Symposium MB2 : Materials under Mechanical Extremes

The mechanics of materials under extreme conditions is often viewed as a complex topic, and extreme conditions are often viewed as being associated with complex phenomena. There is some truth to these views. However, it is also true that extreme conditions interrogate the material in unique ways, resulting in the manifestation of dominant mechanisms that can override much of the complexity of the microstructure (consider, for example, how extreme pressures can interrogate the atomic structure of a material within the equation of state formalism). Similarly, extreme conditions can bring to the fore characteristics of the material that are not easily observed in conventional engineering situations. We focus in this talk on the influence of the (broadly defined) defect population in a material on the mechanical behavior of the material, and show how extreme conditions couple with the defect population to generate the observed behaviors. As a specific example, we consider the strength, fracture and fragmentation of ceramics and geomaterials using in situ experimental methods, theoretical analyses and multiscale simulations.

Accessing high strength regimes of metals in small structures is often succinctly stated as “smaller is stronger”. However the mechanisms responsible for this phenomenon can be ascribed to both “excess” dislocations in small volumes due to geometric requirements, or to “starvation” conditions where nucleation of defects takes on a larger role. The formation excess geometrically necessary dislocations may be impacted by intrinsic materials properties, such as stacking fault energy and the stresses required to make new dislocations, and extrinsic defect effects, such as statistically stored dislocation densities and dislocation multiplication mechanisms. Nanoindentation was carried out at both room temperature and elevated temperatures on four different pure metals (Co, Ni, Ir, and Pt) in a variety of microstructural conditions. The hardness versus depth data from these experiments were fitted using a conventional power law formulation for each material set (combined effects of temperature and existing dislocation structure). The effects of extrinsic defects, particularly the likely dislocation density, dominate the perceived size effect parameter over those likely due to intrinsic properties such as stacking fault energy. A model is presented to suggest that a series of mechanisms, rather than a single mechanism, is responsible for the perceived size effect as a function of length scale, beginning by regions dominated by sparse sources where nucleation is dominant, at increasing length scales it is controlled by the ability of dislocations to cross slip, and then the onset of work hardening dominates the effect. The volume of material sampled is also indicative of the ability to isolate the onset of plasticity at near-theoretical strengths, and a statistical approach to the yield point phenomena is linked to demonstrate the length regimes that may be expected to control the deformation mechanisms is presented.

The compression strength of a brittle material influences its ballistic performance, but measuring the intrinsic compression strength is very difficult because the effects of the loading fixture can cause stress concentrations leading to premature failure. Often the compression strength is inferred from hardness values but this is not not appropriate fro ceramic materials. Additionally, the compression strength is an input parameter in numerous modeling and simulation packages. Dumbell-shaped specimens of boron carbide were machined to induce failure from within the gage section while minimizing the stress concentration at the fillets. Quasi-static and dynamic tests were conducted and the compression strength was measured. Quasi-static experiments were performed using a screw driven load frame and recording the fracture with a high speed camera, and dynamic experiments were performed using a split-Hopkinson pressure bar setup and failure was recorded using an ultra-high speed camera. At both strain rates the compression strength was 6.1 GPa but the strength at the dynamic rate had a larger standard deviation. Approximately half the samples failed prematurely via axial cracking so future testing will confine the ends of the sample to reduce this. A fragmentation analysis has been performed on samples from both strain rates.

The mechanical response of many low symmetry HCP materials such as titanium are known to respond through both plastic mechanisms of dislocation slip and deformation twinning. The twinning response to differing loading conditions is the subject of this study. The twinning process is known to be one of nucleation and growth. Grain boundaries play a prominent role in the nucleation of specific twins. Growth of twins requires the motion of twin boundaries and is believed to be dominated by the motion of partial dislocations comprising those boundaries for each twin variant. Although nucleation models exist based upon the hypothesis of the grain boundary role in twin nucleation. Twin growth models are presently limited to phenomenological representation. A series of experiments have been performed on polycrystalline high-purity titanium under three different loading rates (0.1 /s, 1.0 /s, 1000 /s), strain levels (0.01, 0.02), and three different principal plate directions. The purpose of these experiments were to articulate relationships between these variables and the grains and grain boundaries. These results will be presented in that context. New models for the representation of twin nucleation and growth with be discussed based upon the experimental results presented.

Predicting the dynamic properties of metals requires an understanding of how defects evolve in samples deforming at high strain rates. More specifically one needs to know how the rates of nucleation and propagation vary as a function of strain rate for dislocations and twins, as well as how their propagation within grains and across grain boundaries changes with rate. Since current in situ straining TEM techniques are limited to low or quasi-static strain rates, a high-rate, in situ straining technique is desired. Here we present such a technique using novel specimens, and a new straining stage that enables strain rates over 4x10^3/s, and the Dynamic TEM at the Lawrence Livermore National Laboratory. During straining we record 9 frames with inter-frame delays ranging from 50ns to 5us. The samples are prepared from thick films of pure copper and magnesium alloys using femtosecond laser machining and ion milling, and the new TEM holder uses two piezo-electric actuators working in bending. Nearly identical samples have been strained at quasi-static and high rates so that the impact of strain rate can be assessed directly and changes in dislocation and twin evolution will be reported.

Although nanoscale twinning is an effective means to enhance yield strength and tensile ductility in metals, nanotwinned metals generally fail well below their theoretical strength limit due to heterogeneous dislocation nucleation from boundaries or surface imperfections. Here we show that Au nanowires containing angstrom-scaled twins (0.7 nm in thickness) exhibit tensile strengths up to 3.12 GPa, near the ideal limit, with a remarkable ductile-to-brittle transition with decreasing twin size. This is opposite to the behaviour of metallic nanowires with lower-density twins reported thus far. Ultrahigh-density twins (twin thickness<2.8 nm) are shown to give rise to homogeneous dislocation nucleation and plastic shear localization, contrasting with the heterogeneous slip mechanism observed in single crystalline or low-density-twinned nanowires. The twin size dependent dislocation nucleation and deformation represent a new type of size effect distinct from the sample size effects described previously.

Dislocation evolution and mode transitions in tantalum compressed at pressures above 50 GPa under hydrostatic using diamond anvil cell and dynamic-pressure conditions using gas-gun impact and laser-shock techniques have been studied using transmission electron microscopy (TEM) technique. Results generated from tantalum compressed up to 90 GPa will be presented to reveal that mode transitions from dislocation glide to shear transformations (i.e., twinning and omega transformation) tend to occur in tantalum that was shocked under dynamic-pressure condition. It is accordingly proposed that mode transitions occur in shocked tantalum so as to accommodate insufficient dislocation flux arising from the exhaustion of dislocation sources when dynamic-recovery reactions for dislocation annihilation and cell formation are largely suppressed. The exhaustion of dislocation sources takes place when the stress required for the dislocation multiplication exceeds the threshold stresses required for the shear transformations; its effect on dynamic fracture will also be discussed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Advanced structural alloys, such as Mg, Ti, Mo and W alloys, have special mechanical properties (lightweight, refractory, etc.) but suffer from the disadvantages like low strength/ductility/toughness or mechanical performance degradation due to environmental effects. These mechanical problems are largely due to the intrinsic properties of interatomic interactions experienced in their lattice structures. For example, alloys with bcc or hcp lattice usually lack easy-access plastic deformation systems compared with those with close-packed fcc lattice, resulting in brittle deformation and poor formability. Lattice instability, which describes how the lattice structure under the variation of external conditions (such as extreme stress, etc.) loses its stability and transforms into other structures due to small disturbance, is critical to determine these intrinsic properties. First principles calculations will be applied to investigate lattice instability and its effects on the nucleation of deformation defects in these advanced alloys. Two examples will be investigated: the elastic and phonon instability in bcc alloys and their effects on the competition between the fracture and shear deformation; the elastic instability and ideal strength in hcp alloys and its effects on the nucleation of deformation twinning. The results will provide physical insights on how lattice instability and its dependence on chemical compositions determine the macroscopic mechanical properties.

Dislocation cores control a number of properties and in particular the dislocation mobility, which in turn is central to the time-dependent response of the material under stress. The modeling of dislocations and of their mobility using ab initio density functional theory calculations has made tremendous progress these past few years. In this talk, we will review some of these advances in body centered cubic (BCC) and hexagonal close packed (HCP) metals. In particular, we will discuss in BCC transition metals our new understanding of the screw dislocation two-dimensional Peierls potential and its close connection to the well-known deviations from Schmid law. In HCP metals, we will show how DFT calculations identified stable and metastable cores for the screw dislocation, how these cores are related to pyramidal secondary slip and and how an inversion of their stability between Zr and Ti explains the profoundly different plastic behaviors exhibited by these metals.

HCP materials like magnesium (Mg) and titanium (Ti) and their alloys due to their high strength to weight ratio are promising candidates for next generation armor and structural materials. The successful design of better impact resistant materials relies on a fundamental understanding of deformation and failure behavior of these materials under conditions of high strain rate and shock loading. The mechanical response of HCP metals to impact loading conditions is determined by the nucleation and interaction of different defect structures (twins, stacking faults, dislocations etc.). Of particular importance is the role of twinning in the deformation response of HCP metals due to their complex anisotropic crystal structures and lack of well-defined slip planes.
Large scale molecular dynamics (MD) simulations have been carried out to investigate the evolution of defect structures during high strain rate deformation of nanocrystalline Mg systems with grain sizes of 50 nm and 100 nm. A new method is developed that enables the characterization and the investigation of the nucleation and evolution of various types of defect structures including the twin faults and stacking faults in HCP metals. MD simulations suggest that the deformation behavior of nanocrystalline Mg occurs primarily through the nucleation of tension twins as well as compression twins at the grain boundaries. The nucleation and evolution of these twins in various grains is observed to depend on the grain orientation (Schmid factor) and grain size of the metal. The mechanisms for the nucleation as well as the evolution of twins will be presented. While MD simulations can provide insights in the nucleation and evolution mechanisms of twins, the capability of these simulations is limited to system sizes of up to a few hundred nanometers and time scales of a few hundred picoseconds. The ongoing efforts in the applicability of the quasi- coarse-grained dynamics (QCGD) method to extend the time and/or length scales of current MD simulations to predict dynamic failure behavior of HCP systems at the mesoscales will also be presented.

Resistance to creep deformation is essential for metallic materials used in high temperature applications like Ni-base superalloys. Deformation during dislocation creep involves two classes of processes, displacive and diffusive, i.e., dislocation glide and dislocation climb over obstacles, respectively. Coupled displacive-diffusive processes are particularly difficult to study using classical Molecular Dynamics (MD) techniques, due to the short timescales typically achievable by the simulation method which precludes the study of long-range diffusion. Other simulation techniques such as the nudged elastic band technique can be used to calculate the activation energies of specific vacancy jump processes. These are related directly to the rate at which a process occurs, which can then be used as an input parameter for other simulation methods at the atomic scale, such as kinetic Monte Carlo (kMC) simulations. This allows for the description of the evolution of a system in which decoupled diffusion processes are occurring with rate-dependent probabilities, but it cannot be used to reveal new elementary mechanisms.
Using semi-empirical interatomic potentials such as those described by the embedded atom method, molecular dynamics simulations at high temperature allow for the study of the time-evolution of a system in which diffusion occurs. The relevant mechanisms can thus be studied concurrently to deepen our understanding of diffusion processes at the atomic scale. By measuring atomic mean squared displacements over long periods of time, diffusion coefficients for bulk diffusion and diffusion along dislocation cores can be calculated. These diffusivities can then for example be used to parametrize more advanced simulation techniques such as diffusive molecular dynamics, which are better suited to studying mechanisms involving diffusion.
Here we present a combined kMC and MD exploration of the influence of dynamic dislocation motion as a result of the dislocation interacting with lattice phonons through what is called the flutter effect on diffusion along dislocations. This is done in an attempt to combine pipe diffusion models to the vibrating string model describing dislocation oscillations, in an effort to better understand the high-temperature mechanical response of materials under load.

Magnesium has drawn increasing interest as a lightweight material for applications in construction, transportation and aerospace industries. However, its broad application is limited by low strength and poor ductility. Low strength is caused primarily by easy dislocation basal slip, and poor ductility is due to the low activity of non-basal slips, especially the pyramidal slip. Mechanical testing has shown that alloying can strengthen the materials and enhance the activity of non-basal slip. However, the dominating dislocation slip mechanisms under different loading conditions and the associated solute effects are still poorly understood. This understanding is crucial to property prediction, parameterization of constitutive models, and materials design for performance improvement and cost reduction.

We studied the mobility of both edge and screw dislocations on the basal, prismatic, and pyramidal planes in Mg/Al and Mg/Y alloys using atomistic simulation methods with semi-empirical MEAM models. Simple shear was applied at high strain rates (comparable to experimental strain rate of about 10⁴s^-1). Critical resolved shear stress (CRSS) was estimated at temperatures from 0K to 700K, and with solute concentrations from 0 to 7 at.%. Both solute hardening and solute softening were observed and compared favorably with available experimental results. Solute hardening for in-plane basal slip follows Labusch statistics and its thermo-mechanical behavior was consistent with Kocks model.[1] Solute softening was observed for prismatic slip and pyramidal slip, and it is due to out-of-plane dislocation motion like cross-slip or climb. For prismatic slip, solute atoms facilitate the constriction formation during cross-slip, resulting in a lower unlocking stress for yield. For pyramidal slip, solute atoms assist jog formation during the slip to reduce the CRSS. A common feature for prismatic slip and pyramidal slip is the coupling between in-plane dislocation motion and out-of-plane motion, which enables the construction of a 2D energy landscape for the development of thermo-mechanical model based on thermal activation theories. Lastly, double cross-slip and super-jog formation were observed as potential Frank-Read dislocation multiplication sources for experiment interpretation and constitutive modeling.

[1] Yi, P., R.C. Cammarata, and M.L. Falk, Atomistic simulation of solid solution hardening in Mg/Al alloys: Examination of composition scaling and thermo-mechanical relationships. Acta Materialia, 2016. 105: p. 378-389.

