Near-Earth asteroid, impact hazards, impact risk response, monitoring and early warning, on-orbit disposal, international cooperation

The behavior of M₂SC (M = Ti, Zr, Hf) under tensile and shear deformations was investigated using the plane-wave pseudopotential density functional method. The microscopic mechanism that determines the structural stability is studied using the results of electronic structure calculations. It is shown that the structural failure of M₂SC can be ascribed to the breakage of weak covalent M – S bond under uniaxial tensile tension, and the plasticity of these ternary materials can be triggered by electronic instabilities at finite shear deformations. Layered structural stability of M₂SC is determined by the strength M – S bond under tensile tension, which is less resistive to shear deformation than to tensile strain. The stress-strain relationships of these ternary materials are presented and compared with corresponding binary carbides. Our study demonstrates that the ideal shear strength for M₂SC is limited by the electronic instabilities which in turn lead to elastic instability. M₂SC ceramics are predicted to be intrinsically brittleness in nature based on low bulk-modulus-to-shear-modulus ratios and negative Cauchy pressure ($C_{12}-C_{44}$ ). We propose that the brittleness of M₂SC originates from larger movement of a dislocation in a glide plane and this can be quantized by larger Peierls stress, which is in turn initiated by novel layered structure for M₂SC ceramics.