First-principles investigations of the thermodynamics of binary alloys using a cluster expansion have so far neglected the presence of vacancies. Here, a local cluster expansion as a perturbation to the standard binary cluster expansion was used to model the equilibrium vacancy concentration in a binary alloy as a function of temperature and alloy composition. This approach was applied to a first-principles investigation of the face-centered cubic Al1–xLix alloy (for x-values less than 0.3) which, at x = 0.25, exhibited L12 superstructure ordering. The equilibrium vacancy concentration was predicted to be sensitive to the bulk alloy composition x in the ordered L12 phase, varying by more than an order of magnitude in a narrow interval of x at intermediate temperatures. In both the solid solution and in the ordered L12 phase, the vacancy preferred a nearest-neighbor shell rich in Al. In the L12 ordered phase, the vacancy predominantly occupied the Li sub-lattice. The type of short-range order around a vacancy was expected to affect the mobility of the constituents of the alloy and therefore its interdiffusion coefficient.

Vacancies in Ordered and Disordered Binary Alloys Treated with the Cluster Expansion. A.Van der Ven, G.Ceder: Physical Review B, 2005, 71[5], 054102 (7pp)