Deformation modes and ideal strengths of ternary layered Ti2AlC and Ti2AlN from first-principles calculations

Deformation and failure modes were studied for Ti2AlC and Ti2AlN by deforming the materials from elasticity to structural instability using the first-principles density functional calculations. We found that the TiC0.5/TiN0.5 slabs remain structurally stable under deformations, whereas the weak Ti-Al bonds accommodate deformation by softening and breaking at large strains. The structural stability of the ternary compound is determined by the strength of Ti-Al bond, which is demonstrated to be less resistive to shear deformation than to tension. The ideal stress-strain relationships of ternary compounds are presented and compared with those of the binary materials, TiC and TiN, respectively. For Ti2AlC and Ti2AlN, their ideal tensile strengths are comparable to those of the binary counterparts, while the ideal shear strengths yield much smaller values. Based on electronic structure analyses, the low shear deformation resistance is well interpreted by the response of weak Ti-Al bonds to shear deformations. We propose that the low shear strengths of Ti2AlC and Ti2AlN originate from low slip resistance of Al atomic planes along the basal plane, and furthermore suggest that this is the mechanism for low hardness, damage tolerance, and intrinsic toughness of ternary layered carbides and nitrides.