Microstructural evolution and solidification behavior of (CoCrNi)_100-x Nb _x medium-entropy-alloys

In this work (CoCrNi)(100-x )Nb- x (x = 0-23.08 at.%) medium-entropy alloys were designed to investigate the solidification process and principle. The solid solubility of Nb in CoCrNi equiatomic MEA was discovered to be less than 0.46 at.%, and a fully eutectic structure was obtained at Co28.9Cr28.9Ni28.9Nb13.3 alloy. Fcc and Laves phases are the main component phases in the (CoCrNi)(100-x )Nb- x alloys. With the increasing Nb content, the volume fraction of fcc decreased, accompanied by the increment of Laves phase. Based on CALPHAD and experimental results, the top-left corner of the (CoCrNi)Nb pseudo binary phase diagram was achieved. Hypoeutectic (CoCrNi)(100-x )Nb- x (x < 13.3 at.%) alloys undergo the following solidification process: at first, primary dendrites with fcc structure nucleate and grow in the melt, then, eutectic reaction occurs and a mixture of Laves and fcc eutectic structure is obtained. Conversely, for hypereutectic (CoCrNi)(100-x )Nb- x (x > 13.3 at.%) alloys, the primary dendrite of Laves phase form, followed by the eutectic reaction of L -> fcc + Laves.