Construction and application of metastability field diagrams for non-equilibrium processing of alloys

Abstract

Isothermal metastability field diagrams first proposed by Baker and Cahn depicting the thermodynamic limits to co-existing interfacial solid and liquid compositions have been critically analysed. The properties of these diagrams under different limiting conditions have been derived. Construction of paired diagrams by simultaneous consideration of both the competing solid phases of a binary eutectic has been demonstrated to be useful. The possibility of the recurrence of eutectic-like solidification as in the Pb-Sn system, or formation of continuous solid solutions as in the Ag-Cu system, at moderate undercoolings can be inferred from the paired diagrams. A novel construction of non-isothermal metastable field diagram has been proposed to determine the thermodynamic limits to the distribution coefficient in alloys during freezing as a function of undercooling. Regions of solute and solvent napping and the diffusional domain can also be delineated in these diagrams.