 particles in the second network boundary mainly by hindering phase boundary sliding. In addition, the nano-Ti5Si3 particles were dissolved to smaller-sized Ti5Si3 particles during creep deformation due to high temperature and external stress, which can further continually enhance the creep resistance. Finally, the creep rate during steady-state stage was unprecedentedly decreased, not stable or increased, which manifested superior creep resistance of the composites. The calculated results indicated that the dislocation climb is the dominant mechanism for the composites tested at 550 oC/(250-350) MPa and 650 oC/(100-250) MPa.

Magnesium (Mg) and its alloys are the lightest structural metals, but the low ductility at low temperature prevents them from massive manufacture process and extended engineering applications. Therefore, it is urgent to improve the ductility of Mg. Here, to provide guidelines of accelerating this improvement, we proposed a hierarchical high-throughput screening (HHTS) approach for alloy design that is based on full general stacking fault (GSF) energy surface (γ-surface) derived from density functional theory. We simplify the HHTS approach by examining only the unstable stacking fault energy (γ_us) on γ-surface. We applied this approach to determine the promising dopant elements in Mg-X binary alloys which may activate the pyramidal dislocations by decreasing GSF energy and thus improve the ductility of Mg. From our HHTS approach we sifted out ten promising elements, including Hg, Tl, Sn, Sb, Bi, Te, As, Pb, In and Ca. Particularly, Mg-Te alloy has the lowest γ_us of 156 mJ/m² among all binary alloys, which is 25% lower than that of Mg (209 mJ/m²), suggesting that it is the most promising alloy to enhance the ductility of Mg.

Understanding evolutions of transport properties in undercooled liquids and their interplay with their structural features represents an important issue for solidification processes of metallic alloys, such as crystallization and formation of quasi-crystalline or amorphous phases [1–4]. In the present work, we focus on various classes of Aluminum alloys such as Al-Ni [5], Al-Cu [6], Al-Cr [7] and Al-Zn-Cr [8] that are important for many automotive and aeronautic applications due to their light weight, good forgeability, high thermal conductivity, and low corrosion. Using ab initio molecular dynamics, we simulate the undercooling process of these alloys during which we monitor the structural and atomic transport properties. We find that diffusion, viscosity and structural relaxation time undergo a crossover between an Arrhenius and non-Arrhenius behavior at a temperature T_X during the slowing down, which corresponds to an onset of dynamic heterogeneities (DHs) that develop. The structural features display characteristics compatible with the occurrence of the icosahedral short-range order (ISRO) as well as the development of a medium range order (MRO) upon cooling. The interplay between the ISRO and MRO and the dynamic heterogeneities is examined. The differences and similarities between these alloys is also discussed.

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Ni-base superalloys have a long history of use in jet turbine engines, and efforts to improve their performance in that application are ongoing. It is known that the precipitation of the Ni₃Al gamma prime phase within the disordered FCC gamma phase strengthens the overall material. However, in the high-temperature environment found inside a turbine engine, creep can cause the gamma prime phase to transform to different, weaker phases along stacking faults, which leads to a deterioration of performance. In the gamma prime phase, one mode of creep deformation is the formation of stacking fault ribbons, which consist of intrinsic stacking faults further shearing into antiphase boundaries (APBs). It is also known that certain alloying additions exhibit segregation to gamma prime stacking faults. If elements that segregate to the intrinsic stacking fault but not to the APB could be identified, the inclusion of such elements could lead to improved creep strength in these alloys.

To investigate this possibility, a density functional investigation was performed on the segregation of W, Mo and Cr to both a superlattice intrinsic stacking fault (SISF) and an APB. It was found that W, Mo and Cr all exhibit segregation to the SISF. In contrast, for the APB, Cr was either energy-neutral or segregated, depending on the presence of additional nearby Cr or on the specific lattice site on which it was placed. However, Mo and W did not segregate to the APB. Due to the segregation of W and Mo to the SISF but not to the APB, the inclusion of these elements could provide a degree of protection against creep-related deterioration.

Single crystal Ni-based superalloys are an essential material for application in gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. These properties originate from their typical γ/γ' microstructure, and by manipulating different alloying elements, the γ/γ' morphology, misfit and corresponding properties will change significantly. A turning point was the addition of 3 wt.% Re in the 2^nd generation of single crystal Ni-based superalloys, which doubled its creep life. Despite the significance of this improvement, the reasons for the so-called ‘Re effect’ continue to be debated in the field of superalloys. It is of great scientific and economic interest to understand the role of Re in extending the lifetime of single crystal Ni-based superalloys, and for the development of new superalloys. Here, we show direct evidence of Re, as well as Cr and Co, segregating to crystalline defects (dislocations and stacking faults) formed during creep deformation inside γ', using combined transmission electron microscopy and atom probe tomography. Phase field modelling is employed to support our interpretation of the effect of Re on dislocations and stacking faults. The new observations and understanding regarding the ‘Re effect’ will help design future Ni-based and Co-based superalloys.

Alloy Allvac 718Plus is a relatively new superalloy developed to improve the properties of the widely-used superalloy Inconel 718 (IN 718). The strength of IN 718 significantly decreases at temperatures of 650°C and above due to the transformation of the metastable, γ’’ (tetragonal, D022 structure) phase into the δ phase (orthorhombic, D0a structure). Allvac 718Plus, which was designed to address this issue exhibits service temperature up to 704°C (55°C more than that for IN 718 and close to Waspaloy) and its formability is similar to IN 718 and better than that for Waspaloy. The major changes in chemistry for Allvac 718Plus include a three times increase in aluminum (Al) content and the substitution of 18wt.% iron (Fe) with 9wt.% cobalt (Co) and 10wt.% Fe. The change in the (Al+Ti)/Nb ratio favors the formation of the γ’ as the major strengthening phase in Allvac 718Plus. Another significant phase in Allvac 718Plus is the Nb-rich phase δ, which preferentially forms at grain boundaries. The δ phase plays a major role in controlling the grain growth during processing and fabrication. The volume fraction of the γ’ and the δ phase can be controlled by solutionizing followed by aging. In the present work, the effect of solutionizing and aging on the resulting microstructure and the corresponding mechanical strength of Allvac718Plus was examined. Samples were solutionized at two different temperatures, 1000°C and 954°C, and then aged. To study the effect of aging temperature on solutionized samples, three different aging treatments were performed as follows: 1) one-step aging at 788°C for 16 h followed by faster cooling, 2) one-step aging at 704°C for 16 h followed by faster cooling, 3) Two-step aging at 788°C for 8 h followed by cooling to 704°C for 8 h then air cooling. Metallographic samples of the as-received and heat-treated samples were prepared to identify different phases using optical microscopy and scanning electron microscopy. Electron backscatter diffraction and energy dispersive spectroscopy was performed to investigate the composition of the various phases. Room temperature and elevated-temperature tensile and hardness testing will be performed on the aged samples to investigate the effect of the microstructure on the strength. The data obtained will be compared with the existing data and will help in understanding the dependency of time and temperature on the microstructural changes and mechanical properties.

