Atomic Simulation Of The Mechanical Properties Of Nano-polycrystal Metal Materials

Since the first preparation of nanocrystalline materials is reported by Gletier, the adventof nanocrystalline metals opens up an exciting new field of research. The nanocrystallinemetals are being actively investigated due to their exceptional and unique mechanicalproperties compared to coarse-grained metals. So far, the studies concerned with themechanical properties and deformation behavior of nano-polycrystal metals are mainlyfocused on face-centered cubic (FCC) metals. The relative studies on hexagonal close-packed(HCP) metals still are very limited. In addition, the structure of grain boundary (GB) isgenerally considered to be an important factor in determining the plasticity, strength anddeformation mechanism of nanocrystalline metals. In order to improve the mechanicalproperties of polycrystalline metallic materials, people put forward the concept of interfacecontrol and design, and hope to improve the mechanical properties of polycrystalline metallicmaterials through special interface design in metal material. Although the study of themechanical properties of polycrystalline HCP metal has made some achievements, but thereare still many deficiencies, many phenomena remains to be further reveal. In a certain extent,this restricts the application of HCP metal and its alloy materials in aviation, aerospace andother fields of Engineering.Here, we investigate the effect of temperature, stacking fault spacing, twin spacing, grainsize and interfacial structure (twist boundary, twin boundary, stacking fault boundary,amorphous boundary) on the deformation behavior of HCP metal Mg under tension loadingusing our MD simulation codes. In addition, the effects of twist GB on the mechanicalproperties of two types of nanocrystalline Cu under tensile loading are investigated by MDmethod. For the purpose of visualizing defects in metal material, colors are assigned to atomsaccording to the local crystal structure of atoms by common neighbor analysis. This isimplemented by using the OVITO soft. The main conclusions are briefly described asfollowing:（1）The nucleation and propagation of dislocations plays a dominant role during thedeformation of nanotwinned Mg. For smaller twin boundary spacing, the Young’s modulus and yield strength of nanotwinned Mg exhibits a remarkable dependence on the twinboundary spacing, regardless of temperature. The yield strength of nanotwinned Mgdecreases with increasing temperature, regardless of twin boundary spacing. The yieldstrength of nanotwinned Mg is associated with the dislocation storage ability of material andthe repulsive force between twin boundaries and dislocations.（2）The effect of stacking fault spacing on the Young’s modulus of HCP metal Mgcontaining stacking fault is very slight, regardless of temperature. The yield strength can beobviously enhanced with decreasing stacking fault spacing and there is an optimal stackingfault spacing for mechanical properties of Mg with stacking fault. For relatively hightemperature, the formation and growth of new grain and twin become the main plasticdeformation mechanism of Mg with smaller stacking fault spacing. At low temperature, thenucleation and glide of dislocations plays a dominant role during the deformation of Mg.（3）We study the effect of grain size, temperature and stacking fault on the mechanicalproperties of nano-polycrystal Mg under tensile loading. The average flow stress ofnano-polycrystal Mg decreases with decreasing grain size, exhibiting a breakdown in theHall-Petch relation when grain size is smaller than a critical size. The results show that theGB sliding is the main deformation mechanism for nano-polycrystal Mg with a small grainsize. Furthermore, we find that the deformation behavior of nano-polycrystal Mg obviouslydepends on temperatures. The results indicate that at300.0K, the nucleation and growth oftwins are the predominant deformation mechanism for nano-polycrystal Mg, and that thenucleation and growth of twins and new grains are the predominant deformation mechanismfor nano-polycrystal Mg containing stacking fault. In other words, the stacking fault inducesthe nucleation and growth of twins and new grains. However, the dislocation nucleation andslip are the predominant modes of the plastic deformation for nano-polycrystal Mg at10.0K,regardless of stacking fault.（4）The yield strength of nanocrystalline Mg is obviously affected by the amorphousboundary spacing. The strength of the material increases with the decrease of amorphousboundary spacing, presenting a Hall-Petch effect at both10K and300K. The effect ofdeformation mechanism of nanocrystalline Mg on temperature is very obvious. A stressplatform and following stiffness softening, as well as a linear strengthening in the plastic stage are observed when the amorphous boundary spacing below8.792nm at10K. However,the second stress peak is not observed for the models at300K. Instead, the flow stress inplastic stage is a nearly constant value.（5）We investigate the effects of twist GB on the mechanical properties of two types ofnanocrystalline Cu under tensile loading. The results indicate that the plasticity ofbicrystalline Cu with a high twist angle is much better than that of structure with low twistangle. However, the plasticity of trifurcate crystal Cu is better in low twist angle structure.