SOLIDIFICATION KINETICS AND METASTABLE PHASE FORMATION IN BINARY TI-AL

Near-equiatomic alloys of Ti-Al were solidified at various bulk undercoolings using electromagnetic levitation. Detailed thermal histories were acquired during experiments using optical pyrometry with sampling rates as fast as 500 KHz. Solidification and other high-temperature transformation pathways were deduced from the thermal data and microstructural analysis. Recalescence rise times were used to determine semiquantitative primary solidification kinetics for the different phases. Primary beta solidification was observed at compositions well into the equilibrium alpha regime; this is presented as part of a near-equiatomic nucleation domain diagram that shows the primary solidification phase (beta, alpha, ordered gamma, or disordered gamma) that results for each combination of nucleation temperature and composition. Solidification kinetics are faster for primary beta (V(max) almost-equal-to 15 to 18 m s-1) than they are for primary alpha (V(max) almost-equal-to 10 to 12 m s-1). For urdercoolings less than about 150 K, the primary solidification kinetics are about an order of magnitude slower for gamma than for alpha. However, at an undercooling of about 150 K, the solidification kinetics for gamma increase discontinuously. This discontinuity is associated with a change in the primary solidification phase from ordered gamma (V(max) almost-equal-to 0.5 m s-1) to disordered gamma (V(max) almost-equal-to 10 m s-1).