PLASTIC-FLOW AND MICROSTRUCTURE EVOLUTION DURING HOT DEFORMATION OF AGAMMA-TITANIUM ALUMINIDE ALLOY

The hot workability of a near gamma titanium aluminide alloy, Ti-49.5A l-2.5Nb-1.1Mn, was assessed in both the cast and the wrought condition s through a series of tension tests conducted ever a wide range of str ain rates (10(-4) to 10(0) s(-1)) and temperatures (850 degrees C to 1 377 degrees C). Tensile flow curves for both materials exhibited sharp peaks at low strain levels followed by pronounced necking and flow lo calization at high strain levels. A phenomenological analysis of the s train rate and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and an a pparent activation energy of similar to 411 kJ/mol. At low strain rate s, the tensile ductility displayed a maximum at similar to 1050 degree s C to 1150 degrees C, whereas at high strain rates, a sharp transitio n from a brittle behavior at low temperatures to a ductile behavior at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling the growth and co alescence of cavities and wedge cracks in specimens deformed at strain rates of 10(-4) to 10(-2) s(-1) and temperatures varying from 950 deg rees C to 1250 degrees C. The dynamically recrystallized grain size fo llowed a power-law relationship with the Zener-Hollomon parameter. Def ormation at temperatures higher than 1270 degrees C led to the formati on of randomly oriented alpha laths within the gamma grains at low str ain levels followed by their reorientation and evolution into fibrous structures containing gamma + alpha phases, resulting in excellent duc tility even at high strain rates.