The challenge of raising the steam inlet temperature of fossil-fired power plants calls for creep-resistant steels with a Cr content higher than 9% in order to achieve sufficient corrosion and oxidation resistance. However, it has been found that in 11-12% Cr ferritic-martensitic creep resistant steels strengthened by fine (V,Nb)N particles, precipitation of thermodynamically stable Z-phase particles, CrMN (M=V,Nb,Ta), in long-term service is unavoidable and detrimental. Usually, Z-phase particles are coarse and brittle and grow at the expense of the desired fine (V,Nb)N particles. A promising solution to this problem is provided by the idea to exploit the Z-phase as a thermodynamically stable strengthening agent. Hence the challenge is to control the precipitation of the Z-phase such that fine and long-term stable particles are formed.
We present atomistic simulations, using density function theory, which reveal the essential mechanisms underlying the formation of Z-phases. The picture that evolves consists of the diffusion of Cr atoms into MN particles and their subsequent clustering in a layered arrangement which finally yields the transformation of the nitride particles to Z-phase particles. We study the thermodynamic stability of Z-phase, related structures and predecessors as well as the basic phase formation mechanisms.

Aluminum alloys are commonly used in manufacturing of automotive and aircraft components due to their light weight and good overall strength in a variety of extreme environments. To improve the hardness, strength, and corrosion resistance of such metals and maintain their ductility, surface severe plastic deformation (SSPD) techniques such as surface mechanical attrition treatment (SMAT) have been used to create gradient structured (GS) metals where the grain size changes from ultra-fine nanosized grains to micrometer-sized coarse grains gradually. Here we will present our study that probes the microstructural evolution, mechanical response and thermal stability of SMAT-processed aluminum alloys. Specific focus will be placed on strengthening mechanisms (e.g. the competition between grain refinement, residual stress and texture) and the retention of properties after exposure to elevated temperatures. The results presented will facilitate advanced design of other advanced metallic alloys and composites for extreme environments.

While the strength scaling behaviour of micron-sized single crystalline (sxx) specimens is well investigated, controversial results are reported in literature for miniaturized polycrystalline specimens. Attempts on pre-strained as well as ultrafine-grained (ufg) micropillars were made to identify the determining length scales responsible for the observed size effects. Considering a grain size to pillar size ratio, large deviations in the strength scaling behaviour are observed. While for low ratios bulk strengths are obtained, high ratios lead to a more sxx-like deformation behaviour. In this work, we concentrate on the strength scaling behaviour of ufg versus sxx micropillars using in-situ SEM microcompression tests at room and elevated temperature. Pillars with sample sizes ranging from 0.2 to 4 µm were FIB milled to include the influence of the free surface. Furthermore, the strength scaling behaviour of a recovered ufg microstructure was investigated to study the contribution of a varying dislocation density in the ufg condition on the scaling exponent. Finally, these results are compared to high temperature nanoindentation experiments to elucidate influences of the free surface on the strength-scaling behaviour of ufg and sxx bcc metals, as well as rate controlling deformation mechanism evidenced from analysis of strain rate sensitivity and activation volumes.

Magnesium alloy AMX602 (Mg-6%Al-0.5%Mn-2%Ca) that has undergone the Spinning Water Atomization Process (SWAP) followed by extrusion has shown potential in several applications due to its light weight, high strength, moderate ductility, and corrosion resistance; however, it is not clear what the dominant deformation mechanisms are under quasi-static and dynamic loading conditions and as a function of temperature for this fine-grained material. The objective of this work is to characterize the dynamic behavior of AMX602, and understand the dominant deformation mechanisms from initial yield up to moderate strain levels.

In addition to quasi-static experiments, compression split-Hopkinson pressure bar tests were performed at strain rates of ~10³ s^-1 and different temperatures along the three principal material directions to probe the material behavior. To explain the observed behavior, a rate-dependent Sachs analysis based on the experimentally determined texture was used to predict the dominant deformation mechanisms under various loading rates, testing temperatures and directions. Both experiments and simulations suggest that extension twinning is an important deformation mechanism for loading paths where a substantial fraction of the c-axes in the grains undergo extension. This is in contrast to observations by other authors suggesting that small grain sizes suppress extension twinning under quasi-static loading rates. This material possesses moderately anisotropic strain hardening behavior under dynamic loading conditions despite the near isotropy in the initial yield behavior due to differing deformation mechanisms and their associated strain hardening rates. The strain hardening becomes more isotropic as the temperature increases due to the activation of slip mechanisms.

Cu/Nb metallic multilayers with individual layer thicknesses in the range 7 nm to 63 nm were manufactured by accumulated roll bonding. Micropillars of square cross section of 5 x 5 µm² and an aspect ratio 2-3 were milled with a focused ion beam. The mechanical properties of the Cu/Nb multilayers were determined by means of in situ micropillar compression tests within a scanning electron microscope. Test were carried out at different strain rates (10 ^-2 to 10^-4 s^-1) and temperatures (25C to 400C) and the yield strength, strain rate sensitivity and activation volume were determined from these tests for each multilayer as a function of temperature. In addition, the deformation and fracture mechanisms were ascertained from in situ observations during deformation and from transmission electron microscopy analysis of foils extracted from the deformed micropillars.
A transition in the deformation mechanisms from confined layer slip to dislocations crossing the interface was found at ambient temperature when the layer thickness decreased below ≈20 nm and the maximum yield strength was found when interface crossing was dominant. The yield strength decreased and the strain rate sensitivity increased at 400C and the mayor differences (as compared with ambient temperature) were found in the multilayers whose deformation was controlled by dislocations crossing the interface as a result of the activation of diffusion mechanisms. Thus, the optimum mechanical properties at high temperature were obtained for nanolaminates with an intermediate layer thickness, in which confined layer slip was the dominant deformation mechanism at all temperatures.

Between 2006 and 2009, 20% of the screened combat troops had some degree of Traumatic Brain Injury (TBI). Helmets are often considered the first line of defence against TBI and are the primary form of head protection in military applications. Both the helmet shell and liner influence the performance of the helmet under blast and impact conditions. Under blast conditions helmet liners are able to absorb some of the incoming energy and mitigate the blast to a certain degree. Further investigation into helmet liners has the potential to show how performance can be improved particularly under blast conditions. The helmet liner performance is a function of the microstructure and the properties and failure that follow from the microstructure. In order to make progress towards developing improved combat helmets to better address TBI, this project seeks to investigate the relationship between microstructure and material properties in order to understand material failure and performance. In this submission, we will discuss on-going activities on understanding the dynamic failure of rate-dependent foams supplied by leading manufactures. We present performance metrics typically used to categorize foams in blast and impact, make links to dynamic failure processes, mechanical properties, and microstructure obtained through experimentation and characterization. We discuss the implication of our results to the design of new advanced liner materials for improved Traumatic Brain Injury protection.

Nanocrystalline (NC) materials, have exceptional mechanical properties owing to their small grain size. However, this high strength generally comes with dramatic losses in other properties, such as creep resistance, which limits their practical utility. The large volume fraction
of grain boundaries (GBs), are predominantly responsible for these losses and the deformation is said to be controlled by GB based mechanisms such as sliding, diffusion etc. In this study we report for the first time the creep response of NC-materials with unprecedented property combinations, i.e., high strength with extremely high temperature creep resistance in a mechanically and thermally stable NC-alloy. The unusual combination of properties in these alloys is achieved through the pinning of unique Ta based nanoclusters that contributes to the
negligible grain coarsening. Furthermore, this bulk NC-Cu-10at.%Ta alloy that is able to achieve/retain high strength and creep resistance at a high homologous temperature of 0.64 (600 °C) entirely due to the exotic microstructural configuration. The processing is achieved through
high energy ball milling and subsequently consolidated via equal channel angular processing (ECAE), the as processed microstructure consists of an average Cu matrix grain size of 50 ± 17.5nm and a wide dispersion of Ta particle sizes, ranging from atomic nanoclusters to much larger precipitates. The plasticity in these materials is influenced by these Ta nanoclusters. In summary, this study points to a new beginning for innovative fundamental and applied science in designing NC-alloys with a range of exceptional properties. Further, the presented research on NC-Cu-10at.%Ta alloys proves immiscible based systems produced from non-equilibrium processing represent a new generation of materials for several applications that include extreme conditions.

Lightweight steel based composites with density below 6.0 g/cm³ were fabricated by the isothermal melt infiltration process. The microstructure and the interfacial characteristics between the matrix and the reinforcement of a novel high volume fraction TiC particulate reinforced (>55%) SKD11 alloy has been investigated using the scanning electron microscopy and the transmission electron microscopy. The TiC reinforcement with size of 5~10 μm had uniform distribution without defects in the SKD11 matrix. The mechanical testing results indicate that the TiC reinforced composite display superior mechanical properties, such as high hardness (74 HRC), ultimate tensile strength (935 MPa), elastic modulus (369 GPa), high temperature tensile strength at 700^oC (728 MPa), and ultimate compressive strength (3.3 GPa). The tribology test of the composites against WC ball showed lower friction coefficient, smaller wear depth and width compared with the unreinforced matrix or composites which produced by the powder metallurgy method. Increases in mechanical properties are attributed to the effective load transfer from the matrix to the TiC because of the strong interfacial bonding. This interfacial stability results from the partial dissolution of the TiC during the process.

The tensile test is considered as an accepted test for the specification of materials both at ambient and elevated temperatures. Different types of heating techniques are available today in order to attain the desired temperature during elevated temperature tests, but very little effort has been put towards studying the influence of these different heating techniques on the mechanical properties of the materials obtained from these tests. As a result, two classes of experiments were conducted in this study in order to investigate the effect of different heating techniques on the mechanical behaviour of the Ti-6Al-4V (Ti-64) alloy during high temperature tensile tests. In the first class of experiments, the specimens were heated using a Severn Thermal Solution Split high temperature furnace and in the second class of experiments, a state of the art induction heater was used to heat the specimens up to the target temperature (max = 0.9*T_m). In both cases, the specimens were heated at different heating rates to the prescribed temperatures and then stretched until failure under different strain rates. In addition to the effect of strain rates and heating rates on the mechanical behaviour, this study also investigated the influence of specimen heating techniques used on the elastic moduli, yield strength, ultimate tensile strength and fracture strength of the Ti-64 alloy. Metallographic analysis was carried out, which investigated the effect of heating technique, strain rate, heating rate and temperature on the microstructure of the alloy. With induction heating offering the possibility to heat up materials to extremely high temperatures, irrespective of size and shape of the test specimen, the results from this investigation are expected to pave the way towards extending the limits of high temperature materials and its testing.

Many surface chemical reactions occurring at high temperature and pressure depend strongly on nanometer scale structure and composition. Most nanoscale observations of surface reactions rely on far field measurement of scattering events from the surface, placing significant limitations on the temperatures, pressures, and environmental compositions of studied reactions. There is a need to develop techniques to study nanoscale surface chemical reactions compatible with high temperature, high pressure, and arbitrary environmental composition to understand interfacial processes in real world environments. Here we have developed a scanning tip technique to measure the chemical kinetics and thermodynamics of surface desorption with nanometer scale resolution for temperatures up to 1000 K and surface stresses exceeding 1 GPa. Oxygen removal from graphene was driven and continually measured as a function of applied temperature, load and time by constantly monitoring the evolving friction change during the reaction. Rates obtained from several isothermal reactions followed first order kinetics with an activation energy of 0.7 ± 0.3 eV. To reduce the error in activation energy and to determine the reaction order directly from data, we developed a nanoscale thermal desorption microscopy (ThDM) technique, in which the tip temperature or force can be independently ramped, enabling single scan reaction rate measurement. Non-isothermal desorption revealed an activation energy of 0.62 ± 0.07 eV and a reaction order of 1, providing more accurate results consistent with isothermal method and 10-100x faster measurement. Modifying the applied load shifted the energy barrier non-linearly from 0.67 to 0.21 eV and the rate of decrease of activation energy diminished with increasing load, indicating a reduced susceptibility of oxygen removal at high loads. To assess the stress dependence of activation energy further, oxygen removal was monitored with increasing force for various tip temperatures. Applying non-linear mechanochemical models revealed that the mechanical compliance of the molecular system decreases with applied load, further supporting the strengthening of oxygen groups with increasing applied loads. Thus, we showed that heated AFM tips can simultaneously drive and measure chemical reactions at high temperature and pressure with quantitative determination of fundamental thermodynamic constants and reaction kinetics in ambient environments, and can interrogate reaction pathways not possible with bulk techniques. Such comprehensive understanding of surface reactions driven by multi-physical sources is crucial for development of more efficient chemical reactors, fuel cells, batteries and supercapacitors, flexible electronics, lubricants and biomaterials.

Cold spray (CS), an additive manufacturing process utilizing extreme plastic deformation arising from high-speed collision events between fast-moving powders and substrates, has been highlighted as a unique manufacturing and repair solution for high-performance mechanical parts. Although many metals and alloys have been successfully processed using the CS techniques, the accurate dynamic responses of individual metallic particles related to the consolidation and deformation characteristics are still largely unknown. Although various numerical approaches have been expected to facilitate fundamental understanding of the extreme deformation mechanisms, their results are still limited due to the lack of referenceable experimental results with precise experimental parameters.
We conducted the laser induced single particle impact experiments to study the extreme dynamics of aluminum 6061 particles during individual impacts, and to provide precisely defined critical parameters for computational analysis. Single aluminum particles around 20 μm in diameter were accelerated up to 1 km/s using high-power laser ablation. Light pulses, generated by a femtosecond oscillator were used to take ultra-high-speed photographs (maximum 80 million frames per second) and accurate kinetic information of the impacting aluminum particle was acquired from the photographs.
Two different target substrates, sapphire and aluminum 6061, were used to study the deformation of the impacting particles. Since the entire plastic deformation occurred solely on aluminum spheres due to the high modulus of sapphire, aluminum-sapphire collisions provided a very simple environment for numerical simulations and reference data for the more complicated aluminum-aluminum collisions. As an in-situ characterization, we measured the rebound speed of aluminum particles as a function of its impact speed. The coefficient of restitution, which depends on the impact speed, evidently showed two different transition points related to the high-strain-rate and hydrodynamic regimes in aluminum deformation. In the aluminum-aluminum case, one more transition point around 860 m/s was the critical velocity, over which no rebounding particle was observed. The xenon-plasma focused ion beam cross sectioning of deformed aluminum particles was performed without critical gallium contamination and the electron diffraction backscatter diffraction mapping of the cross sections provide the entire deformation field depending on the collision speeds. Moreover, cross sectional images allow us to investigate the microstructural changes of the particle and the target under high-speed impact so as to study the phenomena that are responsible for the dynamic consolidation.

