EQUILIBRIUM PHASE-DIAGRAM OF FE-CR-MN TERNARY-SYSTEM

The isothermal phase diagram of Fe-Cr-Mn ternary system at 923 K was investigated by means of a metallographic observation, conventional X-ray diffraction and an electron probe X-ray microanalysis. In particular, attention was directed towards the phase stability of the austenite (y) phase. The a phase was detected even in the alloy containing 6 % Cr, indicating that the y/y + a phase boundary at 923 K located in the lower Cr content region of the ternary system. The γ/γ + σ phase boundary determined experimentally in low Mn alloys (up to 20 wt% Mn), was consistent with the prediction using a critical Md value of 0.89, the value chosen following our previous experiments. Here, Md is the compositional average of the Md value which is the d-orbital energy level of transition metals calculated by a molecular orbital methods. On the other hand, the γ/γ + α-Mn phase boundary appeared in the more Mn-rich alloys and moved towards the lower Mn-content region with increasing Cr content. As the result, the γ single-phase region became rather narrow. It was concluded from these results that the γ single-phase region in Fe-Cr-Mn ternary system was not so broad as was reported previously. © 1990, The Iron and Steel Institute of Japan. All rights reserved.