First-principles phase stability calculations of pseudobinary alloys of (Al,Zn) ₃Ti with L1 ₂, D0 ₂₂, and D0 ₂₃ structures

The thermodynamic and mechanical stability of intermetallic phases in the Al ₃Ti-Zn ₃Ti pseudobinary alloy system is investigated from first-principles total energy calculations through electronic density-functional theory within the generalized gradient approximation. Both supercell calculations and sublattice-cluster-expansion methods are used to demonstrate that the addition of Zn to the Al sublattice of Al ₃Ti stabilizes the cubic L1 ₂ structure relative to the tetragonal D0 ₂₂ and D0 ₂₃ structures. This trend can be understood in terms of a simple rigid-band picture in which the addition of Zn modifies the effective number of valence electrons that populate bonding and anti-bonding states. The calculated zero-temperature elastic constants show that the binary end members are mechanically stable in all three ordered phases. These results point to a promising way to cost effectively achieve the stabilization of L1 ₂ precipitates in order to favor the formation of a microstructure associated with desirable mechanical properties.