Phase changes in 15 mainly austenitic iron-manganese-chromium alloys of different compositions were investigated in the temperature range between—196 and 1000°C after different thermomechanical treatments. Different physical measurement techniques were employed to investigate the structural changes occurring during heating and cooling and after cold work: electrical resistivity techniques, differential thermal analysis, magnetic response, Vickers hardness, and X-ray diffraction (XRD) measurements.
Depending on composition, thermomechanical pretreatment, and the measurement temperature of the different alloys, variable amounts of γ, ε, α, α-Fe-, and α-Mn-phases were detected, and the respective transition temperatures for the ε↔γ, α↔γ α↔Fe↔γ, and α-Mn↔γ transformations were determined approximately. The transition temperatures for the transformations ε↔γ and α↔γ in the ternary alloys were found to be almost the same as those established for the binary iron-manganese phase diagrams. Depending on alloy composition, the εdef phase can be induced by cold working, and the εdef transformation occurs at a higher temperature than the reversible ε↔γ transformation. The α-phase, introduced either by cooling below the Ms temperature or by cold work, transforms during subsequent heating at least partly into α-Fe, which in turn transforms back to γ.
The phase boundary between the α-Fe-phase and the γ-phase of the iron manganese alloy is approximately maintained if chromium is added to the two component materials. The new phase boundary between the α-Mn-phase and the γ-phase extends far into the austenitic γ-region. For these reasons the γ-phase field of the iron-chromium-manganese alloys is very small below 600°C and much narrower than reported in the literature.