A unified model for yield strength and plastic behavior of nanovoid evolution in tungsten based on molecular dynamics simulations

Abstract

Void evolution is closely related to the damage and failure of materials under dynamic conditions, such as high strain rate. The rearrangement of atoms on the entire void surface results in microscopic plastic behaviors like dislocation emission and twinning on the surface and further macroscopic plastic flow nearby. In this work, molecular dynamics simulations are conducted to study the compression flow stress and the corresponding plastic behavior of nanovoid inside monocrystal tungsten as a function of crystallographic orientation, void size, and strain rate. Colorful plastic deformation mechanisms, like shear loops, prismatic dislocation loop, twinning, amorphization, etc., are observed at different strain rates and for compression along various crystallographic orientations. A Kelvin tetrakaidecahedron, formed before yielding, is utilized to determine the critical shear stress for initiation of plastic deformation on void surface within the framework of yield criterion and facture mechanics. A unified model is thus proposed based on the macroscopic hardening effect raised by plastic strain and strain rate, and the microscopic plastic behavior of dislocation emission related to the crystallographic orientation and nanovoid size. The newly proposed unified equation gives a good recount on the yield strength and plastic behaviors obtained from molecular dynamics simulations, but also shed lights on the future works of crack members, inclusions, voids, and porous materials at extremely high strain rate.