Molecular dynamics simulations of grain interactions in shock-compressed highly textured columnar nanocrystals
Molecular dynamics simulations of grain interactions in shock-compressed highly textured columnar nanocrystals
复制标题
DOI：
10.1103/physrevmaterials.3.083602
复制
发表时间：
2019-08
影响因子：
3.4
通讯作者：
P. Heighway;D. McGonegle;N. Park;A. Higginbotham;J. Wark
中科院分区：
文献类型：
--
作者：
P. Heighway;D. McGonegle;N. Park;A. Higginbotham;J. Wark
虽然大量的实验和计算研究证明了准静态加载条件下颗粒相互作用引起的各种现象，但在相对剧烈的冲击压缩条件下，对这些相互作用的关注要少得多。在当代冲击压缩实验的背景下，考虑颗粒相互作用是必不可少的，这些实验利用高纹理(因此强各向异性)目标的独特X射线衍射图案，以询问局部结构演变。我们用大规模的分子动力学模拟研究了冲击波压缩、体心立方钽纳米晶中的颗粒相互作用效应，该纳米晶具有柱状结构和强纤维织构。我们的研究表明，在单轴压缩引起的边界上的千兆尺度应力梯度的驱动下，相邻的颗粒在垂直于激波的方向上协同变形，从而能够达到横向剪应力减小的状态。我们比较了两种不同的柱状几何形状(由它们的正方形或六角形截面区分)的松弛程度，并量化了伴随而来的横向弹性应变的变化。我们进一步证明，在较低的冲击压力下，协同变形能够取代普通的塑性变形机制，在一定条件下，在较高的冲击压力下，能够激活新的机制。
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.