Stability of ordered L12 and B2 precipitates in face centered cubic based high entropy alloys - Al0.3CoFeCrNi and Al0.3CuFeCrNi2

Adding a small amount of Al to the well-known high entropy alloy (HEA), Al0.1CoCrFeNi, to form Al0.3CoCrFeNi, leads to the precipitation of highly refined ordered L12 precipitates, which are stable at 550 °C. However, the L12 precipitates (stoichiometry of (Ni,Cr)3(Al,Fe,Co)) are de-stabilized and replaced by coarser B2 precipitates on annealing at 700 °C. Contrastingly, in the Co-free, Al0.3CuCrFeNi2 high entropy alloy, the L12 precipitates are stable at both 550 °C (~ 5 nm) and 700 °C (~ 50 nm), and exhibit a stoichiometry of (Ni,Cu)3(Al,Fe,Cr). These results lead to interesting insights into the stability of ordered phases in HEAs as a function of composition and temperature.