INFLUENCE OF TEMPERATURE TRANSIENTS ON THE HOT WORKABILITY OF A 2-PHASE GAMMA-TITANIUM ALUMINIDE ALLOY

The hot deformation behavior, microstructure development, and fracture characteristics of a wrought two-phase gamma-titanium aluminide alloy Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were inv estigated with special reference to the influence of temperature trans ients immediately preceding plastic deformation. Specimens were soaked at 1321 degrees C or 1260 degrees C, cooled directly to test temperat ures of 1177 degrees C and 1093 degrees C, and upset under conditions of constant strain rate and temperature. Plastic flow behavior and mic rostructure evolution occurring in tests involving prior temperature t ransients were compared with those occurring in specimens which were d irectly heated to the test temperature and upset under identical defor mation conditions. Flow curves associated with prior exposure at 1321 degrees C exhibited very sharp peaks and strong flow softening trends compared to those obtained under isothermal conditions, i.e., involvin g no temperature transients. During cooling from 1321 degrees C, the m etastable Lu phase undergoes limited or complete decomposition into al pha/alpha(2) + gamma lamellae, depending on the final temperature (117 7 degrees C/1093 degrees C). Subsequent hot deformation leads to parti al globularization of the lamellae together with extensive kinking and reorientation of lamellae. In contrast, isothermal deformation at 117 7 degrees C/1093 degrees C preserves the fine, equiaxed microstructure , through dynamic recrystallization of the gamma grains. Cracking obse rved in specimens deformed at 1093 degrees C and 1.0 s(-1) after expos ure at 1321 degrees C has been attributed to the low rate of globulari zation as well as the occurrence of shear localization. Plastic flow b ehavior observed in this work is compared with that observed in severa l single-phase and two-phase gamma titanium aluminide alloys in order to identify mechanism(s) responsible for flow softening.