Effect of copper phase on the deformation behavior of CoCrCuFeNi high-entropy alloys studied by in situ TEM tensile tests

This study investigates the influence of the Cu phase on the deformation mechanisms of a CoCrCuFeNi high-entropy alloy (HEA) through in situ transmission electron microscopy tensile testing. Micromechanical tests were performed on lamellae with a tailored FCC HEA/Cu/FCC HEA sandwich structure, featuring a coarse Cu grain encapsulated by the FCC solid-solution matrix. Our results demonstrate that the Cu phase is the predominant site for deformation localization and the primary origin of failure. Direct observations revealed dislocation activity and necking within the Cu phase, culminating in transgranular fracture. The measured ultimate tensile strength of the lamellae (360–450 MPa) was significantly lower than that reported for the bulk alloy, highlighting the detrimental effect of the soft Cu phase on the overall strength. These findings conclusively identify the Cu phase itself—rather than the FCC matrix or their interfaces—as the most vulnerable component in the CoCrCuFeNi HEA under tensile load.