Order-disorder transitions in the Fe₂VAl Heusler alloy

A Rietveld analysis of neutron powder diffraction patterns obtained in situ during temperature scans shows that Fe₂VAl crystallizes at room temperature in the fully ordered L2₁ structure and transforms at 1080 °C and at 1190 °C into partially disordered B2 and fully disordered A2 variants respectively. The low temperature stability of the L2₁ structure as well as the two high temperature L2₁ → B2 → A2 transitions are theoretically predicted by a combination of ab-initio (electronic structure and phonons) and thermodynamic calculations performed on special quasi-random structures. Pronounced cold work effects and the pre-transitional antisite defects which are observed respectively at 25 °C under mechanical stress and at high temperature (above 800 °C), appear to be generic effects in the family of Heusler alloys.