Mechanical stress has been widely used as a tool to tune materials properties and investigate fundamental phenomena of materials under extreme conditions. One of the most exciting aspects in semiconductor research is using strain to engineer the electronic, optical and optoelectronic properties of materials such that device functionalities and performance can be further expanded and improved. Silicon, as the most important semiconductors in modern technology, has been widely studied under mechanical stress. It is known that uniaxial and biaxial tensile stresses can widen the absorption band of silicon and improve its carrier transport [1]. Such stress states have been achieved by either epitaxial growth of silicon on a substrate with larger lattice parameters [2] or mechanically stretching a silicon nanowire [3]. But tensile stress (negative pressure) in all directions have not been reported or achievable for silicon. Here, we present novel studies of an unusual state of negative-pressure developed in a single crystalline silicon microwire developed from the thermal mismatch with a surrounding glass capillary. [4]

The single crystal silicon wire was created by laser heating an amorphous silicon wire deposited in a 1.7 μm silica glass capillary by high pressure chemical vapor deposition [5]. The full strain tensor of the microwire was investigated using the synchrotron X-ray micro-beam Laue diffraction (μ-Laue) with additional energy-scanned monochromatic X-ray measurements. The results show that the three eigenstrains are +0.47% (corresponding tensile stress is +0.7 GPa) along the fiber axis and +0.02% (corresponding tensile stress is +0.3 GPa) in the cross-sectional plane. Finite element modeling indicates that the strain originates from the thermal expansion coefficients mismatch between the silicon and silica. Although the magnitude is small, this effect is already able to alter the band gap energy of silicon by -30 meV as predicted by density functional theory (DFT) calculations and was further confirmed through photoconductivity measurement. We note that, experimentally, this type of strain/stress state is entirely new for silicon and it could provide useful views for researchers who are trying to explore new phases of silicon [6] through deformations.



This work is supported by the Penn State NSF-MRSEC Center for Nanoscale Science.



References

1. Yang et al. Semicond. Sci. Technol. 19, 1174-1182 (2004)
2. Wolf, Semicond. Sci. Technol. 11, 139-154 (1996)
3. He et al. Nat. Nanotechnology, 1, 42-46 (2006)
4. Ji, X. et al. Appl. Phys. Lett., submitted (2016)
5. Baril et al. Adv. Mater. 22, 4605-4611 (2010)
6. Zhang et al. Appl. Phys. Lett. 97, 121906 (2010)

Electronics generally are not specifically designed to perform in extremely transient high impact scenarios. This research focused on the development of silver-decorated carbon black-based polymeric nanocomposites with properties of high conductivity, flexibility and shock absorption. The processing and fabrication of Ag-CB (silver-carbon black)/Epoxy (thermosetting epoxy polymer) and Ag-CB/TPU (thermoplastic polyurethane) will be discussed. Both Ag-CB/Epoxy and Ag-CB/TPU mixtures with solvents showed shear-thinning behavior, which is an important characteristic for direct printing of traces and additive manufacturing. The mechanical properties of the nanocomposites were measured using dynamic mechanical analysis (DMA). The morphology of the nanocomposite was investigated by TEM; Ag-coated carbon blacks or silver nanoparticles were well-connected to form the network for excellent electrical conductivity. These nanocomposite materials were also successfully used to print flexible circuits using a 3D-printing machine. The electrical resistance changes for the
Ag-CB/Epoxy on PDMS, Ag- CB/TPU on PDMS, and Ag-CB/TPU on PET under strain were studied. The r e s u l t s indicate the promise of these highly conductive polymer nanocomposites as an alternative solution for electronic materials under high impact scenarios.

Ab initio local-energy and local-stress schemes [1, 2] have been applied to first-principles tensile tests (FPTTs) [3] of Σ9 tilt grain boundaries (GBs) in Al and Cu, where the variations of local energies and local hydrostatic stresses of all the atoms have been clarified during the deformation and failure processes. In the FPTTs, the GBs in Al and Cu showed quite different behaviors in spite of similar initial configurations. For the Al GB, there are two stages of deformation before the failure. In the first stage, the back bonds of the interfacial bonds are mainly stretched, due to special high strength of the interfacial bonds. In the second stage, the interfacial bonds start to be greatly stretched due to highly concentrated stresses, while the stretching of the back bonds is suppressed. The atoms at the interfacial, back and bulk bonds show quite different variations of local energies and local stresses in each stage, because the behavior of each atom greatly depends on the evolution of each local structure, due to high sensitivity of Al sp electrons on local environment. The Cu GB has much higher tensile strength, while there occurs natural introduction of stacking faults (SFs) via the {111}<112> shear slip in the bulk regions between the interfaces before the maximum tensile stress. This is caused by the smaller SF energy or lower ideal shear strength of Cu, while this is triggered by highly accumulated local energies and stresses at the interface atoms. After the SF introduction, the local energies and stresses of all the Cu atoms show tendency to become similar to each other during the tensile process in contrast to the inhomogeneity in the Al GB. The origins of different tensile behaviors between the Al and Cu GBs are discussed from different bonding nature, dominated by sp electrons in Al and d bands and s electrons in Cu. We can conclude that the local-energy and local-stress schemes are quite effective in the analysis of FPTTs of defective systems.
[1] Y. Shiihara, M. Kohyama and S. Ishibashi, Phys. Rev. B 81 (2010) 075441.
[2] H. Wang, M. Kohyama, S. Tanaka and Y. Shiihara, J. Phys. Condens. Matter 25 (2013) 305006.
[3] S. Ogata, Y. Umeno and M. Kohyama, Modell. Simul. Mater. Sci. Eng. 17 (2009) 013001.

In the cold spray process, powder feedstocks are accelerated to supersonic velocities by high pressure gas jets. Above some material-specific critical velocity, the particles will bond to the substrate upon impact. Bonding in cold spray is usually attributed to an adiabatic shear instability that occurs at the particle/substrate interface. This is thought to be a competitive process between thermal softening and rate effects together with work hardening. The net effect is to produce localized interfacial jetting with highly localized shear near the interface, and more generalized deformation of the particle away from the interface. The microstructural effects that occur during cold-spray of metallic alloys are of great interest because they give an insight into both the particle/substrate bonding and the origins of the mechanical behavior in the resulting deposit. It is difficult to draw general conclusions about these effects because the deformation mechanisms and the resultant microstructural details vary significantly between different classes of alloys.
In our work we have used a combination of multi-scale modelling and experimental approaches to investigate the cold spray of precipitation-hardened Al alloys from gas atomized powder feedstocks. This is part of a major multi-institution ARL-funded program whose aim is to develop robust processing-microstructure-property models for cold-sprayed Al alloys. Here we will present a summary of our characterization studies on single-pass cold spray trials of Al 6061 onto substrates of the same alloy in a standard microstructural condition. A combination of scanning electron microscopy (SEM), focused ion beam (FIB) sectioning, and transmission electron microscopy (TEM) studies has been used to investigate the microstructural features in the feedstock powder particles and in individual powder splats. These data are used to reveal the nature of the microstructural development in the earliest stages of cold spray deposition. It is shown that the silicide and iron-bearing phases adopt a characteristic distribution in the powder, and that these act as internal markers to reveal the localization of shear within the splats. The absence of these phases in the redeposited jetted material, and the adoption of characteristic columnar grain structures in such regions, is used to show that localized melting is occurring at the interface under the deposition conditions used in this study.

The cold spray process utilizes a high pressure gas jet to accelerate metallic particles to supersonic velocities. Upon impact with a surface, these particles will bond to the substrate above the material’s critical velocity. The collision causes extremely severe plastic deformation, which results in non-equilibrium microstructures. Therefore, the mechanical characterization of these impacted particles is of great interest because the unusual microstructure leads to unique deformation mechanisms, which is related to the intrinsic mechanical properties of cold sprayed coating.

In our work we have used a combination of multi-scale modelling and experimental approaches to investigate the cold spray of precipitation-hardened Al alloys from gas atomized powder feedstocks. This is part of a major multi-institution ARL-funded program whose aim is to develop robust processing-microstructure-property models for cold-sprayed Al alloys. Here we will present a summary of our micro-mechanical studies on single-pass cold spray trials of Al 6061 onto substrates of the same alloy with the velocity of ~1 km/s. The mechanical properties of these splats has been studied by utilizing Ga ion focused ion beam to machine micro-pillars with various diameters from individual splats. These pillars were then deformed by micro-compression in situ in SEM using a purpose-built deformation stage. We found a strong size effect in the submicron diameter range, but negligible size effect in pillars above 1 um in diameter. We explain these two distinct size effect regimes in terms of microstructural length scales such as grain/dendrite size, precipitation, and dislocation densities. We compared these findings with the micro-mechanical properties of undeformed single Al alloy particles. This study will give an insight in fundamental understanding in high strain rate deformation mechanisms during the cold spray process.

Mechanical strength of solid solution strengthened bcc alloys is significantly affected by solute concentration and strain rate and temperature even in dilute alloys. Since these effects on mechanical strength are highly non-linear, theoretical prediction of the mechanical strength of alloys is still challenging task. In this study, we developed atomistically informed kinetic Monte Carlo (kMC) model for screw dislocation motion in bcc substitutional alloy. Frequencies / activation energies of all the possible events contribute to the dislocation motion, such as kink nucleation and migration, are computed by NEB using atomistic model, and these activation energies are applied to the kMC model. By using developed kMC model, we computed yield stress as a function of the solute concentration, strain rate and temperature. The obtained yield stresses agree well with experiments. We also investigate the solute atom effect on dislocation velocities and dislocation structure change during its motion, e.g. cross-slip, jog formation, and non-Schmid behavior.

Diffusion aluminide bond coats are compositionally and microstructurally graded materials with significant variation in engineered mechanical properties across the cross-section, utilized for high-temperature protection of superalloys. These bond coatings exhibit three-layered microstructures: Outer layer contains intermetallic PtAl₂ and Cr-rich fine precipitates, Intermediate layer contains B2-(Ni,Pt)Al and Inner layer is interdiffusion zone containing coarse precipitates in B2-NiAl matrix. This study focuses on understanding the variation in mechanical properties and deformation mechanisms as a function of temperature. An in-situ SEM nanomechanical instrument, PI 87xR SEM PicoIndenter with an integrated high-temperature stage and an active tip heating was used to conduct uniaxial compression of pillar samples. Using the displacement-controlled feedback mode of the system, the pillars were compressed to 5-12% strain at a strain rate of 10^-2 s^-1 at room temperature (RT) as well as several elevated temperatures up to 800°C.
The stress-strain curves from the tests indicate that plastic response is characterized by major strain hardening at room temperature and limited strain hardening at higher temperature. The surface of the bond coating pillars shows evidence of grain boundary sliding at higher temperature. Elastic moduli of the bond coating remain nearly constant at RT to 800^oC whereas yield stress of the coating decreases to ~50%. Transgranular fracture appears on the pillar surface at higher strain at room temperature whereas intergranular fracture dominates deformation at higher temperature.

The cold spray process involves acceleration of metal particles at supersonic velocities on to a metal substrate. The impact results in severe plastic deformation of the particle and the substrate, and in-turn, bonding of the particle to the substrate. This impact-induced bonding of metal particles forms the basis of the process for coating, repair and additive manufacturing technologies. The deformation response of the particle and substrate under these impact conditions is very complex and involves defect nucleation and evolution as well as heat generation and transfer. A critical challenge in the development and optimization of the cold spray process is the current understanding of the dynamic evolution of the microstructure during particle impact. The short time scales (microseconds) and length scales (tens of microns) involved make it extremely challenging to characterize these mechanisms using experiments. While molecular dynamics (MD) simulations allow the investigation of these phenomena at the atomic scales, their applicability is limited to system sizes that are a few hundred nanometers and time scales of a few hundred picoseconds.

To meet this challenge, a novel mesoscopic model, called quasi-coarse-grained dynamics (QCGD), is used that extends the time and length scale capabilities of MD simulations to the mesoscales. The QCGD method is based on solving the equations of motion for a chosen set of representative atoms from an atomistic microstructure and retaining the energetics of these atoms using scaling relationships for the atomic scale interatomic potentials as would be predicted in MD simulations. These scaling relationships in the QCGD method allow the modeling of impact of Al particles with sizes up to tens of microns on to a metal substrate and enable the investigation of the dynamic evolution of microstructure during impact. The scaling relationships, and the effect of particle size, temperature of the particle and impact velocity on the predicted microstructural evolution will be presented. This effort is combined with experimental approaches to investigate the cold spray of precipitation-hardened Al alloys from gas atomized powder feedstocks. This is part of a major multi-institution ARL-funded program with an aim to develop robust processing-microstructure-property models for cold-sprayed Al alloys.

Fracture toughness and strength are known to be mutually exclusive mechanical properties of a material -- most materials with higher toughness have low strength (such as aluminum), whereas materials with higher strengths have low toughness (such as glass or ceramics). In order to design materials with novel functionality, it is critical to enable engineering toughness and strength individually and collectively in a controlled manner. To achieve such mechanistic goals, we exploit elasto-geometric heterogeneity and tailor the macroscopic toughness-strength correlation in carbon nanotube reinforced glass matrix by imparting hierarchical architectures. To carry out this study and capture the mechanical behavior of various architectures across different length scales, we develop an integrated approach by combining molecular dynamics and density functional theory simulations. Upon generating a set of geometrically identifiable nanotube configurations (that are triangular, rectangular and pentagonal in arrangement), toughness-strength correlation is investigated with and without the presence of nanoscale defects such as nanopores or nanocracks. The results show that - in certain patterns - the positions and the number of carbon nanotubes can extraordinarily change the fracture strength, stiffness and toughness. For crack propagation in the medium, it is found that two identical but asymmetric configurations can have equal fracture strength and stiffness but dissimilar toughness. Secondly, for a given volume fraction of the nanotube reinforcement, various geometric configurations with distinct symmetries exhibit configuration dependent toughness. We find that as the rotational symmetry operations increase in the assembly of nanotubes, their macroscopic toughness becomes rotationally invariant. Interestingly their correlative effects are predictable as a function of the geometric parameters. Subsequently, for a given number of nanotubes with specified chiralities, we can predict the architecture that optimizes the macroscopic toughness, strength and stiffness. The talk will discuss these results and prescribe possible pathways for controlling the correlation among stiffness, strength and toughness.

