PHASE-EQUILIBRIA AND TRANSFORMATIONS IN INTERMEDIATE TITANIUM ALUMINUM-ALLOYS

The phase equilibria and solid state phase transformations observed in Ti-Al alloys between 40 and 50 at.% Al are reported. It is shown that the three-phase invariant between the high temperature alpha phase (disordered h.c.p.) and the lower temperature ordered alpha2-Ti3Al (D0(19)) and gamma-TiAl (L1(0)) phases is eutectoid in nature (alpha --> alpha2 + gamma) but that it is difficult to impossible to produce this reaction experimentally due to differences in the transformation kinetics of the two product phases. The decomposition of the high temperature alpha phase is shown to be a strong function of composition and cooling rate and the results are used to construct schematic CCT diagrams. It is also shown that gamma precipitation can be suppressed in 40, 42 and 45Al alloys by rapid quenching from the alpha field but not in the 48 and 50Al alloys; in the latter, the gamma tends to be either lamellar or massive in nature. Finally, the nature of the discontinuous coarsening of the lamellar (alpha2 + gamma or alpha + gamma) structures to produce serrated, interlocking grain boundaries and the lack of heterogeneous nucleation of gamma at grain boundaries is described and related to the thermal histories, driving forces, and crystallography of the system. The various results are then used to shed light on the behavior observed in previous studies on alloys in this system.