We proposed new methodology for estimation of high-temperature properties of structure alloy by molecular dynamics simulation. Using this method, solid-liquid interfacial properties of Fe-Cr alloy are investigated with an EAM potential (Bonny et al., Philo. Mag. 91 (2011) 1724). After estimating melting points of pure Fe and Cr of the EAM potential, solidus and liquidus compositions with respect to temperature were estimated as follows. The solid-liquid coexisting system with the same concentration (X_S = X_L) as the initial state was relaxed under the constant pressure and temperature (NPT) condition. Calculations were carried out until the propagation of the solid-liquid interface due to solidification and solute partition did not proceed. It is considered that converged concentrations in solid and liquid phases correspond to the equilibrium concentration at the set temperature. Therefore, the same calculations were performed for various temperatures. By connecting solidus and liquidus concentrations with respect to temperature, the phase diagram for the Fe-Cr alloy for the EAM potential was successfully derived.
Next, a solid-liquid coexisting system with equilibrium concentrations in the obtained phase diagram are relaxed at equilibrium temperatures to investigate the temperature dependence of the solid-liquid interfacial energy. The capillary fluctuation method (Hoyt et al., Phys. Rev. Lett. 86 (2001) 5530) is employed to estimate the solid-liquid interfacial energy including its anisotropy. The solid-liquid interfacial energy increases with increasing equilibrium temperature. On the other hand, regarding the anisotropy effect, the interfacial energy of solid-liquid interface with (100) orientation is larger than those of (110) and (111) orientations at all temperatures. In summary, the methodology proposed in this study can be easily applied to other novel high-temperature structural alloys including refractory alloys and Co-based superalloys.

Multicomponent solid solution alloys (e.g., high entropy alloys) are at the cutting edge of metals research due to their outstanding mechanical properties such as the refractory characteristic in BCC HfNbTaTiZr alloys. At high temperature, the major deformation mechanism of BCC is dislocation creep, and it is directly related to the dislocation mobility. However, until now, studies have been most devoted to kink-pair nucleation and kink migration mechanism in multicomponent BCC alloys, and seldom discussed the dislocation mobility in them. Therefore, we conducted large scale molecular dynamics to study dislocation mobility in multicomponent BCC alloys including AlCoNiFe and AlCoNiFeTi by applying embedded-atom method (EAM) potential. To look into the dislocation mobility, we measured the dislocation velocities of multicomponent BCC alloys under different stresses, components, and temperatures. By this work, we could figure out the influence to the dislocation mobility and further affect the creep.

High-entropy alloys (HEAs), are multi-component random solid solution alloys with nearly an equiatomic composition, have been receiving tremendous attention due to the excellent cryogenic temperature ductility, superior mechanical strength and good wear resistance, exceptional damage tolerance. These excellent and distinguishing mechanical properties originates the curious properties of the lattice defects, such as vacancy, dislocation, twin, grain boundary, and so on, owing to the characteristic non-equilibrium quasi-random / short-range ordered atomic arrangement. To reveal the fundamentals of the mechanical properties, we studied the relation between the local atomic arrangement and the defect behavior and properties, such as vacancy diffusion, dislocation motion, deformation twinning and phase transformation, and grain boundary stability, using first-principles, molecular dynamics and Monte Carlo methods. We show that the local atomic arrangement, which is specified by the short-range order parameter and the local atomic strain, strongly correlated with the defect behavior in HEA.

High entropy alloys (HEAs) are solid solution materials composed of a large number of components (five and more) available in equiatomic or nearly equatomic proportions. HEAs are considered to show “high entropy effect, severe lattice distortion effect, sluggish diffusion effect and cocktail effect”(B.S. Murty (2014),M.C. Gao(2016)). Since the propotion of each elements in HEAs changes those mechanical behavior, the most suitable assortment can be chosen for each objectives as a structual material by investigation.
Molecular dynamics(MD) simulation is the powerful tool to investigate those atomic effects and the influence of propotion because propotions of each atoms are easily decided for comparison of mechanical behaviors. The second nearest neighbour(2NN) modified embedded atom method(MEAM) potential is used to our MD simulation. Each parameters of 2NN-MEAM potentials of five elements Co, Cr, Fe, Mn and Ni pure and binary systems are taken from published literature.
In this study, influences of Cr proportion for mechanical behaviors at non-equiatomic Co-Cr-Fe-Mn-Ni high-entropy alloy were observed. Models are monocrystals and fcc structures. Six models with different Cr proportions were set to 5, 10, 15, 20, 25, and 30 percents as Co, Fe and Mn propotions were set to 20 percents. As a result, increase of Cr proportion rised monocrystal young module at each models. Moreover, Seven models with different Cr proportions were set to 5, 10, 15, 20, 25, 30 and 35 percents as Co, Ni and Mn propotions were set to 20 percents. Increase of Cr proportion rises young module at models less than Cr 25 percents. Increase of Cr proportion also rised ultimate tensile strength at models less than 25 Cr percents. The highest value of ultimate tensile strength is 20.6 GPa at the equiatomic model. At models more than 20 Cr percents, decrease of Cr proportion brought down each young moduli and each ultimate tensile strengths. In the future work, influences of other components for mechanical behaviors will be revealed as well as Cr. We are also planning to invstigate relationship between atomic propotions and dislocation activities.

We report on microstructural and mechanical characterization of a commercially-available V_0.2Nb_0.2Mo_0.2Ta_0.2W_0.2 bulk alloy. w-2θ X-ray diffraction spectra revealed multiple reflections corresponding to a single-phase, body-centered cubic polycrystal with a lattice constant of 0.319 ± 0.001 nm composed of nanoscale grains. Spatially-resolved energy dispersive X-ray spectra acquired from the sample indicated that the alloy is compositionally homogeneous (heterogeneous) elemental composition at millimeter (micrometer) length scales. Nanoindentation tests carried out using Berkovich diamond indenter yielded hardness and elastic modulus values of 9 ± 1.4 GPa and 207 ± 19 GPa, respectively. In situ scanning electron microscopy based uniaxial compression tests conducted at room-temperature on cylindrical pillars of sizes d between 0.3 µm and 1 µm revealed that all the pillars undergo plastic deformation. From the load-displacement data, we find that the yield strengths of the pillars increase with decreasing d from 3.3 GPa at d = 1 µm to 9.0 GPa at d = 0.3 µm. Work hardening exponents increase over two-fold with increasing d from ~0.3 at d = 0.3 µm to ~0.8 at d = 1 µm. We attribute the observed behavior to size-dependent variations in displacement burst frequency and slip events. We expect that our results provide new insights into the nanomechanical response of body-centered cubic alloys.

