The influence of Fe variations on the phase stability of CrMnFexCoNi alloys following long-duration exposures at intermediate temperatures

The equiatomic CrMnFeCoNi alloy exhibits many desirable properties, but its susceptibility to the formation of embrittling intermetallic phases, makes it unsuitable for structural applications at elevated temperatures. As a result, there has been increasing interest in developing alternative alloys from the CrMnFeCoNi system that avoid this limitation. Here we present a detailed study of phase stability in a CrMnFexCoNi series of alloys, where x = 0, 0.5, 1.5 (in atomic ratio), following long-duration heat treatments of 1000 h at 900 and 700 °C, and up to 5000 h at 500 °C. Each alloy was single phase fcc following homogenisation. After exposure at 900 °C, large σ phase precipitates were present in the CrMnCoNi alloy, but alloys containing ≥0.5 Fe remained single phase fcc. At 700 °C, the alloys investigated all contained the σ phase. Cr-bcc precipitates were also present in the CrMnCoNi and CrMnFe0.5CoNi alloys and Cr carbide precipitates featured in the CrMnFe1.5CoNi alloy. Heat-treatment of the CrMnCoNi alloy at 500 °C caused a partial bulk decomposition of the fcc matrix, which produced a fine-scale intergrowth of phases: σ, NiMn-L10, Cr-bcc and a secondary solute-depleted fcc phase. In the alloy containing 0.5 Fe, cellular regions consisting of a NiMn-L10, Cr-bcc and solute-depleted matrix phase, developed along the grain boundaries. NiMn-L10 and Cr-rich precipitates also formed on grain boundaries in the 1.5 Fe alloy. From these experimental observations, it was clearly established that Fe stabilises the fcc matrix relative to the σ and bcc phases.