CALCULATIONS OF ALPHA/GAMMA PHASE BOUNDARIES IN FE-C-X(1)-X(2) SYSTEMS FROM THE CENTRAL ATOMS MODEL

The alpha/gamma phase boundaries in Fe-C-X(1)-X(2) quaternary alloys (where X(1) = Mn and X(2) = Si, Ni, and Co, successively) are calculated from the Central Atoms model, as generalized to multicomponent systems by Foo and Lupis. The interaction parameters are evaluated from the Wagner interaction parameters in ternary iron alloys reported in the literature or estimated from the interaction parameters in binary alloys. Two equilibrium conditions, para- and orthoequilibrium, are utilized. In the Fe-C-Mn-Si system, a mixed state of equilibrium, in which orthoequilibrium is achieved with respect to C and Si while the other two substitutional elements (Fe and Mn) are assumed to be immobile (paraequilibrium), is also considered. The calculated phase boundaries are employed to evaluate the free energy change for the nucleation and the growth kinetics of proeutectoid ferrite in these alloys in companion articles.