BCC High Entropy Alloys (HEAs) consist of many elements distributed at random on the BCC lattice. The collective fluctuations in solute/dislocation interaction energies, even in dilute binary BCC alloys, lead to the spontaneous energy-lowering formation of a kinked/wavy structure for both screw and edge dislocations, respectively, over characteristic lengths ζ_c,screw and ζ_c,edge. Dislocation motion starting from the kinked/wavy structure is determined by the energetics at scale ζ_c. New general theories for both screw and edge motion in BCC alloys starting from this basic phenomenon are presented. The screw theory is sketched briefly, and shown to accurately predict strength versus composition and temperature in Nb-Mo and Nb-W binary alloys. As for BCC elements, the necessary inputs are difficult to establish, especially in more complex alloys. Key comparisons to simulations help demonstrate major features of the theory. More importantly, the edge theory shows that edge strengthening can be sufficient to compete with screw strengthening. Moreover, edges can control strengthening, especially at high temperatures, in some BCC HEAs. The edge theory, for which all inputs can be computed easily, explains (i) the exceptional retention of strength measured in MoNbTaW and MoNbTaVW at temperatures up to 1900K, and (ii) why the V-containing alloy is stronger. The edge theory can be reduced to a simplified analytic form that enables efficient computationally-guided design of new alloy compositions predicted to have high retained strengths and strength-to-weight ratios. Several new compositions are proposed. The combination of both screw and edge theories enables assessment of strengthening versus composition and temperature across the entire domain of Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr BCC HEAs.

Refractory high-entropy alloys (HEA) show great potential for high-temperature structural applications, particularly involving additive manufacturing. It is important to gain improved understanding of their mechanical properties. Valuable insights and predictions can be gleaned from atomistic simulations, but these are hampered by both the physical complexity of HEA (mandating numerous simulations with large models) and computational cost (driven by the need for accurate quantum-based density functional theory (DFT) to obtain reasonable accuracy). Using a simplified Al-Nb-Ti equi-composition metal alloy (bcc) as our examplar, we present a systematic approach to investigate mechanical strength, beginning with DFT to compute converged elastic properties in random composition alloys, and then using DFT data and machine-learning methods to generate SNAP (Spectral Neighbor Analysis Potentials) interatomic potentials for accurate finite-temperature molecular dynamics simulations of hardness and tensile strength of complex multi-component systems. We refine and assess DFT supercell-based approaches, using the SeqQuest code and PBE functional. The Al-Nb-Ti alloy proves to be physically different than its constituent species, and numerically more robust in calculations than elemental or crystalline (B2) alloys. For instance, the AlNbTi random bcc alloy is much stiffer in shear compared to crystalline sub-alloys. Computed properties of the equi-composition alloy are well converged with small (54-atom) random occupancy cells, are insensitive to the particular assignment of elements to sites in the cells, are robust to modest deviations from equi-composition, and (notably the shear moduli) converge much more rapidly against crucial aspects such as k-point sampling. This advantageous numerical behavior makes these modest supercell models ideal for generating the large training database needed for SNAP potential development. Each DFT model takes just a couple minutes to compute, and collectively encompass an accurate converged description of shear behavior crucial to predictive atomistic simulations of strength. — Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

We present a phase transformation strengthening mechanism in CrCoNi, a ternary derivative of the CrMnFeCoNi high entropy alloy. CrCoNi alloy exhibits a remarkable combination of strength and plastic deformation, even superior to the CrMnFeCoNi high-entropy alloy. We connect the magnetic and mechanical properties of CrCoNi, via a magnetically tunable phase transformation. While both alloys crystallize as single-phase face-centered-cubic (fcc) solid solutions, we find a distinctly lower-energy phase in CrCoNi alloy with a hexagonal close-packed (hcp) structure. Comparing the magnetic configurations of CrCoNi with those of other equiatomic ternary derivatives of CrMnFeCoNi confirms that magnetically frustrated Mn eliminates the fcc- hcp energy difference. This highlights the unique combination of chemistry and magnetic properties in CrCoNi, leading to a fcc-hcp phase transformation that occurs only in this alloy, and is triggered by dislocation slip and interaction with internal boundaries. In addition, we discuss the importance of average stacking fault energies (SFE) on deformation mechanisms in alloys of various compositions. The average SFE will be compared against the change in local bonding environment and the implication on competing deformation mechanisms will be discussed.

In 2004, the face-centered-cubic (fcc) single-phased CoCrFeMnNi was discovered and very promising mechanical properties were demonstrated. Since then, new interesting compositions of high and medium entropy alloys (HEA and MEA) were tested, some of them having superior mechanical properties compared to the CoCrFeMnNi alloy such as CoCrNi but also similar or even lower properties. Through thermodynamic calculations, it was shown that the fcc solid solution is stable for a large range of compositions in the quinary Co-Cr-Fe-Mn-Ni system [1]. These studies motivate a more systematic and comprehensive exploration of this multi-component system. This remains complex because the composition domain to explore is very large and because the mechanical properties are due to several strengthening mechanisms (e.g. : due to solute or Hall&Petch effect), which are difficult to separate.

In this context, the objective of this study is both to explore the mechanical properties due to solid solution strengthening (SSS) within the fcc domain of Co-Cr-Fe-Mn-Ni system, and to provide physical understanding of their evolution with alloy composition. To do so, based on thermodynamic simulations, 24 compositions were selected within the fcc domain to cover as much as possible the quinary system and subsequently processed with a simplified and reproducible metallurgical route. The lattice parameter was measured by X-ray diffraction and nanoindentation was selected as an efficient tool to measure only the SSS. Then, using a recently developed model for SSS in HEA [2], yield strength predictions were performed using as inputs only experimental data, including the lattice parameters measured here. The sensitivity of the predictions to the inputs will be exposed. Afterwards, experimental nanohardness and predicted yield strength will be compared. Then, the trends of evolution of SSS with composition will be connected to physical quantities such as elastic constants and lattice misfit. Finally, the different strategies for exploring solid solution strengthening of HEA will be discussed.

References:
[1] G. Bracq, M. Laurent-Brocq, L. Perrière, R. Pirès, J.-M. Joubert, I. Guillot, The fcc solid solution stability in the Co-Cr-Fe-Mn-Ni multi-component system, Acta Mater. 128 (2017) 327-336.
[2] C. Varvenne, A. Luque, W.A. Curtin, Theory of strengthening in fcc high entropy alloys, Acta Mater. 118 (2016) 164-176.