The technological importance of silicon has motivated many researchers to study the high- pressure phases for the last few decades. Presence of hydrostatic and deviatoric stresses under an indenter makes nanoindentation an ideal tool to study pressure induced phase transformation in silicon. Combination of Raman spectroscopy with nanoindentation gives more insight into the phase transition pathways. In this study a unique experimental configuration was adopted, where nanoindenter was coupled with a Raman spectrometer and real time Raman spectra was collected during indentation. Silicon undergoes diamond cubic structure to β-Sn phase under loading (at 10-12.5 GPa) and during pressure release β-Sn transforms to BC8 and R8 phases and sometimes amorphous silicon depending on the unloading rate. In this study in situ and ex situ Raman map’s generated from the contact region shed lights on the residual phase distribution. The effect of loading rate and holding time on the distribution of end phases were studied in detail.

A monolayer graphene can intrinsically ripple even at 0 K, offering a new route of engineering the properties of graphene via rippling. In this study, we investigate the effects of crumpling on fracture behaviors of graphene employing molecular dynamics simulations. We showed that the crumpled graphene sheet can be produced by introducing defects such as di-vacancies and dislocations, with the crumpling configuration manipulated by varying the concentration and spatial distribution of defects. We found that crumpling can enhance the fracture toughness of graphene for both mode-I and mode-II loading. This toughening phenomenon is shown to originate from crumpling-induced smearing of stress intensity at the crack tip, which is further complemented by crumpling-induced alternation of the effective Young’s modulus and shear modulus of graphene. Our findings suggest a new avenue on defect engineering of graphene.

Materials at temperatures higher than 40% of the melting point, start to be affected by creep. Self-healing creep steels are a promising novel concept to improve the creep life of steels for high-temperature applications. Experimental work has not only proved the possibility of utilizing the concept to extend life-times of creep alloys, but they have also helped to understand the mechanisms governing creep better [1]. Diffusivity calculation performed using ab-initio techniques are combined with finite-element modelling to model the healing of creep voids. We performed ab-initio calculations to obtain prefactors and activation energies for diffusion in bcc-iron.
In a finite-element model, Fick’s laws for diffusion have been used to obtain the solute flux in the direction of a void on a grain boundary, based on a solute concentration gradient. The diffusivities in the grain boundary and in the bulk, solubility of the solute and possible supersaturation and the geometries of the problem have been varied in order to discern the relative contributions of each of these parameters. The size of a void affects the fraction of neighbouring grain boundary area compared to bulk and this affects the relative importance of grain boundary and bulk diffusivity. A critical ratio between grain boundary and bulk diffusivity contributions is found to control the relative importance of grain boundary and bulk diffusion.
The amount of supersaturated solute present, combined with the diffusivities in bulk and on grain boundaries are vital for the healing rate of a void on a grain boundary. A comparison is made with other creep void growth models, which deal with diffusional void growth and void growth through lattice strain. Void growth models for the most part ignore the influence of solute diffusion towards a void on the void growth rate is not considered. Especially for supersaturated solutes effect can be significant and directly affects the strain-rate for creep and the time to failure. This new insight clarifies diffusional creep and the self-healing mechanism in creep steels.

[1] S. Zhang, C. Kwakernaak, W. G. Sloof, E. Brück, S. van der Zwaag, N.H. van Dijk,, Adv. Eng. Mater. 17 (2015) 598

Boron carbide is a common component of both personal and vehicle composite anti-ballistic armor plating. Plate samples have been well-studied from a ballistics perspective, but further energy dissipation can be achieved by fragment failure. In place of studying penetration response in-situ, a fundamental study was performed on boron carbide powder, first sieved between 125 and 150 um, then tested via the thick-walled cylinder (TWC) technique. These experiments were aimed to determine the effect of high strain rate coupled shear and shock loading on the powder, as well as to establish the granular response and damage under two distinct loading conditions (global strains). Recovered samples were analyzed and characterized via optical and scanning electron microscopy. X-ray computed tomography scans were used in conjunction with steel tracer particles mixed in with the boron carbide powder to capture particle motion over the domain of the experiment, and then compared to detailed mesoscale simulations. The characterizations of boron carbide powder were compared to simulations and experimental geometries to determine dominant granular failure mechanisms.

Nanocrystalline metals, due to their high strength, stiffness, and wear resistance, are an emerging class of materials for applications in the next-generation damage-tolerant structural materials. The applicability of these materials, however, is limited by significant coarsening of the grains at high temperatures. Recent efforts to introduce second phase solute species to pin and stabilize the grain boundaries have shown significant promise in enhancing the thermal stability of nanocrystalline metals. For example, Ta has proven to be an effective stabilizing solute, resulting in suppressed grain growth as well as enhanced mechanical strength of nanocrystalline Cu system. The enhanced mechanical strength of these solute-strengthened Cu alloys may open up the possibility of unprecedented performance improvements in the design of damage-tolerant materials. Design and optimization of these Cu-Ta alloys can be significantly accelerated by improvements in the understanding of the deformation and failure mechanisms under extreme conditions of shock loading. In this study, large scale molecular dynamics (MD) simulations are performed to investigate the deformation and failure behavior of Ta solute strengthened bulk nanocrystalline Cu systems under shock loading conditions. The MD simulations suggest that the presence of Ta affects the evolution of defects (dislocations, faults) during shock compression as well as void nucleation, growth and failure (spallation). The spall strength of the Cu-Ta system is found to be determined by the concentration of Ta as solute atoms as well as by the distribution of Ta in the microstructure. In addition, the simulations suggest that the strengthening of the nanocrystalline Cu is more pronounced at larger grain sizes. The evolution of defects and the variations of the spall strength for varying Cu-Ta microstructures will be presented.

In a variety of applications ranging from surface finishing to additive manufacturing by cold spray, micro-particles impact a substrate at high velocities. In this work we aim to study the unit process of such applications, namely, single-particle impacts studied in real time. We employ an in-house-designed setup to accelerate single micrometer-sized metallic particles to supersonic velocities and have them impact a metallic substrate with a strain rate as high as 10⁹ s^-1. Using a high-frame-rate camera with a time resolution as short as 3 ns, we track individual powder particles and observe the impact and deformation in real time. This capability enables us to directly observe that, if the particle impact velocity exceeds a material-dependent threshold, the particle tends to show an impact-induced instability and adheres to the substrate. We have directly measured this critical adhesion velocity for Al, Cu, Ni and Zn particles. We also model this process, using a finite element approach to simulate the impact behavior of particles in order to unravel the critical physics leading to adhesion. Together the numerical and experimental results speak to a physics-based interpretation of critical adhesion velocity that spans different metals, powder particle sizes and temperatures. Our single-particle-level explorations of supersonic metal-on-metal impact provides us with the fundamental understanding of the unit process of a prospective impact based additive manufacturing technology.

Extreme materials science related to ballistic impacts are not only critical for defense applications but are also demanded for civilian applications such as cold spray techniques utilizing the kinetic consolidation of metal powders. Because the kinetic consolidation is based on continuous supersonic collisions of single micrometer-size metal particles to a desired target substrate, strong material nonlinearity and multiple deformation mechanisms are largely inter-coupled. Therefore, precise understanding of the kinetic consolidation has been a challenge due to the lack of a proper experimental method to simulate the microscopic supersonic collision event and corresponding numerical modeling. Here, we report precisely controlled single aluminum 6061 (Al) microsphere collisions at 50-1,000 m/s with both in situ and postmortem characterizations.
We performed highly-controlled single micro-sphere impact experiments, in which a single polycrystalline Al spherical particle was accelerated to a high velocity using the laser induced projectile impact test method. The size of each micro-sphere was measured prior to the impact experiment and an ultrafast optical microscopy system utilizing a femtosecond laser was used to quantify sphere’s impact and rebound speeds, v_i and v_r. By employing rigid (sapphire or Al₂O₃) and deformable (Al) target substrates, we created two different experimental environments, one-body and two-body plastic deformation cases, respectively. We observed several characteristic transition points in the trends of v_r(v_i) and the coefficients of restitution, C_r (v_i), which were related to high-strain-rate responses of Al spheres and their bonding to the substrate. Post-impact dimensions, as a function of v_i, indicated that the Al particle’s plastic deformation was created under very high strain rates order of 10⁷ /s. The deformed particles were cross-sectioned using xenon plasma focused ion beam milling without gallium contamination. The high resolution cross-sectional electron micrographs and the electron diffraction backscatter diffraction mapping showed localized densification, extreme grain deformation, plastic flows across the grain boundaries, and bonding mechanisms. Since our experimental method significantly reduces the uncertainty in critical experimental parameters including the particle mass, impact speed, and impact temperature, demonstrated results provide the validation data to improve the accuracy of various numerical models.

The cold spray process involves the acceleration of powder material to supersonic velocities using a high pressure gas jet. When these particles impinge upon a substrate, they undergo severe plastic deformation. Under appropriate spraying conditions, the deformed particles will adhere to the substrate surface, and thus cold spray can form the basis for coating, repair and additive manufacturing technologies. Unlike the vast majority of spray techniques, the powders do not melt at any stage during the process. This has distinct advantages including: the ability to form and retain ultra-fine grained structures in the deposited material; the preservation of the original powder chemistry with no preferential losses due to vapor pressure differences etc.; and the ability to deposit novel materials such as composites, metallic glasses and other non-equilibrium phases directly from appropriate powder feedstocks.
While the cold spray process has now been developed into a viable commercial technique for certain applications, there are still significant gaps in our understanding of the microscopic processes that occur during particle impact. In our work we have used a combination of multi-scale modelling and experimental approaches to investigate the cold spray of precipitation-hardened Al alloys from gas atomized powder feedstocks. This is part of a major multi-institution ARL-funded program whose aim is to develop robust processing-microstructure-property models for cold-sprayed Al alloys. Our computational approach has been to use quasi-coarse-grained dynamics (QCGD) to extend molecular dynamics methodologies to relevant length scales by treating a sub-set of representative atoms. We show here that the QCGD approach enables us to model the deformation behavior and microstructural evolution during particle impacts on realistic length scales while keeping the computational resources needed within reasonable limits. The outputs of these models have been correlated with experimental observations on single-pass cold spray trials of Al 6061 onto substrates of the same alloy in a standard microstructural condition. A combination of scanning electron microscopy (SEM), focused ion beam (FIB) sectioning and transmission electron microscopy (TEM) studies has been used to reveal the microstructural features associated with individual powder splats. The mechanical properties of these splats has been studied by using FIB to machine micro-pillars from individual splats, and then deforming these pillars in situ in the SEM using a purpose-built deformation stage. The deformation behavior is then related to the operative micro-mechanisms by performing TEM studies on the deformed pillars. The contribution of these computational and experimental studies to our understanding of the physical metallurgy for cold spray of Al alloys will be discussed.

Impact and shock compression have long been used to modify the surface mechanical properties of metals, for example, in shot peening and laser shock peening processes. Recently, it has been shown that a gradient-nano-grained (GNG) structure generated in surface mechanical grinding improves the fatigue properties of metals through progressive yielding. In this study, we demonstrate the creation of an extreme GNG structures in near-defect-free single-crystal silver (Ag) micro-cubes when impacted on hard surfaces at high velocities.
We use an advanced laser induced projectile impact testing (a-LIPIT) apparatus to selectively launch the Ag micro-cubes at ~400 m/s, and directly impact them onto a rigid impenetrable substrate made of thin-film gold coated silicon. The high-strain-rate impact (~10⁸/s) and the high stresses (~ 9 GPa, which is greater than 150 times the yield strength of Ag) generated at the impacted side causes the Ag micro-cubes to deform initially under a hydrodynamic stress state, and subsequently through crystallographic slips at lower stress levels.
The scanning transmission electron microscope (STEM) images of the bottom impacted region reveals new nanocrystalline grains that were created due to impact. Selective area diffraction (SAD) patterns obtained at locations along the height of the sample indicates an extreme GNG structure with grain sizes varying from nanocrystalline to near-single-crystal coarse (sub-micron) grains over a very short distance—from the most severely deformed bottom impacted region to the top less-deformed region. Surface slip steps left from dislocation avalanches exiting the crystal are also clearly visible in the scanning electron microscope (SEM) images. Additionally, the impact along different crystal symmetry directions such as [100], [110], and [111] shows intriguing overall deformation symmetries.
Our observation of the formation of a GNG structure in dynamically deformed Ag micro-cubes shows promising pathways for future development of ultra-strong and tough metals that will not fail catastrophically. The dramatic shape changes observed in impacts along different crystal-symmetry directions also suggest that the resultant GNG structure can be tailored in desired ways by the control of impact direction. Developing a fundamental understanding of the dynamic deformation mechanisms of defect free, single crystal micro-particles is also useful to develop damage tolerant materials for engineering applications in extreme environments, for example, spacecraft that are prone to collisions of micro-meteorites and space debris, and turbine blades that are susceptible to failure from micro-particle impacts.

In the first part of the talk, I will present two recently developed platforms for high temperature nanomechanical testing. The first platform allows for variable temperature and variable strain rate testing of micropillars in situ in the scanning electron microscope. By utilizing an intrinsically displacement-controlled micro-compression setup, which applies displacement using a miniaturized piezo-actuator, we’ve recently extended the attainable range of strain rates to up to~ 10⁴ s⁻¹, and enabled cyclic loading up to 10⁷ cycles. Stable, variable temperature indentation/micro-compression in the range of -4°C to 600°C is achieved through independent heating and temperature monitoring of both the indenter tip and sample and by cooling the instrument frame. At room temperature micro-compression experiments were combined with in-situ EBSD measurments. In situ EBSD allows for the determination of crystallographic orientation with sub-100 nm spatial resolution. Thereby, it provides highly localized information on phenomena such as elastic bending of the micropillar or the formation of deformation twins and plastic orientation gradients due to geometrically necessary dislocations.
A second system allows for measurements at lower loads ex-situ in a dedicated vacuum chamber in the range of -150 °C to 700 °C. The cryo temperature is achieved by means of a liquid nitrogen line, while the high temperature is generated by three independent heat sources for the sample and the two tips of the differential displacement measurement system, establishing an infrared bath in the measurement area.
In the second part several case studies will be presented. Using these new capabilities, we examine the plasticity of nanocrystalline metals. In order to analyse the fundamental deformation mechanisms variable strain rate and variable temperature micro-compression experiments were performed. Activation parameters such as activation energy and activation volume were determined and discussed in view of the most probable deformation mechanism. The fracture behaviour of nanocrystalline ceramics and the fracture behaviour tungsten is currently under investigation and first results will be presented at the time of the conference.

