Effect of Adding Minor Cu Amounts on Stability of Constituent Phases in AlxCrFeMnNi High Entropy Alloy Microstructure

A family of AlxCuyCrFeMnNi (x=0, 0.15, 0.3, 0.6, 0.9 and y=0, 0.07, 0.14) high entropy alloys (HEA) were arc cast and then heat treated for 24h at 1100֯C-1150֯C followed by water quench. The microstructure of low Al alloys (Al0Cux and Al0.15Cux) consisted of FCC and BCC phases. Al0.3Cux showed an additional ordered precipitate phase. High Al alloys (Al0.6Cux and Al0.9Cux) consisted of two BCC phases rich in Cr-Fe and Ni-Al. In the present study, the phases formed in the microstructures were evaluated in light of valence electron concentration (VEC), Hume-Rothery (H-R) and degree of partitioning. Although VEC successfully predicts the impact of Al and Cu on the trend of FCC-BCC phase formation, the parameter does not accurately predict the structure of high Al alloys. A good agreement was observed between H-R rules prediction and the experiments which might be ascribed to the high temperature equilibrium phases developed by the heat treatment. As per these criteria, increasing Cu (up to 3at.%) and decreasing Al promote formation of solid solution phases. Adding minor amounts of Cu avoids the Cu partitioning that besets high Cu HEAs.