| The superplastic behavior of a near-γ titanium aluminide (Ti-45.5Al-2Cr-2Nb) was determined under uniaxial tension in the as-rolled and two rolled-and-heat treated conditions (1177°C/4 hours or 1238°C/2 hours). Flow behavior, failure mode, microstructural evolution, cavitation rates, and the effects of these on superplastic behavior were the primary interests of this research. These relations were established via isothermal, constant strain rate tests conducted at 10−4–10−2 s −1 and temperatures between 900°C and 1200°C. The primary mechanism for hot deformation at 900–1000°C was identified as power law creep (n approximately 3) accompanied by dynamic recrystallization, as indicated by the activation energy values of approximately 300 kJ/mol. The process at 1100–1200°C was grain boundary sliding ( n approximately 2) accommodated by dynamic recrystallization. Significant microstructural changes were found. At 900°C and 1000°C, a small percentage of γ and α2 grains of 3–5 μm diameter statically recrystallized during the test hold period. This was followed by dynamic recrystallization to a finer grain size of both the statically recrystallized grains and the unrecrystallized as-rolled material during tensile deformation. At 1100°C and 1200°C, the microstructural evolution process starting from the as-rolled condition was similar except that during testing there was (1) dynamic grain growth and/or dynamic recrystallization of statically recrystallized grains and (2) dynamic recrystallization of the unrecrystallized as-rolled material. Failure modes were established as predominantly cavitation/fracture controlled rather than localized thinning. Cavity growth was found to be largely plasticity controlled. Experimentally derived cavity growth rates were compared with various equations that predict cavity growth rates as a function of strain rate sensitivity. For all three initial microstructures, the optimum sheet forming temperature in the regime examined was identified as 1200°C, at which the lowest flow stresses and cavity growth rates were observed along with the highest tensile elongations. |
