Stress-induced transformation behavior in near-eutectic (AlNi2)70-xCo30Crx medium entropy alloys

Abstract The present investigation was an evaluation of the microstructure and mechanical properties in as-cast non-equiatomic medium entropy alloys (MEAs) (AlNi2)70-xCo30Crx (with x = 2.5, 10, 17.5, and 25 at.%), showing dual- or multi-phases structure (near-eutectic microstructure). The majority phase of developed MEAs was FCC (L12) phase and the minority phase was the ordered BCC (B2) phase. The t typical eutectic microstructure was formed in the matrix area of all MEAs and a little dendrite structure was identified in microscopic analysis. The dendrite structure changed from the B2 phase to the L12 phase with an increase in Cr contents. When the composition of Cr contents was 10% that consists of a fully eutectic structure, it showed the highest yield strength of 865.7 MPa together with excellent plastic strain. Interestingly, it was found that abnormal deformation behaviors were also observed in the stress-stain curve exhibiting two times of yielding phenomenon typically called “double-yielding”. Through TEM analysis, it was confirmed that the observed double-yielding phenomenon is related to the phase transformation from the B2 phase which constitutes the eutectic lamellar to L10 phases during compressive loading. In this study, we discussed detailed deformation mechanisms that can induce excellent mechanical properties of eutectic MEAs.