THE THERMODYNAMICS AND COMPETITIVE KINETICS OF METASTABLE TAU-PHASE DEVELOPMENT IN MNAL-BASE ALLOYS

The solidification of a metastable ferromagnetic tau phase in undercooled Mn-Al-C alloys is examined in terms of the relative thermodynamic phase stability and competitive kinetics. The heat of transformation DELTAH(t)tau-->epsilon of the metastable tau phase to the equilibrium h.c.p. epsilon phase and the heat of fusion DELTAH(f)epsilon-->l of the h.c.p. epsilon phase in a Mn0.55Al0.433C0.017 alloy were measured to determine the Gibbs free energy differences between the metastable and stable phases as a function of temperature. The results indicate that a minimum amount of undercooling of DELTAT = 87 K is required to form the metastable ferromagnetic tau phase over the equilibrium phases in a Mn0.55Al0.433C0.017 alloy. The thermodynamic evaluation was applied further to develop a complete temperature-time-transformation (TTT) diagram of the Mn0.55Al0.433C0.017 alloy based on a solid-state transformation kinetics analysis of the epsilon --> tau reaction during isothermal treatments. This analysis reveals the critical cooling rate of the order of 20 K s-1 to bypass the metastable tau phase nucleation during solid state transformation in agreement with empirical observations. The combined thermodynamic and competitive kinetics analysis provide a unified guide for the processing of near equiatomic Mn-Al alloys.