Eric Nicholas Hahn

To reveal the relationship between atomic-scale activity and bulk materials properties, we report all-atom mo-lecular dynamics (MD) simulations of the deformation and dynamic failure of mono-and nanocrystalline silicon carbide (SiC) ceramics. We establish a direct link between the reversibility of defects and dynamic tensile strength of a nominally brittle ceramic over a wide range (six orders of magnitude) of strain rates, bridging the simulation regime to current experimental capabilities that enable the observation of lattice dynamics over extremely short timescales. Our results reveal that SiC exhibits a highly reversible deformation twinning mechanism in response to loading along the [001] crystal direction below a critical compression strain. The remarkable reversibility of the active defects allows the crystal to retain its high strength. Beyond a critical strain, the process becomes unstable, and self-activated twin boundary motion is triggered, yielding irreversibly intertwined defects, resulting in a significant reduction in strength.

The elastic-plastic and structural phase transitions of silicon carbide (SiC) under compression loading are investigated using large-scale molecular dynamics simulations. The ramp wave compression applied to single crystal 3C-SiC uses ramp rise times from 10 to 100 ps. Wave-free quasi-isentropic compression loading is also applied with strain rates varying from 10(8) to 10(11) s(-1). Loading is performed along three crystal orientations: [001], [110], and [111]. The effects of different ramp rise time and compressive strain rates on material response are characterized, with special attention payed to anisotropy. SiC under increasing strong ramp compression displays elastic, plastic, and structural phase transition responses. Results show that the plastic deformation and phase transition are strongly strain-rate dependent. With increasing strain rate, the threshold strain and longitudinal stress for deformation twinning is anisotropically increased. The threshold longitudinal and shear stresses triggering plasticity are lowest in [001] direction, followed by [110], and highest in [111] SiC when subjected to the same strain rate. The threshold pressure for the structural phase transition from zinc blende (ZB) to rock-salt (RS) structure increases with the applied strain rate. As a result, the transition from ZB to RS structure is incomplete and inhomogeneous mixed-phase structures are observed over a wide range of applied stresses, even up to similar to 180 GPa, which agrees well with experimental observations.

Shock induced plasticity, structural phase transitions, as well as dynamic failure in nanocrystalline SiC ceramics, with grain sizes varying from similar to 2 to similar to 32 nm, are investigated systematically using large scale molecular dynamics simulations. Shock particle velocities are varied from 1 to 5 km/s in order to study elastic and plastic behavior. Multiple non-monotonic grain-size dependent mechanical properties of nanocrystalline SiC are elucidated. Deformation twinning identified at U-p = 2 km/s is reduced with decreasing grain size with a breakdown between d(G) = 6 to 10 nm. Statistics from grain size effects on the phase transformation from Zinc-Blend to Rock-Salt structure at different particle velocities are obtained. The characteristics of failure shift from classical spall to micro-spall as U-p is increased from 1 to 5 km/s. Spall strengths are evaluated by an indirect free-surface method, akin to experimental measurements, and a direct method evaluating the atomic stress tensor at the point of spallation. Differences between the two methods at high strain rates are discussed in detail. The direct method provides a measure of ultimate spall strength, while the indirect method shows pronounced agreement with the nucleation stress. An unexpected grain size dependence of the tensile strengths is also identified, which is similar to a theoretically predicted trend in nanoscale systems. Our results provide new support to the grain size dependence of mechanical properties of nanocrystalline system at high strain rates, which could benefit the design of nanocrystalline ceramics. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Understanding the effect of grain boundaries (GBs) on the deformation and spall behavior is critical to designing materials with tailored failure responses under dynamic loading. This understanding is hampered by the lack of in situ imaging capability with the optimum spatial and temporal resolution during dynamic experiments, as well as by the scarcity of a systematic data set that correlates boundary structure to failure, especially in BCC metals. To fill in this gap in the current understanding, molecular dynamics simulations are performed on a set of 74 bi-crystals in Ta with a [110] symmetric tilt axis. Our results show a correlation between GB misorientation angle and spall strength and also highlight the importance of GB structure itself in determining the spall strength. Specifically, we find a direct correlation between the ability of the GB to plasticity deform through slip/twinning and its spall strength. Additionally, a change in the deformation mechanism from dislocation-meditated to twinning-dominated plasticity is observed as a function of misorientation angles, which results in lowered spall strengths for high-angle GBs.

Large-scale molecular dynamics simulations are used to investigate shock-induced damage and fracture in 3C-SiC single crystals at an elevated initial temperature of 2000 K and a high tensile strain rate of similar to 10(10) s(-1). Three crystal orientations have been evaluated: [001], [110] and [111]. A comprehensive comparison has been made between cases at 2000 K and at 300 K to address the effects of high temperature on the mechanical performance of SiC under shock loading. Results show that for shock compression, the high temperature decreases the longitude elastic wave speeds as well as the shock stresses. The shock-induced plasticity is mainly in the form of deformation twinning at 300 K, but twinning is absent at 2000 K. The high temperature decreases the structural phase transition threshold pressure in SiC from similar to 90 GPa at 300 K (for all three orientations) to similar to 75 GPa in [001], similar to 57 GPa in [110] and similar to 64 GPa in [111] at 2000 K, with corresponding particle velocities of 2.75 km/s, 2.0 km/s, and 2.25 km/s, respectively, in agreement with trends observed in recent experiments. The spall fracture behavior reveals that high temperature reduces the spall strength with an average spall strength of similar to 20.7 GPa in [001], similar to 21.4 GPa in [110] and similar to 22.5 GPa in [111] at 2000 K in the classical spall regime, which are about 33% lower than strengths measured at 300 K. However, in the micro-spall regime the spall strengths are very similar at both temperatures. The corresponding thresholds of particle velocity to trigger spall decrease at elevated temperature except for [001] loading, as well as the thresholds for generating overdriven phase transition waves. (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

