Effect of A mixing on elastic modulus, cleavage stress, and shear stress in the Ti3(SixAl1−x)C2 MAX phase

Abstract

Solid solution MAX phases offer the opportunity for further tuning of the thermomechanical and functional properties of MAX phases, increasing their envelope of performance. Previous experimental results show that the lattice parameters of ${\mathrm{Ti}}_{3}({\mathrm{Si}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}){\mathrm{C}}_{2}$ decrease, while the Young's modulus increases with increased Si content in the lattice. In this work, we present a computational investigation of the structural, electronic, and mechanical properties of ${\mathrm{Ti}}_{3}({\mathrm{Si}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}){\mathrm{C}}_{2}$ $(x=0$, 0.25, 0.5, 0.75, and 1). The solid solutions were modeled using special quasirandom structures (SQS) and calculated using density functional theory (DFT), which is implemented in the Vienna ab initio simulation package (VASP). The SQS structures represent random mixing of Al and Si in the A sublattice of 312 MAX phase and their structural, electronic, and mechanical properties were calculated and compared with experiments. We study the cleavage and slip behavior of ${\mathrm{Ti}}_{3}({\mathrm{Si}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}){\mathrm{C}}_{2}$ to investigate the deformation behavior in terms of cleavage and shear. It has been shown that the cleavage between M and A layers results in increasing cleavage stress in ${\mathrm{Ti}}_{3}({\mathrm{Si}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}){\mathrm{C}}_{2}$ as a function of Si content in the lattice. In addition, the shear deformation of hexagonal close packed ${\mathrm{Ti}}_{3}({\mathrm{Si}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}){\mathrm{C}}_{2}$ under $\ensuremath{\langle}2\overline{1}\overline{1}0\ensuremath{\rangle}\left\{0001\right\}$ and $\ensuremath{\langle}0\overline{1}10\ensuremath{\rangle}\left\{0001\right\}$ results in increasing unstable stacking fault energy (USFE) and ideal shear strength (ISS) in ${\mathrm{Ti}}_{3}({\mathrm{Si}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}){\mathrm{C}}_{2}$ as the system becomes richer in Si.