Synthesis of A2 and B2 Phases in AlxCoCrFeNi Multi-component System

The two phases of the AlxCoCrFeNi multi-component system, A2 (Im3m, bcc) and B2 (Pm3m, primitive) do not appear as single phases in the entire range studied to date (0 < x < 3). To measure the thermochemical properties of alloys and improve the accuracy of thermodynamic models, single phases need to be synthesized and characterized. Toward this end, we first study the mixture of A2 and B2 phases in the Al2.75CoCrFeNi multi-component alloy. After determining the composition of each phase, we attempted to synthesis each individual phase. The "B2" alloy is rich in Al, Ni, and Co and consists of a single primitive structure with a nonuniform composition. The "A2" alloy is a Cr- and Fe-rich alloy of compositional uniformity on the microscale, but that phase separates on the nanoscale into Al-Ni-Co-rich, nanometer-sized B2 precipitates in an A2 matrix rich in Cr and Fe. This result reveals the profound impact of Al on the stabilization of the B2 phase in the AlxCoCrFeNi system. Although the B2 alloy is adequate for thermochemical study, further efforts are required to synthesize an A2 alloy. In this case, the Al content should be less than 10 at.% and the Ni content should be about 1 at.%. These low contents of Al and Ni should prevent the formation of stable "Al-Ni-like" B2 ordered intermetallics.