Abstract
Rapid solidification processing (RSP) of intermetallic alloys can provide alternative solidification paths and lead to the formation of metastable products. For the intermetallic TiAl with an equilibrium L1 0 structure, RSP has yielded a metastable hexagonal close packed (h.c.p.) α-Ti phase in both fine powder and melt-spun ribbon. Based on transmission electron microscopy observations of a fine antiphase domain structure, the h.c.p. phase orders to the Ti 3Al (DO 19) structure following solidification. A metastable phase diagram analysis indicates that a melt undercooling of about 100 °C is required for partitionless formation of an h.c.p. phase from an equiatomic melt and reveals other possible solidification pathways including the nucleation of metastable disordered body-centered cubic (b.c.c.) and face-centered cubic (f.c.c.) phases. Each of the potential pathways offers further opportunity for microstructural modification by solid-state annealing treatments.
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