Molecular dynamics simulations of grain interactions in shock-compressed highly textured columnar nanocrystals

Abstract

While experimental and computational studies abound demonstrating the diverse range of phenomena caused by grain interactions under quasistatic loading conditions, far less attention has been given to these interactions under the comparatively dramatic conditions of shock compression. The consideration of grain interactions is essential within the context of contemporary shock-compression experiments that exploit the distinctive x-ray diffraction patterns of highly textured (and therefore strongly anisotropic) targets in order to interrogate local structural evolution. We present here a study of grain interaction effects in shock-compressed, body-centered cubic tantalum nanocrystals characterized by a columnar geometry and a strong fiber texture using large-scale molecular dynamics simulations. Our study reveals that contiguous grains deform cooperatively in directions perpendicular to the shock, driven by the gigapascal-scale stress gradients induced over their boundaries by the uniaxial compression, and in so doing are able to reach a state of reduced transverse shear stress. We compare the extent of this relaxation for two different columnar geometries (distinguished by their square or hexagonal cross-sections), and quantify the attendant change in the transverse elastic strains. We further show that cooperative deformation is able to replace ordinary plastic deformation mechanisms at lower shock pressures, and, under certain conditions, activate new mechanisms at higher pressures.

Crystal plasticity finite element simulation of lattice rotation and x-ray diffraction during laser shock-compression of Tantalum

“…The amount of rotation is related to the discrepancy in slip between the two systems, since slip on those systems generates rotation in mutually opposing directions. During the elastic phase of the loading, there is a very small discrepancy in resolved shear stress between the two systems, resulting from two main sources: (1) elastic interactions at boundaries between neighbouring grains [32], and (2) perturbations in the initial alignments the grains from perfect [110] alignment. With Model 2, the plastic relaxation rate near the nucleation threshold is a very rapidly varying function of the resolved shear stress.…”

“…A critical resolved shear stress of 1.48 GPa for spontaneous heterogeneous dislocation nucleation in Tantalum has been suggested. The mechanical twin generation rate may also depend on initial material microstructure: MD simulation of nanopolycrystalline fibre-textured tantalum have shown a tendency for twins to nucleate from grain boundaries [32].…”

Slip competition and rotation suppression in tantalum and copper during dynamic uniaxial compression

“…To study plasticity-induced rotation in tantalum and copper under dynamic loading conditions, we perform classical molecular dynamics (MD) simulations with the open-source code lammps [29]. Atomic interactions in tantalum are modeled using the Ravelo Ta1 potential [30], which successfully reproduces the equation of state, elastic constants, Hugoniot particle velocities [31], and several other high-pressure properties [32][33][34] of tantalum in the megabar regime, and has come to be widely used in atomistic studies of tantalum under extreme loading conditions [14,16,[35][36][37][38][39][40]. For copper, we use the wellestablished Mishin potential [41], which has been used extensively in simulations of shock-loaded copper [42][43][44][45][46][47], and is thus well-characterized in the high-pressure regime of interest here.…”

“…The first of the three cases we consider is body-centered cubic (bcc) tantalum compressed along its [101] direction. Tantalum has garnered considerable interest from the dynamic compression community [14,16,30,39,40,[51][52][53][54][55][56][57] in part due to its high phase stability along the Hugoniot. Under shock, tantalum is thought to retain its ambient bcc structure until it shock-melts at pressures of around 300 GPa [14].…”

“…The remaining shear strains must therefore be relieved either via interactions between neighboring grains (a possibility explored in Ref. [39]), or by activity on additional slip or twin systems. The latter scenario is intriguing, because it would suggest that the crystals studied by Wehrenberg et.…”