Following the introduction of high-entropy alloys (HEAs) with five or more principal elements, dual-phase HEAs have recently received significant attention due to their promising mechanical properties. Theoretical simulations suggest that unique mechanical properties of these alloys arise due to the contribution of localized phase transformation and diverse microstructural behavior of two phases under plastic deformation. In this study, phase transformations and microstructural evolution in a dual-phase AlFeCoNiCu alloy is investigated experimentally during plastic deformation using the high-pressure torsion (HPT) method. The two BCC and FCC phases exhibit diverse behaviors under plastic straining. The FCC phase with low stacking fault energy forms numerous nanotwins and stacking faults and its lattice is expanded by 3 vol%. The BCC phase accumulates dislocations, and its lattice is contracted by 5 vol%. These diverse microstructural/structural evolutions, which are partly consistent with the predictions of theoretical simulations, lead to a high microhardness of 495 Hv in this dual-phase HEA.
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