High entropy alloys (HEAs) are composed of five or more elements of a near-equal molar percentage in solid solutions. This contrasts with most metallic alloys that are designed based on the addition of a small amount of impurities, or alloying elements, to improve the properties of pure metals. A major character of HEAs is severe lattice distortion. The elements that make up the HEAs have different atomic sizes. These size differences inevitably lead to distortion of the lattice. Lattice distortions contribute to the free energy and impede dislocation movements and lead to pronounced solid solution strengthening.
Here we show that the impedance of dislocation motion by lattice distortion in the HEAs lead to mesoscopic dislocation interactions insider nanopillars of hundreds nm in diameter. The manifestation of mesoscopic dislocation dynamics enables their observation by transmission electron microscopy (TEM), and thus provide an unprecendeted opportunity for studying dislocations and dislocation dynamics. To demonstrate the above effect, we investigated the nature of lattice distortions in both fcc and bcc HEAs, and we performed correlation studies of dislocation dynamics with mesoscopic mechanical properties of the HEA nanopillars.
An example of correlative structure-property study is the observation of dislocation avalanches inside nanopillars of a HEA, Al_0.1CoCrFeNi. Our results show that the avalanches start with dislocation accumulations and the formation of dislocation bands in the HEA. Dislocation pileups form in front of the dislocation bands, whose giveaway trigs the avalanche. The experimental technique employed for the study is based on electron imaging, while a nanoindenter is used to exact the applied force and pillar displacement. Using this approach, we have observed and determined the conditions for the multiplication of dislocations, dislocation pinning, as well as a broad range of dislocation motions from nm/s to mm/s.[1]
To determine the nature of lattice distortions, we used electron nanodiffraction. Information about crystal symmetry, lattice strain and atomic distortion are data-mined and mapped from many (~10⁴) diffraction patterns. Application to the HEA reveals two embodiments of distortion, nm-sized mosaic blocks of paracrystals and strained nano-clusters. Their interaction gives rise to fractal strain field across nanoscopic to mesoscopic scales.[2][3]
References:
[1] Yang Hu, Li Shu, Qun Yang, Wei Guo, Peter K. Liaw, Karin A. Dahmen, and Jian-Min Zuo, 'Dislocation Avalanche Mechanism in Slowly Compressed High Entropy Alloy Nanopillars', Communications Physics, 1 (2018), 61.
[2] Yu-Tsun Shao, Renliang Yuan, Yang Hu, Qun Yang, and Jian-Min Zuo, 'The Paracrystalline Nature of Lattice Distortion in a High Entropy Alloy', arXiv:1903.04082 (2019).
[3] The work is supported by DOE BES (Grant No. DEFG02-01ER45923). RLY is supported by SRC.

Recently, high-entropy alloys (HEAs) attract worldwide attention due to their outstanding properties, showing a huge potential as engineering structural materials. In practical engineering applications, materials are usually subjected to cyclic loading during service, leading to fatigue fracture. However, few low-cycle fatigue (LCF) studies have been reported on HEAs so far. In this study, the LCF behavior of a multiphase HEA, are systematically studied by integrated experiments and fatigue-life prediction models. Real-time in-situ neutron diffraction was employed to dynamically monitor the cyclic-deformation behavior of this HEA. Advanced microscopy techniques, such as transmission-electron microscopy (TEM), electron-backscattered diffraction (EBSD), and scanning-electron microscopy (SEM), were used to characterize the microstructural evolution during LCF. The in-situ real-time insight into the cyclic-deformation behavior of this HEA will pave the way to design innovative HEAs with outstanding fatigue resistance.

Concentrated complex alloys (CCAs), also usually referred as high-entropy alloys (HEAs) emerges as a deep paradigm shift in conventional metallurgy. Such solid solution phases are based on multi-element combination, instead of the regular one accepting a unique base element. Recent investigations have recently emphasized promising mechanical such as high yield stress, excellent resistance to wear, corrosion or creep. Among this new family of metallic materials, the refractory HEAs are focusing a lot of interest due to its good ability to retained high strength at elevated temperature, making potential unpreceded structural applications accessible. Nevertheless, the viability of HEAs as structural material requires to assess their mechanical properties under typical loading, such as fatigue one.
In the present work, the fatigue resistance of equimolar HfNbTaTiZr HEA is investigated in air and at room temperature. An outstanding fatigue strength is obtained under four-point bending fatigue, such as fatigue limit at 10⁷ cycles exceeding the uniaxial yield stress is found. Further investigations are also carried out in axial loading. An intergranular fatigue crack initiation and transgranular crystallographic crack propagation mechanisms occur. Analysis of the crack path in terms of crystallographic planes is also included, revealing that the crack spreads along both {011} and {112} slip planes. This particularity implies singularities in the crack propagation phenomenon in comparison with other metalllic materials prone to similar crystallographic crack propagation.

In situ transmission electron microscopy is carried out to investigate deformation mechanisms in the medium- and high entropy alloys (CrCoNi and CrMnFeCoNi), which exhibit very high hardening behavior after plastic yielding, leading to high strength and toughness. With the deformation of CrCoNi, we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dual function: conventional twin-boundary (TB) strengthening from the interaction of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide. While, for CrMnFeCoNi, we found attacking faults and full dislocation process are dominating the plastic deformation, which serves the hardening process for the alloy.

High-Entropy Alloys (HEA) show outstanding mechanical and physical properties, which would not have been expected regarding their simple crystal structure and the fact that they are single phased. Despite, there are already many systems known exhibiting this behavior, the mechanisms, responsible for their exceptional properties, were not completely clarified yet. In this study we report on the novel precious metal based HEA: AuCuNiPdPt, which crystallizes in the Cu-type face-centered cubic structure (fcc) and is single phase without chemical ordering after homogenization and recrystallization treatments. This alloy can be cold worked up to a logarithmic deformation degree of φ=2.4 applying rotary swaging. During compression tests it further deforms homogeneously up to an engineering strain of ε=30 %. The yield strength ranges from 820 MPa in the recrystallized state to 1170 MPa when prior strain hardened by cold work with a logarithmic deformation degree of φ>0.6. The texture with almost random orientation distribution in the recrystallized state develops towards a mixture of <111> / <001> fiber components during cold work. This is a similar behavior like what has been seen for conventional fcc alloys, such as Cu-based alloys or austenitic steels. Further, AuCuNiPdPt shows localized dislocation slip in the majority of all grains at low strains, leading to pronounced stacking faults and slip bands. In grains with a (near-) <111> orientation, extended stacking faults develop towards deformation twins even with higher order causing significant grain refinement and finally to the aforementioned work hardening behavior. The results from mechanical testing are verifiably correlated to the microstructure by means of APT, SEM (EBSD, EDX), TEM as well as XRD analysis.

High entropy alloys (HEAs) is a relatively recently identified class of metallic alloys [1] which, in contrast to its conventional counterparts, have four or more elements mixed in near-equimolar composition to form a single-phase alloy [2]. High entropy alloys of refractory elements like Nb, Ni, Zr, Ta, Cr, Ti have reported exhibiting superior mechanical properties while having low-density [3]. In this work, we investigate the generalized fault energies of the BCC High Entropy Alloy system NbMoTaW using Density Functional Theory and compare the competing deformation mechanisms of slipping and twinning. A 23 layered (1-21) type plane structure was generated and systematically sheared along the [111] direction to get stacking faults and twins. Energy landscapes for twinning (Generalised Planar Fault Energy curve) and for slipping (Generalised Stacking Fault Energy curve) was calculated for NbMoTaW and compared with the component elements. These energy landscapes include parameters like unstable stacking fault energy for slipping and twinning, stable stacking fault energies, twin boundary migration energy, etc. It has been found that the stacking fault and planar fault energy values for the HEA are lower than that of all component elements. Twinnability, the ratio between Twin Boundary Migration energy and the difference in unstable stacking fault energies (γ_TBM/Δus), of the HEA system was calculated from the simulated energy landscapes to investigate the competition between formation of full dislocation and emission of partial dislocations from subsequent atomic layers. This ratio was found to be 0.13 suggesting extensive deformation twinning can happen in the HEA system. Origin of low fault energies in NbMoTaW was investigated using Integrated Crystal Orbit Hamilton Population (ICOHP) which is the molecular bond order analog in solid state. For each pair of interactions in the HEA system, bond weighted Density of States was computed, integrated up to the Fermi level and then compared with the interactions (Nb-Nb, Mo-Mo, Ta-Ta, W-W) in pure component elements. Among the component element interactions, Nb-Nb has the highest ICOHP energy (weakest interaction) and lowest Stacking Fault Energy. W-W interaction is the strongest and it in-turn has the highest stacking fault energy. Except for W-Mo, W-Ta, interactions in NbMoTaW was found to be as weak as the Nb-Nb interactions which explain the reduced fault energies. As the interactions involving Tungsten are stronger than the other interactions, stacking fault and twin formation is less likely to happen near Tungsten rich regions of the alloy. Since deformation twinning is preferred as suggested by the twinnability parameter, the twins formed are likely to be Tungsten deficient due to these strong interactions.

