Mechanical instability and ideal strengths of layered M2SC (M = Ti, Zr, and Hf) compounds

Abstract

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 (C12−C44). 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.