Phase relationships in the Fe-Cr-Ni system at solidification temperatures

Abstract

The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of the Fe-Cr-Ni system as part of a larger study of the Fe-Cr-Ni-C system. The investigation consisted of measurements and modeling of tie-lines and the liquidus surfaces of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria and the peritectic surface involving all three phases in the iron-rich corner of the Gibbs triangle bounded by 0 to 25 wt pct Cr and 0 to 25 wt pct Ni (bal Fe). The temperature ranged from the melting point of iron (1811 K) to about 1750 K. Compositions for tie-lines were obtained from liquid-solid equilibrium couples and temperatures for the surfaces were obtained by differential thermal analysis. Parameters for modeling the system were then selected in the subregular solution model to minimize the square of the difference between experimental and calculated tie-lines. With one ternary parameter employed for each phase, calculations by the model are in excellent agreement with the tie-line and liquidus measurements and in fair agreement with the temperatures for the peritectic surfaceL + δ/L + δ + γ. The usefulness of the model is demonstrated by calculation of the solidification paths of selected alloys in the composition field investigated for the limiting cases of (a) complete equilibrium followed by the alloy system, and (b) no solid diffusion (i.e., segregation) with equilibrium maintained at the solidifying front and complete mixing in the liquid phase.