References
[1] Yeh, J.W., et al. Advanced Engineering Materials 6.5 (2004): 299-303.
[2] Ye, Y. F., et al. Materials Today 19.6 (2016): 349-362.
[3] Senkov, O. N., et al. Acta Materialia 61.5 (2013): 1545-1557.
[4] Shang, S. L., et al. Acta Materialia 67 (2014): 168-180.
[5] Dronskowski, Richard, and Peter E. Blöchl. The Journal of Physical Chemistry 97.33 (1993): 8617-8624.

Compared with traditional alloys, high entropy alloys have excellent properties such as simple phase composition, higher strength, higher hardness, better wear resistance, better corrosion resistance, and unique electromagnetism as a novel alloy. In this work, Effects of Al and Cu content on the microstructure and properties in Al_xCoCu_1-xFeNi_0.8 alloys were investigated by XRD, SEM, EDS, TEM, microhardness testing, tensile and compression testing. In Al_xCoCu_1-xFeNi_0.8 (x=0.25, 0.3, 0.4, 0.5, 0.75) high-entropy alloys, the phase composition of alloys gradually changes from a single face-centered cubic (FCC) structure to a body centered cubic(BCC) structure and a little amount of FCC structure with the increase of Al content and the decrease of Cu content. Microstructure of the as-cast Al_xCoCu_1-xFeNi_0.8 (x=0.25, 0.3, 0.4, 0.5)alloys presents the typical dendritic morphology, and microstructure of as-cast Al_0.75CoCu_0.25FeNi_0.8alloy presents the cellular crystal structure and the dendritic morphology. EDS results show that Cu element tends to be segregated in the inter-dendrites. Al_0.75CoCu_0.25FeNi_0.8alloyhas the best comprehensive mechanical properties. The compressive yield strength and compressive strength are 1585MPa and 2049MPa, respectively. The fracture strain is 16.4%, and the hardness is 619.4Hv.The heat treatmentsof Al_xCoCu_1-xFeNi_0.8 (x=0.25, 0.3, 0.4, 0.5, 0.75) alloys at 800⁰C for 0.5h, 6h, 12h were investigated, too. The inter-dendritic phase was obtained by eutectic reaction and grain boundary generated spinodal decomposition after heat treatment, forming a net structure. After 0.5h of heat treatment, the nano-sized precipitate phase is generated. With the increasing of heat treatment time, the nano-meter precipitate phase is dissolved, and the spinodal decomposition occurs in Al_0.25CoCu_0.75FeNi_0.8alloy. The highest yield strength is obtained at 0.5h heat treatment. and BCC phase is still the main phase of Al_0.75CoCu_0.25FeNi_0.8 alloy. After heat treatment for 6h, the yield strength of Al_0.75CoCu_0.25FeNi_0.8 alloy is as high as 1719MPa.

The largely unexplored phase space of high entropy alloys (HEAs) offers potential for transformative enhancement of metallic material properties, but the breadth of this phase space also invites an approach for high-throughput evaluation of candidate alloys. One possibility is combinatorial thin film screening. Different combinations of CrMnFeCoNiCu and VNbMoTaW thin film samples were prepared by simultaneous magnetron sputtering of the elements (all 6, or subsets of 5) onto silicon wafer substrates, to identify potential FCC and BCC HEAs, respectively. Due to the arrangement of the sputtering targets, a wide chemical composition gradient was achieved in the films. The various areas of these films exhibited different phases, some of which were non-equiatomic single-phase regions. In order to screen the crystal structure and chemical composition of these potential HEAs, multiple characterization techniques were used: scanning electron microscopy, energy dispersive x-ray spectroscopy, X-ray diffraction and electron backscattered diffraction analysis. Based on the results of this combinatorial method, potential single-phase HEAs were identified and successfully produced in bulk form via arc-melting followed by thermomechanical processing. A second possibility for high-throughput evaluation of candidate alloys is slow cooling from the melt, which allows the alloy system to find its stable composition(s). Equiatomic CrMnFeCoNiCu was melted and then cooled at a slow, controlled rate, resulting in a multiphase microstructure consisting of Cr-rich needles inside a matrix that was itself an HEA. This matrix was used as the basis for fabricating new alloys with similar composition. These approaches yielded bulk alloys that exhibited improved mechanical properties in comparison to their equiatomic counterparts. This presentation will describe the identification of candidate HEAs with desired microstructure (stable single-phase alloy), their fabrication in bulk form, thermomechanical processing, and mechanical testing with a focus on tensile properties.

The effect of magnetic ordering on stacking fault energy (SFE) is of great interest and significance for designing high entropy alloys (HEA) with targeted mechanical properties. To reveal the influence of magnetic ordering on SFE and correspondingly its influence on the deformation mechanisms and mechanical properties, we designed a class of novel non-equiatomic quinary HEAs by a combination of ab initio simulation and experimental validation. A wide compositional space of Cr₂₀Mn_xFe_yCo₂₀Ni_z (x + y + z = 60, at. %) was probed by density functional theory (DFT) calculations to search for potential alloys displaying the TWIP (twinning induced plasticity) /TRIP (transformation induced plasticity) effects. Several representative HEA compositions with low computed SFEs were metallurgically synthesized, processed, and probed for microstructure, deformation mechanism and mechanical property evaluation. It is found that the Mn content plays a key role in the stabilization of antiferromagnetic configurations which strongly contribute to the SFEs and eventually lead to the prevalent deformation behaviour observed. The impact of valence electron concentration (VEC) on TWIP/TRIP behaviour in Mn-containing HEAs is also probed and discussed.

