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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.
phase diagram, iron-chromium-manganese, γ-phase, α-Fe-phase, α-phase, ε-phase, ε, def, -phase, α-Mn-phase, transition temperatures, cold working
Senior research physicist, Commission of the European Communities, Institute for Advanced Materials, Ispra, Varese
Senior research physicist, Commission of the European Communities, Institute for Advanced Materials, Petten, ZG