FLOW SOFTENING AND MICROSTRUCTURE EVOLUTION DURING HOT-WORKING OF WROUGHT NEAR-GAMMA TITANIUM ALUMINIDES

The hot-working behavior of two wrought ingot-metallurgy near-gamma titanium aluminides was established using the isothermal, hot compression test. Experiments were conducted in both the two-phase (alpha + gamma) and single-phase (alpha) regimes at strain rates typical of conventional metalworking operations (0.1 to 10 s-1). As for conventional titanium alloys, the flow stress showed a strong dependence on temperature and strain rate. In addition, the stress-strain curves revealed substantial levels of flow softening. Under subtransus conditions, the majority of this softening was ascribed to deformation heating effects and, secondarily, to microstructural effects. By contrast, microstructural changes, associated with the dissolution of remnant gamma grains and redistribution of solute, appeared to lead to the majority of the softening observed in the nominally single-phase alpha regime. Metallography on as-received (isothermally forged). heat-treated, isothermally upset, and upset and heat-treated samples revealed the persistence of segregation which appears to be a signature of the solidification process characteristic of near-gamma titanium aluminides.