The recent synthesis of twin interfaces in two covalent-bonding materials, cBN and diamond, has improved their thermal stability and fracture toughness and meantime introduced a new record for material's hardness. The continuous hardening of these materials by decreasing the twin-spacing even lower than the critical thickness (about 15 nm), which was a turning point to softening behaviour in Cu, is mysterious. Here, we show a similar observation of hardening in nanotwinned Pd polycrystalline samples at room temperature and reveal that there exists a transition temperature for materials, below that the softening will be replaced by hardening behaviour. Our large molecular dynamic and finite element simulations show that below this transition temperature, thermally-activated source-controlled plasticity will be substituted by the stress-driven one. Since the stress-concentration at grain boundary-twin intersections for nucleation of partial dislocations gets higher value by increasing the twin spacing, twinning migrations are progressively observed in grains with thicker twin interfaces by decreasing the temperature. Higher amount of stress concentration is caused by lower elastic-field interaction of close intersections when twin interfaces become far from each other. Higher the bond's strength, higher transition temperature is captured by our simulations and predicted by our theoretical modelling. These results give an insight for observing hardening in covalent-bond materials in which the transition temperature is anticipated to occur at values higher than the room temperature similar to Pd.

The number of known materials that can be used for ultra-high temperature applications is relatively limited, even when just considering melting point. When considering other performance parameters such as mechanical stability, thermal conductivity, thermal shock and oxidation resistance, the list of viable materials becomes even smaller. To drive this area forward beyond traditional considerations such as improved microstructures and densification, new materials classes need to be explored and characterized. With this in mind, we have been using a combination of high throughput thermodynamic and first principles modeling, experimental prototyping, and advanced bulk processing to explore the viability of using high entropy materials for these applications. This talk will provide an overview of this effort, including recent results on the synthesis and properties of new classes of high entropy oxides, nitrides and borides. These materials are unique in the field of high entropy alloys in that they contain entropic and ordered sublattices, which gives them the potential for materials engineering to meet the demands of ultra-high temperature applications.
This work was supported by the Office of Naval Research through a Multi-Disciplinary University Research Initiative.

Polycrystalline Y₂O₃, is an excellent candidate to replace single crystal alumina windows in harsh enviroments. In this research, reversible transformation has been utilized in grain size refinement in Y₂O₃. This is a two step reaction where 300 um cubic Y₂O₃ polycrystalline material transformed into monoclinic Y₂O₃ with <100 nm grains under high temperature and pressure (1000^oC and 8GPa) for short period of time ( <15 minutes). Then the sample is transformation back to cubic phase at 1 GPa/1000GPa while maintaining the grain size <100 nm.

It has been observed that increasing the holding of the first step reaction for periods > 4 hrs has led to surface modification by growing columnar grains with lower stiffness and hardness than sample interior. Moreover, Chemical analysis by EDS shows that these grains are oxygen deficient compared to sample interior. Moreover, electron diffraction confirms that the columnar grains have cubic symmetry while, sample interior exhibits monoclinic phase. A detailed electron microscopy which includes, high resolution TEM analysis, growth orientation and defect analysis were applied to understand the nature of this new surface effect.

It is widely believed that dislocation junctions are strong contributors to work hardening of single-phase crystals. However, it has not been possible to determine the relative importance of different types of dislocation junctions on the total hardening rate. In principle, this question could be answered by dislocation dynamics (DD) simulations. But systematic studies of junctions and work hardening by DD simulations have been out of reach due to computational limitations. In this work, an efficient new time integrator is used in large-scale DD simulations to study work hardening in single crystal copper. The hardening rates obtained under [001] loading over a range of dislocation densities and strain rates are shown to be consistent with Stage II hardening in experiments. To probe the role of junction formation, specialized simulations are run where only certain kinds of junctions are allowed to form. Of the four possible binary junctions in FCC metals, the glissile junction contributes most significantly to hardening, followed by collinear, Lomer and Hirth junctions. The dislocation microstructures are examined and reveal that the lengths of dislocation lines connecting junction nodes are exponentially distributed. This means that junction formation can be characterized as a Poisson point process in space. A Boltzmann-type model is developed where the line length distribution evolves in time under the action of line bow-out and forest collisions. The Boltzmann-type model not only explains the dislocation line length distributions, but also clarifies the role of different dislocation junctions on the work hardening rate.
This work was supported by Sandia National Laboratories (R.B.S.) and by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0010412 (W.C.). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

A dislocation-density based multiple slip crystalline plasticity formulation and a new computational fracture approach have been used to investigate and predict intergranular and transgranular dynamic fracture in hexagonal cubic packed (h.c.p.) materials with a focus on h.c.p. alloys subjected to larges changes in strains, strain-rates, and temperatures. This validated predictive framework has been used to understand and predict the interrelated effects of dislocation-density interactions, generation, and recovery on the competition between intergranular and transgranular crack nucleation and propagation. The predictions indicate that transgranular fracture is dominated dislocation-density interactions with hydrides and intergranular fracture is dominated by elemental pits that have diffused into the GB

Plasticity in Mg single crystals oriented for c-axis compression is attributed to <c+a> dislocation slip on pyramidal planes. Recent systematic molecular dynamics studies clearly show that pyramidal slip is highly anisotropic, and edge dislocations on the pyramidal plans are immobile at temperatures higher than 300K due to a thermally activated dissociation process on the pyramidal I planes. On the other hand the near screw dislocations are highly mobile and have low critical resolved shear stresses. In the absence of detailed TEM observations at different temperature, a debate still exists on the slip geometry and the microstructural features of deformation for this loading orientation. Here, we present detailed large scale atomistically informed discrete dislocation dynamics simulations accounting for the anisotropy of pyramidal slip in Mg which is atomistically informed at different temperatures from 0K to 600K. Large-scale c-axis compression simulations of different Mg crystal sizes are conducted and the dislocation microstructure evolution in both bulk and micrometer sized magnesium single crystals is examined. The results are compared with recent transmission electron microscopy from deformed Mg single crystals at room temperature. The results show that at low temperatures slip is predominantly on pyramidal I planes, while with increasing temperature slip on pyramidal planes become more active. The simulations also show that climb and basal slip cannot be neglected to explain many experimental observations.

A multi-scale elastic-plastic finite element (FE) and fast Fourier transform (FFT) approach is proposed to study lattice strain evolution in cruciform samples of 316L SS subjected to biaxial loading. At the macroscale, FE simulations capture the cruciform geometry induced coupling between applied forces in the arms and gauge stresses. Results show that uniaxial loading in cruciform samples results in a biaxial stress state with a compressive component along the direction normal to the loading direction. The FE simulation predicted stresses are used as macroscopic boundary conditions to drive the mesoscale elasto-viscoplastic FFT model. The combined FE-FFT approach appropriately captures the in-situ neutron diffraction predicted lattice strain evolution for different applied load ratios for the cruciform samples. A quantitative analysis of the lattice strain evolution of 200 grains is performed for different loading conditions. Results show that the contribution of elastic compliance and plastic slip to the lattice strain evolution highly depends on the applied stress ratio.

Shock-compression of structural energetic/reactive material systems consisting of pressed compacts of dissimilar powder mixtures or multi-layered structures, result in effects that are dominated by heterogeneities associated with differences in elastic and plastic properties of the reactant constituents. Impact experiments are performed using the gas gun or laser-accelerated launching of thin foils coupled with use of nanosecond-resolution piezoelectric PVDF stress gauges and interferometry to measure the stress-wave or particle-velocity profiles and shock propagation speeds. These time-resolved diagnostics provide information about evidence of reaction inferred based on changes in the equation of state and/or the pressure-volume compressibility. However, these continuum-based diagnostics lack spatial resolution necessary to capture the micro- or meso-scale structural evolution of transition states, extent of reaction, localized changes in reactant configuration(s), or transport processes that lead to reaction. Two-dimensional meso-scale numerical simulations employing actual micrographs of starting reactive constituents imported into a multi-material CTH hydrocode, provide qualitative and semi-quantitative understanding of the configurational changes between reactants and their effects on possible reaction mechanisms. The simulations also reveal effects of highly-heterogeneous nature of shock-wave interactions with reactants resulting in forced/turbulent flow, vortex formation, and even micro-scale dispersion and solid-state mixing as possible processes promoting reaction. However, there exists no scale-specific validation of these processes. In this presentation, use of meso-scale in-situ diagnostics employing quantum dots and photonic crystals, to experimentally measure spectral signatures characteristic of localized stress/strain effects of shock interactions with heterogeneities, will be discussed as a possible means for correlations with simulated reactant configuration changes. The understanding generated through such spatially and temporally-resolved in-situ diagnostics, combined with meso-scale simulations, can enable the design of performance-specific structural energetic/reactive materials.

The molecular dynamics method is used to investigate martensitic transformations during shock of single crystal Cu in the <100> orientation. To explore the influence of shock strength and temperature on martensitic transformations, particle velocities between 0.5 and 1.5 km/s for samples with initial temperatures of 5, 300 and 600 K are employed. Generally, the FCC structure uniaxially compresses into a BCC structure behind the shock wave front, and the atomic percentage of BCC Cu increases with increasing particle velocity and temperature. For particle velocities between the Hugoniot elastic limit (HEL) and 1.0 km/s, the BCC Cu quickly transforms into a faulted FCC structure; however, for particle velocities greater than 1.0 km, the BCC Cu transforms into a faulted 9R structure. A BCC to HCP martensitic transformation also occurs for particle velocities greater than 1.0 km/s; the atomic percentage of the HCP phase decreases with increasing temperature.

Pressure-induced bcc-hcp martensitic phase transitions in iron are very complex due to the interplay of the transformational strains and long-range stresses induced by the emerging microstructures. A thermodynamically consistent phase-field approach for multivariant transformations during shock wave propagation is developed at large strains with plasticity. Thermodynamic potential includes the description of elastic and inelastic energy landscapes based on the concept of reaction pathways and the second law of thermodynamics is used to determine the driving force for change in transformational strain gradients. Compared to the pure hydrostatic compression case, the morphological features of the bcc-hcp-bcc transformations in iron are investigated and the importance of the plastic deformations in the shock-induced structural transitions is discussed.

Carbon nanostructures are promising ballistic protection materials due to their low density and superior mechanical properties. Recent experimental [1] and computational [2] investigations on the high-strain rate behavior of graphene revealed exceptional energy absorption properties as well. However, the reported numerical and experimental values differ by an order of magnitude. In this work, we employed a combined numerical and analytical modeling to address this apparent inconsistency. We show that the specific penetration energy decreases as the number of layers (N) increases, from ~25 MJ/kg for N=1 to ~0.30 MJ/kg as N goes to infinity. Our results suggested that the main aspects of the ballistic graphene fracture are size-independent. By analyzing MD trajectories, we obtained more accurate values for dynamic quantities such as the average tensile strain rate [4]. The observed sharp transition confirms the scaling law proposed by Pugno [3].

[1] J.-H. Lee, P. E. Loya, J. Lou, and E. L. Thomas, Science (New York, N.Y.) 346, 1092 (2014).
[2] K. Yoon, A. Ostadhossein, and A. C. van Duin, Carbon 99, 58 (2016).
[3] N. M. Pugno, Acta Materialia 55, 1947 (2007).
[4] R. A. Bizao, L. D. Machado, J. M. de Sousa, N. M. Pugno, and D. S. Galvao - submitted.

Multimillion to billion atom reactive molecular dynamics and quantum molecular dynamics simulations are used to investigate structural and dynamical correlations under thermomechanical extremes.

Cavitation bubbles readily occur in fluids subjected to rapid changes in pressure. We use billion-atom reactive molecular dynamics simulations to investigate chemical and mechanical damages caused by shock-induced collapse of nanobubbles in water near silica surface. Collapse of an empty nanobubble generates high-speed nanojet, resulting in the formation of a pit on the surface. The gas-filled bubbles undergo partial collapse and consequently the damage on the silica surface is mitigated.

Reactive molecular dynamics simulations predict unexpected condensation of large graphene flakes during high-temperature oxidation of nSiC. Initial oxidation produces a molten silica shell that acts as an autocatalytic 'nanoreactor' by actively transporting oxygen reactants while protecting the nanocarbon product from harsh oxidizing environment. Percolation transition produces porous nanocarbon with fractal geometry, which consists of mostly sp2 carbons with pentagonal and heptagonal defects.

While the thermodynamics of tensile amorphization has fascinated scientists over decades, such far-from-equilibrium dynamics remained elusive. Our quantum molecular dynamics simulations show that stishovite indeed amorphizes rapidly within 1 ps under tension. We find a displacive amorphization pathway that only involves short-distance collective motions of atoms, thereby facilitating the rapid transformation.

“This work was supported as part of the Computational Materials Sciences Program funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC00014607.”

The ground state α-phase of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) undergoes phase transition to γ-phase around 3.8 GPa under hydrostatic loading. γ-RDX plays an important role in the detonation mechanism of RDX.¹ While the two polymorphs have been extensively studied in their respective stability range, the atomistic modeling of α-γ phase transition under hydrostatic loading has received much less consideration. In this work, we report α-γ phase transformation under hydrostatic loading using molecular dynamics (MD) with a non-reactive fully flexible Smith-Bharadwaj (SB) molecular potential.

The simulation box consists of α-RDX molecules with non-orthogonal periodic boundary conditions. Initially, the system is equilibrated at 0Pa and 300K using isothermal-isostress (NσT) ensemble and then equilibrated till 450K in increments of 25K at 0Pa. At each temperature increment, the system is equilibrated from 0-2GPa and 2-4GPa in increments of 0.5GPa and 0.1GPa, respectively. At each equilibration point, the pressure-volume-temperature data, the individual energy terms in SB potential, and the atomic positions of all molecules in the system are collected and analyzed to predict the α-γ phase transition.

