Excellent combination of compressive strength and ductility of (CoCrFeNi)_x(Co_0.26Cr_0.07Fe_0.16Ni_0.31Hf_0.4) high-entropy alloys

Instead of conventional CoCrFeNiHf_x high-entropy alloys (HEAs), we investigated the evolution of microstructure and compressive mechanical properties of (CoCrFeNi)_x(Co_0.26Cr_0.07Fe_0.16Ni_0.31Hf_0.4) HEAs with varying x. Increasing x from 0.5 to 1.0, the microstructure changes from hyper-eutectic ones consisting of primary Laves phase and eutectic structure (x < 0.7), firstly to eutectic ones consisting of alternative FCC and Laves phase (x = 0.7 and 0.8), and finally to hypo-eutectic ones consisting of primary FCC phase and eutectic structure (x > 0.8). Interestingly, two different morphologies of eutectic microstructures are observed when x = 0.7 and 0.8 probably due to different fusion entropies of each phase. Increasing x from 0.5 to 1.0 reduces the yield strength from 1661 ± 64 to 688 ± 17 MPa but enhances the compressive ductility due to an increased fraction of soft FCC phase from ~0.39 to ~0.74. The second phase strengthening is the main strengthening mechanism. Importantly, when x = 0.9 and 1.0, these two alloys are not fractured when the strain reaches 0.5, one of which shows a large yield strength of 1028 ± 45 MPa (x = 0.9). Compared with conventional CoCrFeNiHf_x, the studied HEAs exhibit a much better combination of compressive strength and ductility.