Determination of α/γ phase boundaries in the Fe–Cr–Ni–Mn quaternary system with a diffusion-multiple method

•The γ single phase field expands when the Mn content is increased from 0 to 17wt.%.•Mn destabilizes austenite at Cr levels higher than 23wt.%.•The Ni equivalence for Mn is dependent on composition. The phase relationships between the ferrite phase (α) and the austenite phase (γ) were investigated in the iron-rich corner of the Fe–Cr–Mn–Ni quaternary system at 1000°C. Phase boundaries were determined using a diffusion-multiple technique. In this technique, three end members, Fe–Cr, Fe–Cr–Mn and Fe–Cr–Ni alloys, were coupled inside a metal tube, and then heat treated at 1300°C for 24h to create a microstructural state with a composition gradient of Cr, Ni and Mn within the composition range of the end members. This was followed by long time heat treatment at 1000°C for periods of up to 1000h to obtain phase equilibrium states between the γ and α phase in the composition range. The γ+α two-phase field shifts in a way to expand the γ single phase field when the Mn content is increased from 0 up to 17wt.%. Ni equivalent of Mn (NieqMn) is highly dependent on the Cr and Mn contents. A maximum NieqMn value of 0.9 was observed in the range 2–5wt.% Mn at 19wt.% Cr. The NieqMn value is zero at around 23wt.% Cr and becomes negative at higher Cr levels.