Molecular Dynamics Study On Plastic Deformation Behavior And Influencing Factors Of Tungsten Nanocrystalline

As an important plasma facing material, tungsten is widely used in the nuclearfusion reaction apparatus. In order to lower the ductile-to-brittle transition temperatureand to improve its strength and toughness, the process of grain refinement has become amajor trend. It is no doubt that crystal defects have great effect on the performance ofthe plasma facing component. However, its exact mechanism remains to be furtherstudied. In this paper, the influence of vacancy and grain boundaries on the mechanicalproperties and deformation mechanism of nanocrystalline tungsten have been studied bymeans of molecular dynamics simulations.Firstly, lattice parameter, cohesive energy, elastic constants, vacancy formingenergy and surface energy of tungsten are calculated using two potential functions basedon embedded atom method to verify the accuracy by comparison with the experimentalvalues. The results show that the modified F-S potential is more accurate and has highercomputational efficiency, and could be used in the molecular dynamics simulation ofthis paper.Secondly, the influence of vacancy on the mechanical properties and plasticdeformation mechanism of nanocrystalline tungsten at different temperatures have beenanalyzed. The results indicate that the deformation process contains four stages, elasticdeformation, uneven plastic deformation, even plastic deformation and necking stage. Inthe initiating stage of plastic deformation, the crystal shows uniform shear deformedzone and undeformed zone exhibit mirror symmetry which is the notable feature oftwinning. The surface is embossed and the orientation of twinning i（s211）[111].At the temperature of77K and298K, the existing of vacancy produces distortioninside the crystal, hindering the shear and then the yield strength is improved. At thetemperature of800K and1000K, atoms with high energy make the crystal deformationexhibit softening effect. When the strengthening effect of vacancy couldn’t balance outthe softening effect of temperature on the yield strength, it shows lower yield strength.Moreover, yield strength has a strong temperature effect. As the temperature increasing,the yield strength decreases significantly.Finally, molecular dynamics method is used to simulate the tensile deformation ofthe nano-polycrystalline of tungsten at the temperature of298K,800K and1273K. Theresults turn out that models produce plastic deformation after the elastic deformation.When there is complex plastic deformation, the crack grows along with the grainboundary, and break at last.Analysis on the relationship between grain size and deformation stress shows mixed Hall-Petch relationship. When the grain size is refined from15.18nm to11.61nm,yield strength elevates, showing the Hall-Petch relationship. With a further refining to3.57nm, yield strength decreases significantly, showing anomalous Hall-Petchrelationship. Therefore, the critical grain size can be judged as11.61nm.Through the analysis of deformation mechanism of the grain size of3.57nm and15.18nm polycrystalline tungsten, the results show discrepancy of deformationmechanism in these two systems. To those polycrystalline tungsten systems with grainsize less than critical grain size, grain boundary rotation occurs to coordinate thedeformation between the adjacent grains. The following progresses are twinning and theemission of grain boundary dislocation, and also structural transition. When the grainsize is larger than the critical grain size, the emission of grain boundary mainly occurs.Temperature affects the thermal motion of atoms and the dislocation emission capability.Therefore, as the temperature increasing, all the polycrystalline tungsten with differentgrain size show the same tendency of yield strength decreasing.