The demand on materials for various applications is growing continuously, especially in the field of ceramic coatings used to protect various components or tools for hard-to-machine materials. The development of materials that can withstand high thermal and mechanical loads is therefore in focus of many materials science activities. In this respective, a relatively new class of materials, so-called high-entropy materials, gained enormous attraction. These alloys consist of multiple principal elements (more than 5), which lead to a high configurational entropy (> 1.5R, R being the universal gas constant) if single-phased. Typically, these high-entropy materials exhibit four core effects: 1) The high-entropy effect, which significantly reduces the overall Gibbs free-energy (especially with increasing temperature); 2) the severe lattice distortion, stemming from the different atom radii forming the solid solution; 3) the sluggish diffusion, due to the increased activation energy for diffusion as the individual atomic neighbourhood is different; and 4) the so-called cocktail effect, allowing to combine the individual properties.
More recently as for metallic bulk materials, this concept was also introduced to ceramics and especially hard coatings and ceramic thin films.
Here we show the beneficial effect of the high-entropy concept applied to several thin film material systems including nitrides, borides, and oxides. The study focuses on their preparation with physical vapor deposition, their (non)sensitivity to deposition parameters, their thermal stability and mechanical properties. All coatings investigated exhibit outstanding thermal stability and significantly decelerated decomposition and softening processes, outperforming their commonly-used binary or ternary constituents.

Modern materials research receives increasing support from first-principles modeling based on Density Functional Theory (DFT). Such approach gives fundamental understanding, offers efficient pre-screening against various degrees of freedom and provides information where experimental assessments are not feasible. Today the ab initio theory aided materials assay finds its way in almost all areas of advanced materials design and characterization.

Due to the complexity of the problem, DFT modeling was practically missing within the field of High Entropy Alloys (HEAs) for almost one decade after their discovery. Starting from early 2010s, we made a series of attempts to fill this gap and extend the scope of ab initio modeling to HEAs. In the present contribution, I will briefly overview the pioneering applications of alloy theory in the case of magnetic and refractory HEAs, and point out the main known and hidden challenges associated with such efforts. I will present our recent theoretical predictions for the mechanical and magnetic properties of existing HEAs. Special emphasis will be placed on the plastic deformation mechanism in HEAs with close packed crystal structure. A fresh understanding of the twinning in various HEAs will be disclosed.

High entropy alloys (HEAs) have been investigated in recent years due to their superior properties such as high strength, high hardness, and high temperature softening resistance. Meanwhile, the traditional aluminum alloys possess attractive properties including light weight, high corrosion resistance, and high thermal conductivity.
In this project, the ICME approach (Integrated Computational Materials Engineering) is applied to design a new alloy by combining high-entropy alloys and traditional aluminum alloys. In other words, the “high entropy” concept will be utilized in this new alloy with the aluminum FCC matrix. Combining the “high entropy” concept borrowed from HEAs and aluminum FCC matrix shows a totally new composition region, which has higher entropy of mixing comparing with the traditional aluminum alloys. After manufacturing the samples based on the simulation guidance and experimental characterizations, it was found that the designed alloys show promising ultimate tensile strength and elongation.

After the recognition that conventional superalloys have reached their limits in high-temperature applications, there has been growing interest in developing alternative metallic-based structural alloys. Owing to their unique characteristics such as high melting point and excellent softening resistance, refractory metals have attracted significant focus in high-entropy alloys (HEAs) design. However, the investigation of novel refractory HEAs is largely retarded by the traditional trial-and-error alloy design framework, particularly within the non-equiatomic composition regime. To this end, here we present that by making use of the natural thermodynamic mixing characteristics, a series of non-equiatomic refractory HEAs can be developed. We show that these non-equiatomic HEAs exhibit desirable strength-ductility synergy and promising high-temperature performances.

High-entropy alloys exhibit many desirable mechanical properties, including high fracture toughness, wear resistance, and crack resistance. However, the large atomic mass differential between constituent elements can lead to undesired compositional segregation and inhomogeneity. Gravitational effects (e.g., convection and sedimentation) during the melt cause density-driven phase segregation leading to inhomogeneity in the composition of the alloy. This causes reduced mechanical strength and toughness.
The International Space Station (ISS) U.S. National Laboratory offers a unique environment of persistent microgravity and, if desired, exposure to the harsh space environment in low Earth orbit. Microgravity mitigates gravitational phenomena such as buoyancy-driven convection, density-driven sedimentation and segregation, and heterogeneous nucleation at surface interfaces. Early studies of the solidification of high-entropy alloys in microgravity have demonstrated a higher degree of compositional homogeneity compared with terrestrial solidification. In addition, microgravity provides the opportunity to decouple the effects of Stokes sedimentation from Marangoni flow in order to better understand the heat and mass transfer dynamics in the melt and during solidification.
We will introduce the underlying phenomena directing the physics of the high-entropy alloy melt in persistent microgravity and will present the roles of Stokes sedimentation, Marangoni flow, and buoyancy-driven convection during the melt and solidification of high-entropy alloys. Additionally, we will present case studies of high-entropy alloy solidification in microgravity and compare the results with terrestrial experiments. We will also discuss translational lessons learned from microgravity experiments that inform and direct terrestrial research and manufacturing. Finally, we will present opportunities for future microgravity experiments and access to ISS facilities through the ISS National Laboratory.

High entropy alloy (HEA) or Medium entropy alloy (MEA) have attracted extraordinary attention since the single faced-center-cubic (FCC) Cantor alloy with excellent mechanical properties in a cryogenic environment was observed. Based on the cantor alloy, equal-atomic CoCrFeMnNi, nemours Cantor-like alloy systems have been widely studied in the past decade. Adding external elements and adjusting operational element contents, all of the alloy design strategies were to enhance the mechanical performance through different strengthen mechanism. In this work, a FeMnSi-based MEA containing 10% Cobalt, 12.5% Chromium and 5% Nickel (at atomic%) was achieved to be a single FCC crystal structure through vacuum argon melting furnace, and it is realized the improvement in the mechanical properties through tensile test comparing with those of Cantor alloy. By reduction of valuable contents, such as cobalt and nickel, the cost of the novel MEA has been cheapened, and the addition of Silicon into this system has diminished the stacking fault energy (SFE) in order to trigger the deform-induced ε martensite phase transformation over a tensile test. Under the transmission electron micrograph (TEM) observation, various hexagonal close-packed (HCP) martensite phases were investigated among the deformed microstructure. The result from cryogenic tensile under the liquid nitrogen environment illustrated two series of interwoven ε martensite phases that behave both slat-shaped and needle-shaped sizes, which contribute the ultimate tensile strength (UTS) to a value over 1.2GPa. To further study the effect of microstructure evolution on the mechanical property among the elongation process, X-ray diffraction (XRD), electron backscatter diffraction (EBSD) technique have applied in this work.

The controllable incorporation of multiple immiscible elements into a single nanoparticle merits untold scientific and technological potential, yet remains a challenge using conventional synthetic techniques. We present a general route for alloying up to eight dissimilar elements into single-phase solid-solution nanoparticles, referred to as high-entropy-alloy nanoparticles (HEA-NPs), by thermally shocking precursor metal salt mixtures loaded onto carbon supports [temperature ~2000 kelvin (K), 55-millisecond duration]. We synthesized a wide range of multicomponent nanoparticles with a desired chemistry (composition), size, and phase (solid solution, phase-separated) by controlling the thermal shock parameters (substrate, temperature, shock duration, and heating/cooling rate). To prove utility, we synthesized quinary HEA-NPs as ammonia oxidation catalysts with ~100% conversion and >99% nitrogen oxide selectivity over prolonged operations. The unique shock synthesis enables high-entropy mixing and non-equilibrium processing to create HEA-NPs, and shows generality, tunability, and potential scalability. The successful synthesis of HEA-NPs opens a new research area for materials discovery and optimization, where the elemental composition and mixing entropy of nanoparticles can be carefully designed and controlled.