We examine the effect of grain size on the dynamic failure of tantalum during laser-shock compression and release and identify a significant effect of grain size on spall strength, which is opposite to the prediction of the Hall-Petch relationship because spall is primarily intergranular in both poly and nanocrystalline samples; thus, monocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultra-fine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra - and intergranular in the poly and ultra-fine-grained crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is a strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. The length of geometrically-necessary dislocations required to form a grain-boundary (intergranular) void is lower than that of grain-interior (intragranular) void with the same maximum diameter; thus, the energy required is lower. Consistent with prior literature and theory we also identify an increase with spall strength with strain rate from 6 × 106 to 5 × 107 s−1. Molecular dynamics calculations agree with the experimental results and also predict grain-boundary separation in the spalling of polycrystals as well as an increase in spall strength with strain rate. An analytical model based on the kinetics of nucleation and growth of intra- and intergranular voids and extending the Curran-Seaman-Shockey theory is applied which shows the competition between the two processes for polycrystals. [Display omitted]

While silicon carbide (SiC) has been predicted to undergo pressure-induced amorphization, the micro structural evidence of such a drastic phase change is absent as its brittleness usually prevents its successful recovery from high-pressure experiments. Here we report on the observation of amorphous SiC recovered from laser-ablation-driven shock compression with a peak stress of approximately 50 GPa. Transmission electron microscopy reveals that the amorphous regions are extremely localized, forming bands as narrow as a few nanometers. In addition to these amorphous bands, planar stacking faults are observed. Large-scale non-equilibrium molecular dynamic simulations elucidate the process and suggest that the planar stacking faults serve as the precursors to amorphization. Our results suggest that the amorphous phase produced is a high-density form, which enhances its thermodynamical stability under the high pressures combined with the shear stresses generated by the uniaxial strain state in shock compression. (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

It is generally understood that microstructure plays a significant role in determining the deformation response of materials. During shock compression, grain boundaries serve as dislocation nucleation/pile-up/adsorption sites and grain size can alter the width of the shock front. During tensile release, grain boundaries are often "weak links" where spallation occurs. As such, a current deficit in predictive modeling capability is a quantitative description of these locations and their relative ability to serve as void nucleation sites - a challenging component of such a description is that spallation is inherently stochastic in nature. The inclination of the grain boundary plane with respect to the loading direction is thought to be a critical constituent in the resultant stress and failure at the boundary. Non-equilibrium molecular dynamics simulations are used to statistically quantify the influence of grain boundary inclination on the location of void nucleation and to highlight the emergence of stress hotspots at such boundaries. Boundaries oriented perpendicular to the loading direction are more likely to fail, but grain boundary inclination alone is not a complete predictor - i.e. not all perpendicular boundaries fail during spallation.

Several factors can affect the failure stress of a grain boundary, such as grain boundary structure, energy and excess volume, in addition to its interactions with dislocations. In this paper, we focus on the influence of grain boundary energy and excess volume at the boundary on the failure stress of a grain boundary in tantalum from molecular-dynamics simulations. Flyer plate simulations were carried out for a handful of boundary types with different energies and excess volumes. These boundaries were chosen as model systems to represent various boundaries observed in "real" materials. For a small, but representative, set of boundaries explored, no direct correlation was observed between the void nucleation stress of a boundary and either its energy and excess volume. This result suggests that average properties of grain boundaries alone are not sufficient indicators of the failure strength of a boundary.

Strain rate, temperature, and microstructure play a significant role in the mechanical response of materials. Using non-equilibrium molecular dynamics simulations, we characterize the ductile tensile failure of a model body-centered cubic metal, tantalum, over six orders of magnitude in strain rate. Molecular dynamics calculations combined with reported experimental measurements show power-law kinetic relationships that vary as a function of dominant defect mechanism and grain size. The maximum sustained tensile stress, or spall strength, increases with increasing strain rate, before ultimately saturating at ultra-high strain rates, i.e. those approaching or exceeding the Debye frequency. The upper limit of tensile strength can be well estimated by the cohesive energy, or the energy required to separate atoms from one another. At strain rates below the Debye frequency, the spall strength of nanocrystalline Ta is less than single crystalline tantalum. This occurs in part due to the decreased flow stress of the grain boundaries; stress concentrations at grain boundaries that arise due to compatibility requirements; and the growing fraction of grain-boundary atoms as grain size is decreased into the nanocrystalline regime. In the present cases, voids nucleate at defect structures present in the microstructure. The exact makeup and distribution of defects is controlled by the initial microstructure and the plastic deformation during both compression and expansion, where grain boundaries and grain orientation play critical roles. [Display omitted]