Stability of Zr-Al-C and Ti-Al-C MAX phases: A theoretical study
Angeliki Poulou, Thomas A. Mellan, Michael W. Finnis
Abstract
We calculate the stability of the MAX-phase materials ${\mathrm{Zr}}_{n+1}{\mathrm{AlC}}_{n}$ and ${\mathrm{Ti}}_{n+1}{\mathrm{AlC}}_{n}$ in the context of the M-A-X ternary phase diagrams and competing binary and ternary compounds, as a function of temperature, by applying density functional theory (DFT) within the quasiharmonic approximation. By examining the convex hull of free energy we find that the Zr-based MAX phases are thermodynamically unstable at room temperature and below with respect to decomposition to carbide and intermetallics, although with increasing temperature the ${\mathrm{Zr}}_{3}{\mathrm{AlC}}_{2}$ phase becomes stable. On the other hand, the pure ${\mathrm{Ti}}_{2}\mathrm{AlC}$ phase is thermodynamically stable at room temperature, consistent with the success in its synthesis.