High entropy oxide composed of Bi, Sr, Fe, Zr, and Y is synthesized by sol-gel method. Using nitrate-based compound including 4BiNO₃(OH)₂-BiO(OH), Sr(NO₃)₂, Fe(NO₃)₃-9H₂O, N₂O₇Zr-xH₂O and Y(NO₃)₃-6H₂O to synthesize the Bi-Sr-Fe-Zr-Y-O. First, all compounds are poured in a beaker with water and stir it for 1 hour. Then, adding the citric acid to as the chelating agent that catches all metal atoms rotates for half an hour. Next, the solution is adjusted to PH=7 by adding ammonia solution and put it into the oven that is at 90 degree Celsius for 48 hours. After that, it is sintered at 900 degree Celsius for 12 hours in the furnace. Finally, the High Entropy Oxide, Bi-Sr-Fe-Zr-Y-O is formed as nanoparticle.

Although solid-solution strengthening (SSS) is one of the most important mechanisms that dominate the mechanical strength of high entropy alloys (HEAs), the classical theories of SSS, which were established mostly for dilute binary solid solutions, cannot be applied to HEAs, because the definition of “solvent” or “solute” in HEAs is difficult and also because the “solute” concentration is as high as ~20 at.%. It has recently been proposed, however, that the mean-square atomic displacement (MSAD) averaged over the constituent elements can be a good scaling parameter to predict the degree of SSS in HEAs [1]; the MSAD is a measure of the local lattice strain that acts as obstacles for moving dislocations. In the course of investigating atomic environment of each of the constituent species in the Al_0.3CrFeCoNi HEA by X-ray fluorescence holography (XFH), some of the authors (TY & KH) have found that the holograms and reconstructed atomic images drastically change after room-temperature (RT) aging for 18 months, which invokes that certain diffusionless structural relaxation occurs at room temperature in the HEA. The RT structural relaxation will have an impact on the MSAD value, resulting in change in strength/hardness.
In the present study, we have investigated the resonant frequencies, internal friction, Vickers hardness, and MSAD value in Al_0.3CrFeCoNi as a function of RT-aging time after casting, in order to elucidate what kind of structural relaxation occurs during RT aging. The resonant frequencies/internal friction measured by electromagnetic acoustic resonant spectroscopy monotonically increase/decreases with the aging time whereas the Vickers hardness decreases by approximately 5%. These phenomena can be understood by considering a situation where atoms located at unfavorable (metastable) positions just after casting, which are largely displaced from the average lattice points (ALPs), are shifted to more stable positions closer to ALPs with the aid of thermal energy during RT aging. In other words, just after casting the atoms are somewhat loosely bonded to each other with large MSAD, consequently, to exhibit a relatively low elastic modulus (low resonant frequencies) and high hardness, but they become more tightly and regularly arranged with small MSAD during RT aging, resultingly exhibiting higher elastic stiffness (higher resonant frequencies) and lower hardness. The decrease in the MSAD value after RT aging has been exemplified by single crystal X-ray diffraction analysis. This softening behavior observed in the hardness is opposite to the age hardening observed in some aluminum alloys and should be taken into consideration for the practical usage of HEAs as structural materials.
Reference
[1] N. L. Okamoto, K. Yuge, K. Tanaka, H. Inui, and E. P. George, AIP Adv., 6, 125008 (2016).

The increase of temperature typically facilitates the mixing of materials because it enhances the effect of entropy. However, a series of high-entropy, lanthanide oxides, with nine different five-component high-entropy oxides involved, has shown the opposite behavior. Those nine candidates exhibit single-phase, high-entropy crystal structures below 1100 °C, but experience phase separations as well as multiple phase transformations at high temperature, up to 2200 °C. In this study, in-situ, high temperature synchrotron data of this high-entropy lanthanide system was collected. Atomic resolution, high angle annular dark-field (HAADF) imaging and energy-dispersive X-ray mapping (STEM-EDS) were performed on our candidates before and after phase separations. Another five-component, high-entropy, lanthanide oxides system, with similar cation radii differences (δ) compared to the prior system, has been studied. Those candidates remained structurally stable without phase transformation from room temperature to melt. By comparing these two systems, a hypothesis has been made based on their crystal structures, oxidation states and difference in constituent cation radii. This research introduces that the single-phase oxide could possibly be transformed to multi-phases under thermal treatments. Therefore, the possible structural transformation(s) of the constituent mono-cation ceramic (e.g. oxide) need to be considered in the design of high-entropy ceramics.

High-entropy alloys (HEAs) have proven to exhibit remarkable mechanical and structural properties. Beyond HEAs, the high entropy oxides (HEOs) have been proposed as a promising material system, where configurational disorder due to the random distribution of constituent elements into the cation sublattice can act as a driving force to tailor physical properties as well as stabilize new material phases and functionalities. In this talk, we will present on growth and functional properties of single-crystal epitaxial thin films of extremely configurationally disordered ABO₃ perovskites; Ba(Zr_0.2Sn_0.2Ti_0.2Hf_0.2Nb_0.2)O₃, La(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ and (La_0.2Nd_0.2Sm_0.2Gd_0.2Y_0.2)(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃. These new materials conform to standard tolerance factor considerations where multicomponent on A or B-site can be simply being treated as the average ionic radii value. A first investigation on the crystal, magnetic, relaxor-ferroelectric, and thermal properties of these high-entropy perovskite oxide thin films grown by pulsed laser epitaxy will be presented. X-ray diffraction and scanning transmission electron microscopy show single phase growth of these films with excellent crystallinity and atomically abrupt interfaces to the underlying substrates. Atomically resolved chemical mapping confirms a uniform and random distribution of all B-site cations. We will close by discussing how “Entropy-Stabilization” approach provides an ideal opportunity to design materials from a much broader combinatorial cation pallet for fundamental studies in strongly correlated materials where local disorder can play a critical role in determining macroscopic properties.

This work was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

Flexible alloys are most potential in the future, just like the rigid alloys played in the past. The high entropy alloys have been verified to have the following five typical properties: (1) high thermal stability and resistance to heat softening; (2) easier to break the tradeoff between strength and ductility; (3) very low stacking-fault energy; (4) high irradiation resistance; (5) high corrosion resistance. Thus if the flexibel high-entropy alloys developed, that would be a great achievement. Currently flexible high entropy alloys can be the fibers, foils, and films, which can be bended and recoverable. However, as a nonlinear behavior between the entropy and the properties, to develp the flexible high entropy alloys with special properties, is usually a time consuming process, lots of alloy with different compositions need to be prepared and tested. The new flexible high entropy alloys are usually expected to be ductile with solid solution phases, irradiation resistant, excellent phase and microstructural stability, and corrosion resistant. In the paper, configurational entropic principle were used to design the alloys composition with forming solid solution phase of BCC structure, and the properties were optimized by using high-throughput screening technology.

