Evolution of crystal structure in high-entropy AlCoCrFeNi alloy: An in situ high-temperature X-ray diffraction study

A powder of uniform high entropy alloy AlCoCrFeNi was produced by relatively short-term (90 min) mechanical alloying in a planetary mill followed by annealing at 873, 1073 and 1273 K for 5.5 h. Results of high-temperature in situ XRD analyses demonstrated the occurrence of specific crystal lattice transformations taking place in major fcc and bcc phases during annealing and cooling down. Shrinkage of atomic structures was detected for all phases after annealing at any temperature, while precipitation of the σ-phase occurred only at 1073 K. A drift of the (111) peak relative to the (200) peak of the fcc phase at 1273 K allowed us to assume the occurrence of weak martensitic transformations: a high-temperature cubic phase turns into slightly tetragonal phase (a/c = 1.00426) upon cooling down to room temperature. This transformation takes place without mechanical stress or deformation. Despite structural transformations, the major high-entropy phases remained after annealing.