We observe a sharp decrease in the volume around 2.5GPa for temperature beyond 350K, which is indicative of α-γ phase transition. Though the transition pressure is below the experimentally observed pressure of 4GPa, it is consistent with other phase transformation studies of RDX using MD simulations with SB potential.² We also observe an abrupt decrease in the dihedral and improper dihedral energy terms of the SB potential, and a sudden increase in the angle energy around the transition point, which is due to the change in the conformation of RDX during phase transition. After the phase transition, the angle and dihedral energies approximately saturate to the respective energies at 0Pa, which is also predicted in previous MD simulations. There is no change in the bond energy since the bond distance and type do not change between the two polymorphs. The wag angles for two nitro groups of all the molecules remain axial throughout the phase transformation process. For the third nitro group, the wag angle for half of the molecules remains equatorial, whereas, that for the other half evolves into intermediate orientation. These are consistent with the experimental crystal structures of the two polymorphs.

Our simulations indicate that the SB potential is able to capture the α-γ phase transition in RDX given appropriate thermal excitation to induce changes in the molecular conformation. It elucidates the efficacy of SB potential in investigating problems involving α-γ phase transition in RDX.

References
1. Dreger and Gupta, J. Phys. Chem. A 116, 8713 (2012).
2. Munday et al., J. Phys. Chem. B 115, 4378 (2011).

As part of a multiscale modeling effort, we present progress on a challenge in continuum-scale modeling: the direct incorporation of complex molecular level processes in the constitutive evaluation. In this initial phase of the research we use a concurrent scale-bridging approach, with a hierarchical multiscale framework running in parallel to couple a particle-based model (the “lower scale”) computing the equation of state (EOS) to the constitutive response in a finite-element multi-physics simulation (the “upper scale”).
The lower scale simulations of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) use a force-matched coarse-grain model and dissipative particle dynamics methods, and the upper scale simulation is of a Taylor anvil impact experiment. Results emphasize use of adaptive sampling (via dynamic kriging) that accelerates time to solution, and its comparison to fully “on the fly” runs. Results involving chemistry from a particle-based, fully reactive EOS during plate impact are also discussed.

Explosives and propellants are heterogeneous materials consisting of energetic molecular crystals embedded in polymeric binders. These energetic materials can be initiated through a variety of stimuli, including thermal, mechanical, and electrical means, with the non-thermal stimuli believed to generate heat first which then causes thermally induced chemical decomposition. Optimizing the performance of energetic materials for specific applications has to be balanced against safety and reliability considerations. This process requires an improved fundamental understanding of chemical decomposition or initiation (for a particular stimulus). In particular, the need for real-time measurements of local temperatures and microstructures in shocked energetic materials is well recognized. However, the challenging nature of this problem has precluded significant success to date. Our long-term goal is to develop and demonstrate the experimental feasibility to measure the following in real-time: temperature, stress, and microstructural evolution in binder and crystal components of heterogeneous materials under shock wave loading. Next, we plan to use these experimental developments to investigate and characterize hot-spot mechanisms in heterogeneous materials containing organic molecular crystals. As part of this effort, we are characterizing and evaluating Dy(III) and Sm(III) containing molecular complexes as potential in-situ temperature sensors. The complexes can be dispersed throughout polymers and embedded into molecular crystals. The temperature is determined via laser-induced 2-color fluorescence thermometry, measured by fast imaging cameras.

This work presents microscale modelling of defects in methane hydrate based on previous ab initio work at the nanoscale. Methane hydrates are crystalline compounds in which hydrogen-bonded water molecules entrap methane gas within cages at high pressures and/or low temperatures. While they exist abundantly all over the world and are considered a significant alternative energy resource, their mechanical properties have not yet been fully investigated. From ab initio nanoscale simulations, the lack of a dominant slip system was determined, and insights, in terms of the importance of multi-body interactions and the existence of a critical hydrogen bond length, have been obtained for choosing an appropriate force field and for interpreting results in microscale simulations . At the microscale, the effect of cage occupancy on the core structure and mobility of edge and screw dislocations was determined using molecular statics and dynamics. The prominent finding was the very wide spreading of dislocations and their dissociation at different stages of deformation. Point defects in the polyhedral structure of cages were also considered for comparison in terms of mobility. The nucleation and structural evolution of defects is studied along with the brittle failure mechanism of methane hydrate crystals. These results constitute essential input for risk assessment studies of controlled natural methane hydrate production and for the synthesis of these crystals for the transportation of natural gas.

This presentation will review the recent progress in the understanding of plastic deformation in ultra-fine scale metal-based composites. Examples will be presented from a variety of material systems: laser-processed Al-Al₂Cu lamellar eutectic, vapor deposited Cu-TiN and Al-TiN thin films, and Cu-Nb multilayers rolled to large plastic strains. The common aspects in interface-dominated mechanical behavior in ultra-fine scale metallic composites such as unusually high flow strengths, high strain hardening rates and plastic co-deformability will be elucidated through in situ TEM straining experiments and analyzed using atomistic modeling, dislocation theory and crystal plasticity. The strain hardening behavior of confined systems will be interpreted using a three-dimensional crystal elastic–plastic model that describes plastic deformation based on the evolution of dislocation density in the constituent phases. The conditions that lead to morphological and crystallographic stability of interphase boundaries in certain composite systems at extremes of mechanical straining will be highlighted.

Al/SiC metal-ceramic multilayers were manufactured by magnetron sputtering. Different nanolaminates were manufactured with the same nominal values for the Al and SiC layer thickness (in nm): 10/10, 25/25, 50/50 and 100/100 and cylindrical micropillars of 2 µm in diameter and 4 µm in height were milled with a focused ion beam (FIB). Nanoindentation and micropillar compression tests were carried in the Al/SiC multilayers in the direction perpendicular to the laminate at 25C and 100C. In addition, the deformation mechanisms in the 100/100 nanolaminate were ascertained by means of in situ micropillar compression tests in the transmission electron microscope. It was found that deformation was controlled by the plastic deformation of the Al layers that took place by the nucleation of the dislocations at the metal-ceramic interface. The dislocations were absorbed in the opposite interface and the Al plastic flow was constrained by the stiff SiC layer.
The hardness of the multilayers at ambient and elevated temperature was fairly independent of the layer thickness, while the strain hardening and yield strength of the Al layers increased significantly as the layer thickness decreased at both temperatures. Numerical simulations of the hardness and micropillar compression tests were in agreement with the experimental results and showed that hardness was independent of the layer thickness because the Al deformation was fully constrained regardless of the layer thickness. However, the constrain imposed by the ceramic layers during micropillar compression was much higher in the case of the thin nanolaminates (10/10 and 25/25), leading to a very large strain hardening which was not found in the thicker nanolaminates. Under constrained deformation, the mechanical response of the nanolaminate was weakly dependent on the Al yield strength and the influence of the temperature on the mechanical properties was limited.

In recent years two-phase nanolayered composites with individual layer thicknesses varying from 200-300nm down to 1-2 nm have been the subject of intensive study because of their unusual physical, chemical and mechanical properties. For example, with decreasing layer thicknesses (down to nanometer length scales) the mechanical response of these nanocomposites becomes increasingly interface dominated, and they exhibit ultrahigh strengths approaching the theoretical limit for ideal crystals. Moreover if the constituent phases present large differences in strength, elastic modulus and ductility, these multilayers give rise to new possibilities for the deformation mechanisms and properties of the composite as a whole. In this work we explore the possibility of synthesizing multilayered composites where one constituent phase has a low ductility, with a final goal of enhancing both the strength and ductility of the system.

Using physical vapor deposition (PVD) techniques we synthesized a hexagonal close-packed (HCP) – body-centered cubic (BCC) Mg-Nb system (where twinning in Mg leads to its lack of ductility), over a range of layer thicknesses ranging from 5 nm to 200 nm. Testing of such miniaturized poses significant challenges. We utilize a combination of nanoindentation, in-situ SEM compression testing of micro-pillars, and in-situ SEM fracture toughness testing of 3 point bend micro-beams containing these multilayered nano-composites to evaluate their deformation mechanisms. Micropillar testing for three different orientations, with the interfaces oriented normal, parallel and oblique (45^o) to the compression axis, enable us to explore the anisotropy in the mechanical response of the multilayer system, while the fracture toughness of the specimens are measured using the notched 3-point bend tests. These results are compared for varying layer thicknesses as well as under varying ambient temperatures.

Additionally our work shows that at low enough layer thicknesses the crystal structure of Mg can be transformed and stabilized from simple hexagonal (hexagonal close packed hcp) to body center cubic (bcc) at ambient pressures through interface strains,. We show that when introduced into a nanocomposite bcc Mg is far more ductile, 50% stronger, and retains its strength after extended exposure to 200 C, which is 0.5 times its homologous temperature. These findings reveal an alternative solution to obtaining lightweight metals critical needed for future energy efficiency and fuel savings.

Two-phase nanolaminate thin film composites have demonstrated an unusually broad number of desirable properties under extreme environments, such as high strength, high strain to failure, thermal stability, and resistance to light-ion radiation. The microstructures that arise from different synthesis routes such as Physical Vapor Deposition or Severe Plastic Deformation can vary widely in terms of layer morphology, local structrure, texture, and resulting mechanical behavior. Recently we have shown that bi-phase HCP/BCC nanolaminates with individual layer thicknesses approaching 10 nm can be made via severe plastic deformation (SPD) in bulk sizes suitable for structural applications. Mechanical testing of these HCP/BCC nanolaminates shows exceptionally high strength and characterization via a suite of techniques including neutron diffraction, EBSD, and TEM indicates that the crystals are highly oriented. While the cause of these unusual properties can easily be associated with a high density of bimetal interfaces, how the interfaces physically control microstructural evolution and macroscopic properties remains an area of intense research. This presentation highlights our modeling and experimental efforts to understand and link the evolution of the nanostructure, the interface properties, and preferred texture during the SPD process.

Textile composites with ceramic fiber tows woven or braided into two-dimensional and three-dimensional architectures in a ceramic matrix are important material options for many types of structures. These composites represent a new class of integrally woven ceramic matrix composites for high-temperature applications, where both strength and thermal conductivity are important. For high performance and reliability, a key issue is irregularities or geometrical defects in the textile reinforcement, which trigger failure mechanisms and compromise strength and life. Due to the intricate 3D microstructural architecture of the woven composites, damage initiation, propagation and the nature of failure is extremely challenging to investigate and interpret.
Voids and small volumes of local misalignment of the fiber tows with respect to the nominal load axis (or axis of the fabric) and variations in tow position and tow cross-section act as critical defects where cracks tend to initiate. Once these cracks initiate they are known to propagate internally within the bulk of the composite along tow boundaries and also within the tows in the form of splitting cracks. Moreover, the nature of failures drastically differs with the change in the orientation of the loading direction with reference to the tow directions. In order to fully understand the failure mechanisms in such highly three dimensional architectures, it becomes critical to obtain the full spatially resolved three dimensional information of cracks in relation to the underlying microstructure.
3D x-ray microscopy is a highly powerful non-destructive method that allows non-destructive imaging of the sample and enables us to subject samples simultaneously to in-situ mechanical loading. Using the in-situ imaging approach we can acquire data that resolves the cracks initiated at several different sites due to the load and observe them to propagate along certain critical paths which ultimately lead to failure. We present here results from a 3D multi-scale non-destructive investigation performed on a 3 layer angle-interlocked woven composite specimen subjected to in-situ mechanical loading. Employing X-ray microscopy (XRM) in a computed tomography (Micro-CT) approach we performed 3D imaging covering multiple size scales. The multi-scale imaging approach using XRM facilitates switching between a wide range of objectives to adapt to the length scales in which key damage mechanisms operate. For instance, the lower resolution scan provided a global overview of the geometry of the specimen with enough resolution to discern the tows. Using a scout and zoom approach the central notched region of the composite specimen was targeted to image the in-situ damage at a higher resolution. We further demonstrate the advantages of the multiscale approach by non-destructively “zooming-in” onto a specific site resolving further how the crack interacts with individual fibers within the tows in 3D.
Through the successive tomographic volumes collected as a function of load we were able to determine the crack extension for a given load and resolve in great detail how the entire crack network that leads to ultimate failure. Indeed, these results provide three dimensional experimental data that are just not restricted to simple visualization and virtual analysis, but contain rich information that can certainly be used further to extract stochastic information of the composite, compare the results and directly validate the fidelity of predictive modeling codes that simulate failure in complex composite material systems.

The pulsed, coherent x-rays produced by modern synchrotron sources are well suited for studies of the internal structure of materials during failure due to dynamic loading. In this talk we demonstrate the capabilities of synchrotron imaging on ceramics, with an emphasis on boron carbide. Using a high-speed detector system and a Kolsky bar apparatus we are able to image the propagation of individual cracks in three-point bend specimens of boron carbide under dynamic loading, at frame rates of up to five million frames per second. The penetrating power of the x-rays allows us to observe the evolution of the internal structure of the material which we correlate with load-displacement data from the Kolsky bar. For example, we are able to correlate the velocity of the crack tip in the three-point bend specimens with the stress intensity at the crack tip.

The coherent nature of the synchrotron radiation alloys us to employ propagation-based x-ray phase-contrast imaging, which renders gradients in refractive index as intensity variations. In particular, phase-contrast imaging enhances the visibility of cracks compared to traditional radiography in which the contrast is due only to x-ray absorption. Phase-contrast imaging encodes quantitative information about the material being imaged, including the evolving structure of the cracks. As a motivating example we show data on single-crystal quartz loaded dynamically in compression, resulting in complex images due the simultaneous propagation of cracks from different points in the specimen. We discuss the potential to decode structural information, such as the number, size, and orientation of multiple cracks evolving simultaneously. This quantitative information would be useful for the development of constitutive models that incorporate the effects of evolving damage in the material.

Constitutive response of materials under high pressures is typically measured on small specimens using diamond anvil cells. At moderate pressures (2-10 GPa), strength can be measured on a modified Bridgman anvil known as the tri-anvil apparatus. This device was used to measure the pressure dependent shear strength of Mo single crystals oriented for slip in the <111> and <211> directions. Deformation structures observed for these materials vary as a function of pressure with a well-organized dislocation structure being observed for structures deformed at higher pressures. It was determined for the oriented Mo single crystals at pressures from 2-5 GPa, that slip was achieved at lower shear stress for <211> oriented crystals than for those that were deformed along the <111> axis.