The large composition-processing space for high-entropy alloys (HEAs) makes study of them using high-throughput screening methodologies advantageous. The most common method of rapid synthesis is to create combinatorial thin film libraries. For refractory HEAs, an important and necessary property is high-temperature oxidation resistance. Currently it is unknown if high-temperature oxidation in HEAs can be studied in thin films as there is some question as to whether the finite thickness of the film, on the scale of a couple of microns, will affect the ability to measure oxygen penetration and oxide thickness compared to semi-infinite bulk samples. An answer to such question is necessary to justify the translation of thin-film high-throughput refractory HEA oxidation resistance to bulk alloys. In this work we create NbTiTa thin films with a range of thicknesses and subject them to oxidation at varying times and temperatures. As a function of thickness, we look at oxygen penetration depth and oxide thickness to determine the limits of time and temperature at which the thin film becomes bulk-like and no longer exhibits finite size effects.

High entropy alloys (HEAs) have great potential to combine numerous properties for structural applications. Here, mechanical mixing of pre-alloyed CoCrFeMnNi and pure Ti powders are employed to reduce the complexity of powder fabrication and the cost of synthesizing high-purity pre-alloyed powder. The CoCrFeMnNi-0.18Ti HEA is successfully fabricated with homogenous chemical compositions via electron beam melting (EBM), which is one of popular powder bed fusion additive manufacturing technologies. It is revealed that EBM is a promising method to develop new alloy using mixed powder. A detailed investigation on microstructural evolutions through the building direction is conducted by a transmission electron microscope, a scanning electron microscope with energy dispersive spectroscopy and electron backscattered diffraction. Besides of FCC phase, three precipitates, Cr-rich γ, Ni-rich D0₂₄, and σ (CrFe) are identified. The formation of these precipitates and their evolution are discussed based on the rapid cooling and subsequence in situ heat treatment during the EBM process. In addition, the tensile and compressive mechanical properties are also revealed based on the size and distribution of the precipitate.

Zr-based alloys are considered as the most promising materials in the biomedical application fields because of its excellent in vivo performance, acceptable mechanical properties, and great corrosion resistance. However, due to the complexity of composition and nonlinear relationship between the performance and mixing-entropy, efficientive screening of Zr-based alloy with ideal phase structure and properties is a huge challenge.
In this work, a high-throughput method combining co-sputtering and physical mask, was used to prepare the compositional gradient films, which will be used to screen in Zr-based medium entropy alloys with BCC structure. Based on the screening results, the formation, structure and mechanical properties of bulk Zr-based medium entropy alloys were further discussed in detail. This work not only offers novel BCC structured Zr-based medium entropy alloys with prominent properties for practical applications, but also shed light on development of high-throughput preparation technology in general.

Abstract
The improvement in plasticity is usually compensated by the reduction of strength in structural materials, which is a long-standing conflict referred as the strength- plasticity trade-off. In recent years, a new type of structural materials of high-entropy alloys (HEAs) with equiatomic or near-equiatomic concentrations has attracted increasing attention due to their unique mechanical properties. However, HEAs with BCC structure normally exhibit high strength but low plasticity, while the HEAs with FCC structure show inversely high plasticity and low strength. The formation of the mixing structures (i.e, FCC+BCC) is an approach to solve the problem. In this work, a BCC-based AlCoCrFeTi_0.5 high-entropy alloy (HEA) was modified by the addition of Ni to improve the plasticity. The new HEAs have the composition of AlCoCrFeTi_0.5Ni_x (x=0~3 mol%), which were prepared by arc melting following by copper mold casting. The effect of Ni concentration on the microstructures and mechanical properties of AlCoCrFeTi_0.5Nix alloys were systematically studied. It was found that the phase transformation follows the following sequences with the increase of Ni concentration: BCC1 (disordered)→BCC1+BCC2 (ordered)→BCC1+BCC2(ordered) +FCC(ordered). FCC phase appeared at Ni=1.5 mol% and then increased in amount with further increase of Ni content. Consequently, the mechanical properties changed drastically due to the phase transformations. A good combination of strength and plasticity was achieved at Ni=2.5 mol%, in which the proportion of FCC phase and BCC phases (BCC1+BCC2) are nearly equivalent. This high entropy alloy exhibits yield strength of 1.41 GPa ,compressive plastic strain of 31.8% and fracture toughness Kq of 64.3MPa *m^1/2. The toughening mechanism of the HEAs are discussed on the basis of the microstructure evolutions associated with Ni addition.

Owing to their excellent physical properties such high melting temperature, significant high-temperature strength and good thermal conductivity, tungsten and tungsten-based alloys are promising candidates in various applications. They are commonly used as potential plasma-facing material in fusion reactors and electrical contacts. In literature, several experimental observations by TEM revealed the improved thermal stability of W-based binary alloys upon addition of Ti, Ag and Ta atoms [1,2]. These observations have been attributed to the segregation of solute atoms to grain boundaries (GB). The propensity for segregation of these alloying elements is different as indicated by formation of enrichment/depletion regions around GBs. However, the segregants behavior, enhanced stability and its underlying mechanisms are not fully understood. In this work, an extensive DFT calculations have been performed to explore the GB morphological effects on the segregation behavior of Ti, Ag and Ta as examples of 3d, 4d and 5d elements, respectively. Six different GB structures consisting of low/high-angle symmetric tilt [001] GBs are considered. It is found that Ag atoms exhibit substantial tendency of segregation to both types of GBs. In contrast, both Ta and Ti atoms exhibit preferential propensity to segregate to high-angle STGB. The highest segregation energy values obtained for Ti, Ag and Ta are -0.017, -1.53 and -0.2, respectively. The change of metallic bond nature between solute and W atoms are responsible for the variation of the segregation energy for each element. Furthermore, segregation energy results indicate that while Ag atoms tend to segregate within and around the GBs plane, whereas Ti and Ta atoms energetically prefer to reside within the GB core. This behavior was explained using density of states (DOS) and charge transfer analysis. We found that d-d hybridization and charge transfer between solute and host atoms play important role in determining segregation/anti-segregation character for each GB site. A comparison of our DFT results with existing thermodynamical and atomistic models have been made and some improvements should be included for further development.
References
[1] M. Callisti, F.D. Tichelaar, T. Polcar, In situ TEM observations on the structural evolution of a nanocrystalline W-Ti alloy at elevated temperatures, J. Alloys Compd. 749 (2018) 1000–1008. doi:10.1016/j.jallcom.2018.03.335.

[2] A. Iveković, N. Omidvari, B. Vrancken, K. Lietaert, L. Thijs, K. Vanmeensel, J. Vleugels, J.-P. Kruth, Selective laser melting of tungsten and tungsten alloys, Int. J. Refract. Met. Hard Mater. 72 (2018) 27–32. doi:10.1016/j.ijrmhm.2017.12.005.