Theoretical Calculation Of Mechanical Properties Of Lonsdaleite

Covalent material is a very important structural material.Because of its excellent mechanical properties,it is widely used in aerospace,transportation and engineering construction.As is known to all,diamond is the hardest material in nature with high hardness and many excellent properties.However,over the past decade,researchers have found that lonsdaleite has better mechanical properties than cubic diamond through theoretical simulations.However,the understanding of the mechanical properties of lonsdaleite is unclear,which seriously hinders the further development of superhard materials in industrial applications.Therefore,this paper systematically studies the elasticity and plasticity of lonsdaleiteFirst principles calculations were performed to investigate the elastic modulus of lonsdaleite.The results show that the bulk modulus B=431.7 GPa,shear modulus G=525.7 GPa,Young's modulus E=1121.7 GPa,Poisson's ratio v=0.067,which are similar to that of cubic diamond.In order to study the plasticity of lonsdaleite,we calculated the generalized stacking fault energy of different slip planes,and determined the types of dislocations existing on each slip plane based on the study of the stability of the stacking fault structure.The result show that there can be partial dislocation slip modes on the {0001}?{1100}?{1120}?{1122}?{1011} and {1012} slip planes.Based on the above studies,we use the molecular dynamics method to calculate the lattice friction force of each dislocation slip and establish the expression of critical resolved shear stress(CRSS)for different dislocation slip modes.The yield strength and hardness of polycrystalline lonsdaleite with different grain sizes were calculated according to the CRSSs of the dislocation slip modes and the Sachs model.The research results indicated that the hardness of lonsdaleite with a grain size of 10 nm is as high as 390 GPa,when the proportion of plastically deformed grains reaches 80%.As the grain size increases,the hardness value gradually decreases.When the lonsdaleite yields,the contribution of the basal 30° partial dislocation slip mode,the prism ? 0° prefect dislocation and the prism ? 0° partial dislocation plays a decisive role,of which the basal 30° partial dislocation slip mode contribute the most to the yield of grains.This work has certain guiding significance for the future application of lonsdaleite.