Microstructure and mechanical property of novel NiAl-based hypoeutectic/eutectic/hypereutectic high-entropy alloy

In this paper, we investigate the microstructures and mechanical properties of novel NiAl-MoxCrxVx, NiAl-MoyCryFey and NiAl-MozCrzVzFez eutectic high-entropy alloy (EHEA) systems. The microstructures of three alloy systems change from hypoeutectic (primary NiAl + eutectic, x = 7.5,8.33/y = 13.33,14.0/z = 9.0,9.5, hypo-EHEA) to eutectic (x = 8.7/y = 14.5/z = 10.0, EHEA) and then to hypereutectic (primary MoCrV/MoCrFe/MoCrVFe + eutectic, x = 9.0,10.0/y = 15.0,16.0/z = 10.5,11.0, hyper-EHEA). The microhardness of primary MoCrV/MoCrFe/MoCrVFe is higher than that of primary NiAl. It almost has a rule that the yield strength gradually increases from hypo-EHEA to EHEA, and then to hyper-EHEA, which is attributed to a certain amount of high-microhardness primary MoCrV/MoCrFe/MoCrVFe phase in hyper-EHEA. For the EHEAs, the microstructures of NiAl–Mo8.7Cr8.7V8.7/NiAl–Mo14.5Cr14.5Fe14.5/NiAl–Mo10Cr10V10Fe10 are composed of eutectic cells or eutectic dendrites which consist of NiAl (B2) and MoCrV/MoCrFe/MoCrVFe (BCC) lamellar phases, respectively. It is also found that the plenty of fine (Ni,Al)-rich precipitates occur in MoCrFe and MoCrVFe phases (SEM images). Moreover, according to the phase composition, it is determined that MoCrV/MoCrFe/MoCrVFe is a high entropy phase, thus possessing a better strengthening effect on NiAl. Correspondingly, three EHEAs possess the good mechanical properties, and the microhardness and the yield strength exhibit an increased trend: NiAl–Mo8.7Cr8.7V8.7 < NiAl–Mo14.5Cr14.5Fe14.5 < NiAl–Mo10Cr10V10Fe10 EHEAs. The major strengthening mechanisms are the solution strengthening and the precipitation strengthening.