Molecular Dynamics Studies On Plasticity Of Metals Under Torsion

In this paper, we have studied the evolution behaviors of twist grain boundaries(GBs) for FCC metals bulk(Al and Ni) using the molecular dynamics method. And the bimetal(Al and Ni) interface evolution phenomenon under torsion. And we also have studied the plastic deformation behaviors of BCC metal(Fe) at nanoscale. The plastic deformation mechanism of pure metal nanowires(Fe) with a twist GB is also under consideration. The main research contents and conclusions are as follows:(1) We studied the evolution behavior of twist grain boundary for Al and Ni in<100>, <110> and <111> oriented nanowires under clockwise and counterclockwise torsion, respectively. Because of the different orientations of grains, the grain boundary structure show different patterns. In <100> orientation, the initial dislocation structure of grain boundary shows a rectangular dislocation network. It is composed of two groups of dislocation lines which perpendicular to each other. In<111> orientation, the initial dislocation configuration of grain boundary shows a triangle dislocation network, which is composed of three groups of dislocation lines.In <110> orientation, metals present different dislocation structure, the dislocation structure of aluminum is a approximation rectangular dislocation network, the dislocation structure of nickel is a dislocation network composed of the zigzag dislocation lines. Under anticlockwise clockwise twist, the dislocation network begins to shrink, making the dislocation density increase; under clockwise torsion, the dislocation networks are expanding, leading to the decrease of dislocation density.Then we found that the evolution results of the twist grain boundary in two twist directions are asymmetric. We also have studied the evolution behaviors of the bimetal interface under torsion. We found that the dislocations of the Al/Ni bimetal interface are always in the interface, they did not propagate to the grain interior. But the twist grain boundary dislocations of single metal can not only propagate in the grain interface, but also can propagate to the grain interior. Compared to the two cases,we find that the bimetal interface has a certain inhibition effect on the propagation of the dislocations in the grain boundaries.(2) We have studied the plasticity of Fe nanowires under torsion in <100>,<110> and <111> orientation, respectively. And the case with a twist grain boundary is also under consideration. For defect free nanowires, in <100> orientation,dislocations generated on the surface of the nanowires and spread to the interior of the nanowires, and they gradually formed a mirrored "C" dislocation line; with the torsion continues, the dislocation lines interact with each other and eventually formed a three-dimensional dislocation network. In <110> orientation, the dislocation sliding planes formed on the surface of the nanowire gradually, with the rotation continues,the dislocation sliding planes eventually met and formed a larger sliding layer, they cut the nanowire into two parts eventually. In the <111> direction, the nanowires not only form a dislocation line of mirrored "C" shape, but also form two dislocation surfaces. For the nanowire with a twist grain boundary, in <100> orientation, the initial dislocation structure exhibits a rectangular dislocation network, which is composed of two groups of mutually perpendicular dislocation lines. In <110>orientation, the initial dislocation structure is a network of an "X" type, which is made up of four dislocation lines. In <111> orientation, the initial dislocation structure is composed of three parallel dislocation lines. Our research results show that the plastic behavior of defect-free nanowires depends on the orientation of nanowires. The plasticity of nanowire with a twist grain boundary occurs mainly centered on the grain boundary, and presents a similar trend to the grain boundary evolution behavior of FCC metal.