  MAX phases have attracted increased attention due to their unique combination
of ceramic and metallic properties. Point-defects are known to play a vital
role in the structural, electronic and transport properties of alloys in
general and this system in particular. As some MAX phases have been shown to be
stable in non-stoichiometric compositions, it is likely that such alloying
effects will affect the behavior of lattice point defects. This problem,
however, remains relatively unexplored. In this work, we investigate the
alloying effect on the structural-stability, energy-stability,
electronic-structure, and diffusion barrier of point defects in MAX phase
alloys within a first-principles density functional theory framework. The
vacancy (V$_{M}$, V$_{A}$, V$_{X}$) and antisite (M-A; M-X) defects are
considered with M and A site disorder in (Zr-M)$_{2}$(AA${'}$)C, where
M=Cr,Nb,Ti and AA${'}$=Al, Al-Sn, Pb-Bi. Our calculations suggest that the
chemical disorder helps lower the V$_{A}$ formation energies compared to
V$_{M}$ and V$_{X}$. The V$_{A}$ diffusion barrier is also significantly
reduced for M-site disorder compared to their ordered counterpart. This is very
important finding because reduced barrier height will ease the Al diffusion at
high-operating temperatures, which will help the formation of passivating oxide
layer (i.e., Al$_{2}$O$_{3}$ in aluminum-based MAX phases) and will slow down
or stop the material degradation. We believe that our study will provide a
fundamental understanding and an approach to tailor the key properties that can
lead to the discovery of new MAX phases.