The nanometer-scale chemical and structural analysis within the same position using correlative microscopy techniques provides a more incisive perspective of the material being studied that cannot be observed by using a single instrument. A correlative TEM and APT approach was used to characterize an extremely strained carburized 8620 Fe-C-Mn-Si bearing steel to understand its microstructural evolution upon severe shear deformation (SSD). It was found by APT that SSD not only refines the grain size to tens of nanometers by creating non-equilibrium grain boundaries that exhibit carbon segregation, but also creates many 2D linear defects and possible phase transformations within localized regions. The combination of APT analyses with TEM investigations further reveals the following features: (i) compositional and structural characteristics of the grain boundaries after SSD, (ii) interstitial atom segregation content versus the type of grain boundary, and (iii) phase transformation processes. In the end, the molecular dynamics simulation was used to confirm the observed phase transformation process.

The polycrystalline nickel-base superalloy René 88DT has been cycled at 20kHz up to 10⁹ cycles in an ultrasonic fatigue system. Fatigue properties in this low stress regime exhibit considerable variability due to intrinsic features of the polycrystalline structure of the material. Strain localization has been studied by digital image correlation. The presence of annealing twins strongly influences the initiation process. Mesoscale three dimensional datasets gathered via TriBeam tomography have been acquired and are employed to study the statistical variations in microstructure that favor crack initiation.

Cemented carbides, tungsten carbide with binders of cobalt or nickel (WC-Co/Ni), are widely used in the process of petroleum extraction due to their superior hardness, strength, and wear resistance. Consequently failure of these materials by impact fatigue, the result of repeated sub-failure strength impacts, is of practical and scientific concern. Current active research in WC-Co/Ni hardmetals focuses on classical fatigue in effort to optimize defect and grain characteristics for improved life cycles and fracture toughness; however a lack of fundamental understanding establishing the role of the microstructure on physical deformation mechanisms under transient loading remains. Three variants of WC-Co, one with 6% Co binder and a fine grain size, and the other two with >10% Co binder of fine and course grain size, along with a course grain >10% of WC-Ni are examined. The samples are characterized for their microstructural features pre- and post-mortem using X-ray microtomography. The specimens are tested in a modified three-point-bend or compact compression configuration with a pre-crack using a modified Kolsky (split-Hopkinson) bar apparatus under repetitive sub-failure impact loading. Digital image correlation (DIC) and in-situ microscopy are used to examine the deformation and fracture process during impact cycles until failure, and post-mortem fragmentation characterization is completed to determine the cumulative distribution of the resulting characteristic fragment length (which can be related to the microstructural defect and hence failure mechanisms). Initial 2D finite element method (FEM) simulations have been conducted using Abaqus Explicit to validate the experimental setup and provide insight into the transient stress wave propagation with impacts between 3 and 30 m/s and of various orientations and geometries of the pre-crack. The results are provided in the context of a dynamic Paris’ Law.

Nearly 90% of service failures of metallic components and structures are caused by fatigue at a cyclic stress amplitude much lower than the tensile strength of the materials involved. A long-standing obstacle to developing better materials with higher fatigue limit and longer fatigue life has been that metals typically suffer from large, accumulative, irreversible damages in microstructure during fatigue, leading to history-dependent and unstable cyclic responses. Here, through both experiments and atomistic simulations, we report a history-independent and stable cyclic response in bulk Cu samples that contain highly oriented nanoscale twins. We demonstrate that this unusual behavior is governed by a type of highly correlated necklace dislocations formed in the nanotwinned metal under cyclic loading. Our findings reveal a new route to improve the fatigue life of engineering materials through tailor-designed microstructure.

In comparison to fatigue crack growth in air, testing (load ratio of R=0.1, frequency of ν=1 Hz) a low carbon structural steel (JIS-SS400, equivalent to ASTM A36, YS=314 MPa) in 40 MPa hydrogen gas environment increases the fatigue crack growth rate by 13 times, and changes its major fracture feature from striations to mixed intergranular and quasi-cleavage type. The dislocation structures immediately beneath these fracture surfaces were examined by exploiting focused ion beam machining to extract samples from specific sites and zone-axis STEM image conditions to obtain dislocation microstructures. The dislocation structure beneath the striations was low angle subgrains with compact dislocation walls. With hydrogen, beneath the intergranular and quasi-cleavage fracture features exists dislocation cells and dislocation bands, respectively. Dislocation dynamic analysis indicates that the mobile dislocation generation rate beneath the intergranular surface is much higher than that beneath striations generated in the air test. The high mobile dislocation generation rate facilitates hydrogen transport to grain boundaries and increases the hydrogen coverage in the grain boundaries, and thus promotes intergranular fracture. In contrast, the dislocation bands beneath quasi-cleavage features indicate enhanced dislocation activity on slip systems experiencing the highest resolve stress. Combing these results with those from previous studies indicates that specific dislocation patterns are associated with the fracture path and mechanism, and these can be mediated by the presence of hydrogen. The enhanced dislocation activity in the presence of hydrogen will be explained by the hydrogen enhanced plasticity mechanism and the connection between dislocation structure and fracture path will be established.

Aluminum alloys are frequently exposed to harsh environments in service. For instance, the skins of aircraft on carriers are exposed to corrosive saltwater spray combined with the fatigue stresses of flight operations. The combined service conditions can be difficult to understand, leading to inaccurate models for service life prediction and economic loss. Understanding the complex mechanisms of corrosion that operate under a combination of stress and corrosive environment is quite crucial to development of better corrosion resistant alloys. Moreover, it is known that several factors including the underlying microstructure, presence of defects and alloys crystallographic orientation play a dominant role in corrosion related fracture of these alloys.
To investigate the effects of corrosion and fatigue on peak-aged 7475 aluminum alloy, we imaged the corroded samples using laboratory based non-destructive X-ray microscopy. In combination with the conventional absorption contrast tomography we collected an additional dataset using Diffraction Contrast Tomography which enabled reconstruction of the three dimensional crystallographic information of the grains. The samples were mechanically polished, then soaked in covered 3.5 wt.% NaCl and simultaneously subjected to a constant load to initiate stress corrosion cracking. The combined diffraction contrast tomography and absorption tomography provides comprehensive information of the grains within the samples before and after corrosion, enabling a detailed analysis of damage initiation and propagation. The effect of the microstructure on corrosion cracking and whether the crack follows a path adjacent to preferred orientation of grains will be discussed.

Attempts to trigger ferromagnetism in diamagnetic and paramagnetic metals and alloys without changing the chemical composition must devise alternative ways to modify permanently their electronic properties. Tuning the magnetic behaviour of metals by altering their crystal structure, the ultimate responsible for their electronic configuration, may constitute a viable, yet unexplored, strategy to induce ferromagnetism in non-magnetic metals. Here we show that the lattice distorsions induced by grain refinement of pure metals such as zirconium, titanium and hafnium down to the nanocrystalline regime, via severe plastic deformation, lead to the development of new crystalline structures that exhibit room temperature ferromagnetic behaviour. More concretely, density functional theory (DFT) calculations predict that local stretching of the original hexagonal close packed lattice along specific pyramidal directions, due to the presence of internal stresses in the deformed nanostructure, gives rise to the emergence of monoclinic phases endowed with a net magnetic moment. An excellent agreement is found between DFT calculations and experimental transmission electron microscopy (TEM) observations, which provide a first evidence of the presence of the pure monoclinic crystal lattice in group IV metals.

Rare-earth orthophosphates (REPO₄) have been explored for use as oxidation-resistant fiber coatings in oxide-oxide ceramic matrix composites in efforts to increase strain to failure and composite flaw tolerance by reducing fiber push-out stresses. Fiber coatings of xenotime Gd_0.4Dy_0.6PO₄ have been shown by R.S. Hay et al. to substantially reduce fiber push-out stresses compared to coatings of monazite LaPO₄ and xenotime DyPO₄, and show evidence of deformation-induced phase transformation to denser phases. We use diamond anvil cell experiments with in situ Raman spectroscopy to characterize the pressure-induced phase transformation under near hydrostatic conditions in REPO₄s including TbPO₄, DyPO₄, and solid-solution orthophosphates with effective rare-earth radius similar to Tb (Gd_xDy_(1-x)PO₄ with x=0.4, 0.5, and 0.6). All five materials exhibit a bending mode softening with increasing pressure and level crossing prior to the onset of transformation. The solid solutions are shown to transform at similar pressures to TbPO₄ within the resolution of our study, while DyPO₄ transforms at a pressure ~30% higher. Thus, the reduction in fiber push-out stress in Gd_0.4Dy_0.6PO₄ may be partially attributable to the substantial lowering of the transformation pressure.

Hexagonal-diamond carbon, otherwise known as lonsdaleite, is predicted to have exceptional physical properties such as extreme hardness, potentially exceeding that of diamond. Thus understanding the extreme conditions of pressure and temperature that lead to the formation of lonsdaleite is a topic of much interest.

Here we report the synthesis of a stable, recoverable form of almost pure lonsdaleite grown via a strain induced mechanism under static compression in a diamond-anvil-cell, at a temperature much lower than previously thought possible. This is the first successful direct conversion of glassy carbon to any form of lonsdaleite that is retrievable at ordinary laboratory conditions.

The material was analysed using x-ray and electron diffraction, and high resolution electron microscopy. Our results show that the material has a unique structure, distinct from that of cubic diamond, and that it has the correct orientational relationship relative to the graphitic planes of the precursor.

A low energy barrier for a progressive transformation from graphite to lonsdaleite is proposed as the underlying cause of this kinetically driven phase transformation [1], encouraged by the large shear strains present in the non-hydrostatic pressure environment inside the diamond-anvil-cell [2]. In the strain induced scenario we propose, graphene layers slide over each other, and the progressive ‘locking in’ of energetically favourable structures leads to the formation of stable lonsdaleite crystals [3].

References:
1. Xiao, P.H. and G. Henkelman, Communication: From graphite to diamond: Reaction pathways of the phase transition. Journal of Chemical Physics, 2012. 137(10).
2. Levitas, V.I. and O.M. Zarechnyy, Modeling and simulation of strain-induced phase transformations under compression in a diamond anvil cell. Physical Review B, 2010. 82(17).
3. Khaliullin, R.Z., et al., Nucleation mechanism for the direct graphite-to-diamond phase transition. Nature Materials, 2011. 10(9): p. 693-697.

We present the fabrication of molybdenum di-sulfide (MoS₂) thin films using radio frequency high vacuum techniques. Films were fabricated to achieve specific uniform thickness of 400-500 nm over SiO₂ substrate using commercial MoS₂ target, in order to evaluate crystal growth preferential direction a series of small angle x-ray with combination of both scanning electron and transmission electron microscopy characterization in high resolution was completed. Furthermore, sample was subjected to nanoindentantion for evaluation of measurements to understand the parameters of hardness and elastic moduli. The information as obtained by the mentioned fabrication and characterization techniques will allow to design of flexible templates towards development of flexible electronic devices.

The formation and growth of voids in an irradiated material significantly degrades its physical and mechanical properties. Void nucleation and growth involve discrete atomic-scale processes that, unfortunately, are not yet well understood due to the lack of direct experimental examination. Here we present in-situ atomic-scale observation of the nucleation and growth of voids in hexagonal close-packed magnesium under electron irradiation. The voids are found to first grow into a platelet shape, followed by a gradual transition to a nearly equiaxial geometry. The initial growth in length is controlled by slow nucleation kinetics of vacancy layers on basal facets and anisotropic vacancy diffusivity. The subsequent thickness growth is driven by thermodynamics to reduce surface energy. With increasing irradiation dose, some voids continued to grow while others shrank to disappear, depending on the nature of their interactions with nearby self-interstitial loops. These experiments represent unprecedented resolution and characterization of void nucleation and growth under irradiation, and might help with understanding the irradiation damage of other hexagonal close-packed materials, including Zr alloys, which have similar c/a ratio and are extensively used in nuclear reactors.

Single-phase concentrated solid-solution nickel-based alloys are of increasing interest in nuclear energy applications, where materials are subject to high-energy radiation damage. In high-energy radiation damage processes high electronic temperatures are expected; however their effects on the damage production and evolution are not well understood. We use the Two-Temperature Model for Molecular Dynamics (2T-MD) simulations to investigate the effects of the electronic excitations in high-energy cascades in Ni and Ni-based alloys. We compare quantitatively and qualitatively the produced damage under three irradiation conditions: (i) Classical MD simulations (ii) classical cascade simulations with the electronic stopping mechanism activated as a friction term, and (iii) cascades where the full two-temperature (2T-MD) model is implemented, i.e. the electron-phonon interaction is taken into account in addition to the electronic stopping. Our results indicate that the 2T-MD model results in reduced damage while it affects the defect distribution in clusters. Our findings highlight the need to include a model to describe the electronic effects locally, as they can affect the resulting damage and, therefore, the long-term performance of the material.
This work was supported by Energy Dissipation to Defect Evolution (EDDE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

Gisocarbon graphite has been long used in the core of the UK advanced gas-cooled reactors (AGRs) to provide moderation to slow down fast neutrons in order to maintain an efficient fission chain reaction, guide the flow of coolant gases and provide channels for the movement of fuel and control rods. In addtion, it functions as structural core component which is not replaceable and therefore life-limiting. Graphite is also a critical candidate material for several designs of future Gen IV reactors that will operate at much higher temperatures (~1000°C). For a reliable and trustworthy evaluation of the core integrity, to predict potential fracture and to mitigate premature failure of the graphite components, the mechanical response of graphite has to be evaluated in real time, in three-dimensions, under load, and at service-relevant temperatures. However, due to experimental limitations all the in situ 3-D tomography work up to date has been performed at ambient. These experiments can never adequately describe the mechanical behaviour and damage evolution in graphite at realistic temperatures (~650°C for current operating reactors; ~1000°C outlet temperature for Gen IV reactors).
This paper provides an update on the most recent advances made in the high temperature in situ tests of Gilsocarbon graphite using synchrotron radiation x-ray computed micro-tomography at the Advanced Light Source of the Lawrence Berkeley National Laboratory. A unique in situ ultrahigh temperature tomography rig that permits real-time investigation of damage evolution under load at temperatures up to 2000°C was adopted. Gilsocarbon specimens with dimensions similar to those from trepanned surveillance samples were tested, typically 4x4x20 mm, by a three-point bending configuration. Both plain and notched specimens were studied for strength and fracture toughness at room temperature, 650°C and 1000°C, respectively. A full X-ray tomography scan was performed at each step as the specimens were incrementally loaded. These tests have revealed for the first time the three-dimensional deformation and fracture of the nuclear graphite at temperature. In general, the strength and fracture toughness of Gilsocarbon graphite was found to increase with temperature. To understand the underlying physical mechanisms, high temperature Raman spectroscopy mapping (40x40 µm) of the graphite surface were undertaken in a hot cell (with Ar atmosphere) at the Centre for Device Thermography and Reliability, University of Bristol, at room temperature and at 800°C. We found indications that the amplitude of residual strain reduces at high temperature; this could be one of the contributing factors to the nominal high strength measured at temperature. Results will be discussed with respect to the understanding of the high temperature mechanical behaviour of nuclear reactor core graphites.

