Effect of Co on the phase stability of CrMnFeCoxNi high entropy alloys following long-duration exposures at intermediate temperatures

The effect of Co on the phase stability of the CrMnFeCoxNi family of alloys, where the atomic ratio x = 0, 0.5, 1.5, has been experimentally established following 1000 h heat treatments at 900 and 700 degrees C and up to 5000 h at 500 degrees C. All the alloys were single phase fcc in the homogenised condition, except for CrMnFeNi which also contained bcc precipitates that remained present following exposures at 900 degrees C and 700 degrees C. The exposures at 900 degrees C and 700 degrees C also resulted in the formation of sigma phase precipitates in the CrMnFeNi and CrMnFeCo0.5Ni alloys but not in the CrMnFeCoL1.5Ni alloy. These data, in conjunction with results previously published in the literature, conclusively establish that Co stabilises the fcc matrix at elevated temperatures. However, at 500 degrees C, further bulk decomposition in the CrMnFeNi alloy was observed, consisting of a fine-scale intergrowth of a NiMn L1(0) phase and CrFe sigma phase. Grain boundary precipitates were also observed following exposure at 500 degrees C in the CrMnFeCo0.5Ni and CrMnFeCoLxNi alloys. Four different phases were observed on the grain boundaries of the CrMnFeCo0.5Ni alloy (Cr carbide, sigma, FeCo B2 and NiMn L1(0)), whilst only two phases were found on the grain boundaries of the CrMnFeCoLxNi alloy (Cr carbide and NiMn L1(0)). The experimental observations facilitated an assessment of the fidelity of current thermodynamic predictions of phase equilibria. All the phases predicted were observed experimentally and the sigma stability fitted the experimental observations well. However, at lower temperatures, thermodynamic predictions were less consistent with experimental observations, underpredicting the extent of the B2 phase field and failing to predict the formation of the L1(0) phase.