Ordering in the Solid Solution U(Al,Si)3

UAl₃ - USi₃ equilibrium phase diagram has already been analyzed by the present authors in a previous work based in a set of ab initio calculated energies for volume relaxed ordered structures. We evaluate in this work the formation energy of the disordered phase U(Al,Si)₃ through a cluster expansion method from ab initio calculated total energies for full relaxed ordered structures. Total energies were calculated through a DFT based FP LAPW method in the GGA approximation, including scalar relativistic corrections and implemented in the WIEN2K code. Cluster expansion and Monte Carlo simulation for finite temperatures were performed with the aid of the program package ATAT. Ionic relaxation does not introduce variations neither in the transition temperatures nor in the general morphology of the equilibrium phase diagram. However, it allows the stabilization of a wide zone of ordering of the solid solution around the U₂Al₃Si₃ composition, and an ordering at low temperatures at U₂AlSi₅ composition.