Melt separation phenomena in CoNiCuAlCr high entropy alloy containing silver

The effects of Ag additions to AlCoCrCuNi or AlCoCrCuFeNi high entropy (HE) alloys are described. It is shown that Ag additions to these HE alloys results in liquid phase separation (LPS) into two melts, namely, an AlCoCr(Fe) rich liquid (L1) and a second liquid enriched in Ag and Cu (L2). Further, since the L2 has a higher density and a lower melting temperature, it tends to flow to the bottom of the arc melted buttons due to liquid flow (a) along the outer surface of the unmelted L1 and (b) through the interdendritic channels. In addition to the primary macroscopic LPS, secondary (within the different primary liquids) melt separation was observed in the two melts: the upper L1 layer contained L2 spheres which tended to sink to the fusion line while the L2 melt layer found at the bottom of the buttons contained L1 spheres which tended to float to the top of the L2 layer. The L1 microstructure and microhardness resembles that of an Ag-free AlCoCrCuFeNi HE alloy. In spite of the melt separation, relatively simple microstructures were obtained from the two different liquids; these are described in light of the current understanding of this class of alloys. Finally, the possibility of metastable LPS in a Cu-rich HE alloy using electron beam surface melting, which is known to produce significant dynamic undercoolings on the order of 150 °C, was investigated but was not observed.