The Fe-Cr system is the basis for most of structural steels currently considered for the nuclear applications. While simple in appearance, it presents experimental and theoretical challenges that if solved would provide unique insights into the processes controlling microstructure evolution under irradiation. Experimentally, Cr is a low diffusing element in α-Fe, which precludes studying precipitation processes under thermal conditions. Because of the increased concentration of point defects, irradiation could therefore be a mean to accelerate the diffusion-controlled processes and access the low temperature regime where the α′ phase may precipitate. However, the responses of Fe-Cr alloys to irradiation appear to be particularly susceptible to the type of irradiation performed and more specifically dose rate. A series of Fe-Cr alloys were irradiated and characterized after ion and neutron irradiation under a wide range of conditions. We will discuss the role of irradiation and kinetics effects on the observed microstructures.

The development of small scale mechanical testing has opened a large number of opportunities to evaluate material properties for both ion-beam irradiated and neutron irradiated materials. Ion beams tend to have limited penetration depth in materials. Thus it is necessary to scale the mechanical test volume to that of the irradiated region. Neutron irradiated materials are difficult to handle in large volumes due to residual activity. Reducing the sample volume allows for such materials to be handled and tested in facilities that do not have large-scale hot cell capabilities. In addition, small scale mechanical testing allows for the sampling of specific regions of interest such as grain boundaries, weld zones, and areas of crack tip propagation.
While the benefits of small scale mechanical testing are well known, the techniques still need to be developed and fully understood in order to conduct meaningful tests. In this presentation, we will give an introduction to nanoindentation, micro compression testing, micro tensile testing and micro bend testing on ion-beam and neutron irradiated samples. We will cross compare the data with other small scale techniques and as well as with those obtained via large scale, macroscopic tests. This presentation will bridge the gap over different length scales and techniques, enabling quantitative statements of the materials performance. The main focus of this talk is on 304SS but other materials such as SiC/SiC composites, high nickel austenitic steel, and nickel superalloys will be introduced.

Rare-earth orthophosphate (REPO₄) ceramics exhibit incredible compositional flexibility as well as excellent thermal- and chemical-resistance, which makes them candidates for a wide array of applications including proton conductors for fuel cells, waste material storage in the nuclear field, and fiber coatings in ceramic matrix composites. REPO₄s exist in either a monazite (RE elements: 57 – 64, monoclinic crystal structure) or xenotime structure (RE elements: 65 – 71, tetragonal crystal structure) in equilibrium at ambient conditions. Xenotime compositions near the monazite/xenotime border are known to undergo pressure-induced phase transformations and some reportedly have substantially lower indentation modulus and hardness values than expected, showing anomalous unloading behavior in the form of ‘elbows’ where the slope in the load-depth data changes distinctly. In this study, we catalog the indentation behaviour of EuPO₄, GdPO₄, TbPO₄, and DyPO₄, identifying the frequency of anomalous behaviour including pop-ins, pop-outs, and elbows and measuring the mechanical properties of the materials over a range of loading rates and as a function of peak load. It is shown that elbow-type behavior is apparent in all of the examined materials surrounding the monazite/xenotime boundary, including those that are non-transforming; thus we conclude that the presence of an elbow in the indentation data is not a unique identifier of phase transformation in REPO₄s. It is also shown that the mechanical properties of all these compositions approach the modulus values predicted in simulations, provided that the samples are nearly fully dense.

A fundamental understanding of the deformation mechanisms in geological materials is needed to predict the materials behavior for such applications as CO₂ sequestration, waste storage, geothermal heat pumps, and hydraulic fracking, for example. Mica is a geologically abundant mineral group with a layered sheet-like structure that easily shears along its basal plane, and can greatly influence the mechanical properties of its host rock. Thus, understanding the deformation mechanisms of this constituent mineral group is essential in forming the fundamental understanding needed in predicting mechanical behavior of large-scale geological materials. Here we have performed the very first quantitative in-situ TEM mechanical experiments of biotite mica, with the goal of correlating deformation mechanisms to shear stress and loading direction, as well as quantitatively measuring activation and interaction energies of participating defects. Using quantitative stress/strain data, we have found that samples strained in tension parallel to the basal plane show nominally elastic behavior, with no observable dislocation motion or nucleation, until brittle failure. These nano-scale samples reach near-ideal strengths, though the measured mechanical properties, such as Young’s modulus, are comparable to those of bulk-scale mica. Our presentation will focus on our results from slip-oriented samples, as these are expected to yield a quantitative understanding of unit deformation mechanisms in this geological material.

This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND Number: SAND2016-5808 A. The authors gratefully acknowledge Dr. Khalid Hattar for the use of his equipment and facilities: the I³TEM and Hysitron PI-95.

At small length scales, the probability of encountering materials defects shrinks dramatically. In the cases of micro-samples with nearly pristine, defect-free structures, this has allowed researchers [1] to measure strength’s approaching the ideal, theoretical shear strengths of the materials: between G/20 and G/2π. The highest yet of these reported strength values is 75 GPa or G/7, which was measured on <111>-oriented diamond micro-pillars [2]. However, this was measured upon catastrophic failure of the pillars, rather than a dislocation-based plasticity event, which suggests that the fracture may have been premature. This implies that yet higher strengths may be achieved, i.e. G₁₁₁/5.5, as suggested by DFT calculations [3].
Here we investigate this possibility by optimizing conditions for plasticity using <123>-oriented diamond pillars to favor slip and using more advanced pillar fabrication methods to produce pillars of high geometric fidelity. In order to determine the possibility of a ductile-brittle transition in diamond, similar to that observed in silicon [4], high temperature microcompression testing will be used to determine the relationship between strength and temperature and the accompanying activation parameters. Furthermore, we will investigate the possible influence of different dopant species on the strength of diamond.


References
[1] L.Y. Chen, M.-r. He, J. Shin, G. Richter, D.S. Gianola, Nature Materials, (2015).
[2] J.M. Wheeler, R. Raghavan, J. Wehrs, Y. Zhang, R. Erni, J. Michler, Nano Letters, (2015).
[3] D. Roundy, M.L. Cohen, Physical Review B, 64 (2001) 212103.
[4] J. Samuels, S.G. Roberts, Proc. R. Soc. Lond. A, 421 (1989) 1-23.

This talk presents the design and development of a novel nanoindentation system that can perform reliable load-displacement measurements over a wide temperature range (from -150 to 800 °C) emphasizing the procedures required for performing accurate nanomechanical measurements. This system utilizes an active surface referencing technique comprising of two independent axes, one for surface referencing and another for indentation. The differential depth measurement technology results in negligible compliance of the system and very low thermal drift rates at high temperatures. The sample, indenter and reference tip are heated/cooled separately and the surface temperatures matched to obtain drift rates as low as 5nm/min at 800 °C. Instrumentation development, system characterization, experimental protocol, operational refinements and thermal drift characteristics over the temperature range will be presented. Extensive test results on a variety of materials will be shown. Finally, the current status and future roadmap for variable temperature nanoindentation testing will be summarized.

Copper-based high strength and high electrical conductivity nanocomposite wires reinforced by Nb nanotubes are prepared by severe plastic deformation, applied with an Accumulative Drawing and Bundling process (ADB), for the windings of high pulsed magnets. The ADB process leads to a multi-scale Cu matrix containing up to N=85⁴ (52.2 10⁶) continuous parallel Nb tubes with diameter down to few tens nanometers. After heavy strain, the Nb nanotubes exhibit a homogeneous microstructure with grain size below 100 nm. The Cu matrix presents a multi-scale microstructure with multi-modal grain size distribution from the micrometer to the nanometer range.
The use of complementary characterization techniques at the microscopic and macroscopic level (in-situ tensile tests in the TEM, nanoindentation, in-situ tensile tests under neutrons and high energy synchrotron beam) shed light on the individual roles of the microstructure and its multi-scale nature in the recorded extreme mechanical properties and strong stability in extreme environment.
[Acta Mat, 57 (2009), p3157; Acta Mat, 58 (2010), p6504; Adv. Eng. Mat., 14-11 (2012), p998].

In this work we continue our previous investigations of the severe plastic deformation on the magnetic properties of 4-f elements, with special accent on magnetic anisotropy and magnetic transformations. As it shown in [1], severe plastic deformation has a great effect on magnetic properties of 4-f elements. For instance, in Gd a significant increase of the magnetocrystalline anisotropy (up to 2 orders of magnitude) has been observed. The aim of this work is to investigate other important 4-f ferromagnetic elements Er and Ho for a change in physical properties.
The interest in this matter is far from being purely academic. High pressure torsion (HPT) is very interesting technique for designing novel functional materials. Depending on the degree of deformation, magnetic, structural or thermodynamic properties could be varied in severely deformed materials.
As it shown in [1] a significant depression of magnetic and thermodynamic properties occurs in severely deformed samples of Gd. The reason of such behavior is in a giant magnetic anisotropy induced by SPD. This unexpected phenomena drives to a new thermodynamic and magnetic properties of severely deformed Gd ribbons [1] which are inapplicable after the SPD-treatment for magnetocaloric applications without additional heat treatment procedure. The heat treatment regimes are directly connected with the degree of plastic deformation [2]. Qualitatively the same behavior observed for other 4-f elements – terbium and dysprosium [3].
In the talk we report the influence of high pressure torsion on magnetic, structural and thermodynamic properties of Er and Ho samples treated with the help of HPT technique. High pressure torsion was performed under 5 GPa with 5 complete turns at room temperature. This feature is helpful for designing novel magnetic materials (especially hard magnetic materials). Special accent is made for modifying magnetic anisotropy of the HPT treated Er and Ho metals.
Authors appreciate Russian Science Foundation grant 15-12-10008 for financing this work.

References
[1] S. V. Taskaev, M. D. Kuz`min, K. P. Skokov, D. Yu.Karpenkov, A. P. Pellenen, V. D. Buchelnikov and O. Gutfleisch, JMMM 331, 33 (2013).
[2] S. V. Taskaev, V. D. Buchelnikov, A. P. Pellenen, M. D. Kuz’min, K. P. Skokov, D. Yu. Karpenkov, D. S. Bataev and O. Gutfleisch, J. Appl. Phys. 113, 17A933 (2013).
[3] Sergey V. Taskaev, Konstantin Skokov, Vladimir Khovaylo, Dmitriy Karpenkov, Maxim Ulyanov, Dmitriy Bataev, Anatoliy Pellenen. Abstracts of MRS Spring Meeting, MD9.8.04 (2016).

Attempts to trigger ferromagnetism in diamagnetic and paramagnetic metals and alloys without changing the chemical composition must devise alternative ways to modify permanently their electronic properties. Tuning the magnetic behaviour of metals by altering their crystal structure, the ultimate responsible for their electronic configuration, may constitute a viable, yet unexplored, strategy to induce ferromagnetism in non-magnetic metals. Here we show that the lattice distorsions induced by grain refinement of pure metals such as zirconium, titanium and hafnium down to the nanocrystalline regime, via severe plastic deformation, lead to the development of new crystalline structures that exhibit room temperature ferromagnetic behaviour. More concretely, density functional theory (DFT) calculations predict that local stretching of the original hexagonal close packed lattice along specific pyramidal directions, due to the presence of internal stresses in the deformed nanostructure, gives rise to the emergence of monoclinic phases endowed with a net magnetic moment. An excellent agreement is found between DFT calculations and experimental transmission electron microscopy (TEM) observations, which provide a first evidence of the presence of the pure monoclinic crystal lattice in group IV metals.

Mechanochemistry aims at modification of chemical structures through external mechanical stress, and offers an orthogonal strategy to conventional synthetic approaches. While mechanically-induced intermetallic charge transfer reactions have been reported, electron transfer from an organic ligand to reduce the inorganic to its metallic state is relatively unknown. Here we explore pressure-driven mechanchemical reduction-oxidation (redox) reactions of a series of metal organic chalcogenide hybrid materials that resulted in formation of elemental metals, where the reactivity of the inorganic bond was found to be controlled by the arrangement the organic ligands. During hydrostatic compression, the crystalline metalorganic compound copper(I) m-carborane-9-thiolate underwent an irreversible redox reaction at ~10 GPa, forming elemental Cu. In-situ x-ray diffraction and absorption spectroscopy coupled with modeling indicate that the Cu-S inorganic core in the compound was strongly deformed under pressure, resulting in the charge separation on Cu and S atoms that drove forward the redox reaction. During such pressure-driven reactions, the relative positions and orientations of the reactant atoms as well as their surrounding ligands would be expected to affect their reactivity under pressure due to geometric constraints. In this case, the carborane ligands had accessible space during deformation, allowing the Cu-S bond to strain and react. Conversely, a similar compound, copper(I) adamantylthiolate, with a different geometric Cu-S bond and ligand arrangement precluded ligand motion, and showed no reactivity under pressure up to 20 GPa. This demonstrates that mechanochemical reactions can be controlled by tailoring the steric environment around the reactive bond, rather than simply the bond itself. The pressure-driven, steric-controlled redox reaction was applicable to several different transition metal organic chalcogenides with bulky organic groups such as silver adamantyl-, diamantyl- and carboranethiolates. Our discovery provides a previously unexplored model system for pressure-driven reactions, and can provide new insight in the mechanisms of chemical bond activation through mechanical stress.