Liquid phase separation and rapid dendritic growth of high-entropy CoCrCuFeNi alloy

The liquid state stability of high-entropy alloys (HEAs) are usually taken for granted, because the large configurational entropy for equiatomic multicomponent mixtures of metallic elements can even stabilize disordered solid-solution phases. Here we show that liquid quinary CoCrCuFeNi alloy displays a metastable phase separation phenomenon when it is undercooled beyond a critical degree of 223 K. Indeed the high entropy of mixing favours the liquid state undercoolability so that a maximum undercooling up to 381 K (0.23T_L) is achieved. The positive mixing enthalpy of adding Cu into other four elements to form the quinary alloys and their poor affinity account for the liquid immiscibility. Meanwhile, the dendritic growth of high-entropy face-centered cubic (HEF) phase always dominates the crystallization process and attains a crystal growth velocity as high as 6.59 m/s. Furthermore, the separated Cu-rich phase occupies only a minor volume fraction and dissolves much more Ni than the other three elements.