Molecular Dynamics Study On The Influences Of Crystal Defection On Mechanical Behaviors Of Tungsten

Defections are common in crystal materials, The research of the influence of defect on physical and mechanical properties of nano materials is of great significance. In order to study the effects of crystal defects on the mechanical properties of nanocrystalline tungsten, the mechanical behavior of crystal defect(e.g., dislocations, twin crystal, grain boundary) was analyzed by means of molecular dynamics simulations in this paper. The movement of dislocation in tungsten, the mechanism of twin nucleation and propagation as well as the mechanical behavior of bicrystal were studied in this paper.This study begins with the establishment of edge dislocation model based on the linear elastic displacement field of dislocations, then the model’s rationality has been verified through the distribution of the field of theoretical stress. Dislocations are forced to move by uniaxial tension so that their motion mode could be observed, The Peierls stress under 0K temperature is measured to be 4.14 GPa while the critical shear stress for dislocation movement under 298 K was measured as 2.98 GPa. Edge dislocation line is found to move, through sliding and broadening alternately under 0K, while to move in a zig-zag way through kink under 298 K. As the temperature falls, the capacity of forming kink of edge dislocation will reduce, the slipping of dislocation will be difficult, thus the brittleness of tungsten single crystal will increase.In further steps, the deformation process of tungsten nanocrystalline containing a single dislocation under compressive stress has been studied. The results show that, under different temperature, deformation of the crystals were all characterized by the process of “dislocation slipping- material hardening- twin deformation ” when the compressive stress increases. Deformation twins nucleate from the step on the surface created by dislocation slipping, with twin parameter <111>/{121}. Under 77 K, 298 K and 800 K, the yield strength of crystal are 61.1GPa, 52.54 GPa and 35.97 GPa. Compared with tungsten single crystal without dislocation, yield strength are decreased by 18.61 GPa, 4.01 GPa and 2.52 GPa under corresponding temperature. Dislocations tend to exert greater impact on promoting deformation and therefore improve the toughness of tungsten under lower temperatures.In the final part of this paper, the effects of temperature, grain boundary orientation and crystal orientation on the compressive deformation of twin tungsten nanopillar are discussed. It is proved that, under both 298 K and 800 K, in the compressing process of nanopillar with a <110> 90° twist boundary, the twin deformation nucleated from areas of stress concentrating dominates the plastic deformation. Twinning proceeds continuously through the movement of twin dislocation a/6[111?]. Under 298 K, twinning is only found at both loading ends while under 800 K twinning also nucleates at the grain boundary and the <110>90° twist boundary is tilted by 35.3° for the effect of twinning shear. Grain boundary orientation also affects the deformation process. When grain boundary is perpendicular to loading, twinning occur in the grains at both sides of the boundary, but when grain boundary is parallel to loading, twinning nucleated in the grain with larger Schmid factor first; then, twinning and sliping are induced in the opposite grain for the mismatch of the elastic strain across the boundary. During the research on the compressing deformation of Σ3(1?12)symmetrical tilt boundary of the twin tungsten nanopillar, the instability from bending at the boundary is observed at early stage. Fracture takes place consequently where cracking occur with a period of twinning.