Structure of the high-entropy alloy AlxCrFeCoNi: fcc versus bcc

The effect of Al on the crystal structures of the high-entropy alloy AlxCrFeCoNi is discussed using first-principles electronic structure calculations. When the atomic configuration is totally random, AlxCrFeCoNi has the fcc structure. However, the total energy difference between the fcc and bcc structures decreases as the Al concentration increases. In the calculations Cr and Fe stabilize the bcc structure and Ni and Co work as fcc stabilizer in the alloys, as is observed in experiments. Moreover, the interactions between Al and transition metal elements are strongly attractive. As a result, partially disordered structures such as L12, D03 and B2, where the Al atoms are ordered and the transition metal atoms are still random, are more stable than the totally disordered phases. Especially, the energy gain by the D03 structure is large and this leads to the transition from fcc to bcc for strongly increased Al concentration.