High temperature deformation of NbTiAl alloys with σ + γ microstructure

The deformation behavior of an alloy with composition 27Nb33Ti40Al was evaluated at 1000 °C using compression and creep testing. In this alloy, σ-Nb2Al + γ-TiAl duplex microstructures with different degrees of coarseness and morphology but similar volume fractions of the phases were obtained through suitable aging treatments. The compressive flow curves of both microstructures revealed flow softening, which has been attributed to dynamic recrystallization of the γ phase. During deformation, the σ phase underwent a fragmentation process by which it broke up into smaller particles. The degree of fragmentation increased with decreases in strain rate and σ phase particle size. The main mechanism involved in the fragmentation process has been identified as grain boundary sliding. The coarser microstructure exhibited microcracking at high strain rates and large applied strains. The stress-strain rate relationship for both compression and creep tests followed a power law. For the specimen with a fine microstructure, the stress exponents were 1.9 and 3.1 while for the specimens with a coarse microstructure they were 3.2 and 4.5 for creep and compression testing respectively. These results are consistent with observation of grain boundary sliding in the σ phase and dislocation glide and twinning of the σ phase on deformation.