Superplasticity in a large-grained TiAl alloy

The superplastic behaviour was systematically investigated in a large-grained Ti-47Al-2Mn-2Nb-B alloy having nearly equiaxed gamma-phase with grain size of 95 mum, in which a small amount of fine particles of alpha(2) distribute uniformly. Superplastic deformation was examined at a temperature range of 1025-1100 degreesC and strain rates range of 4 x 10(-5_)1.28 x 10(-3) s(-1). The large-grained TiAl alloy exhibits all deformation characteristics of conventionally fine-grained superplastic alloys without the prerequisites, fine grain size and grain boundary sliding. All the values of strain rate sensitivity, m are larger than 0.3. In most cases, an elongation over 200% was gained. A maximum elongation of 287.5% with an m value of 0.39 was obtained at 1100 degreesC and an initial strain rate of 4 x 10(-5) s(-1). Microstructure evolution during superplastic deformation was characterized by optical microscopy, orientation imaging microscopy and transmission electron microscopy (TEM). Metallographic examination has shown that the average grain size of large-grained TiAl alloy decreased during superplastic deformation, after that a much finer grain size of 10 to 3-5 mum could be obtained. Electron back-scattered diffraction analysis revealed that significant grain refinement was obtained at different levels with an increase in the density of low and high angle grain boundaries. A direct evidence for dynamic formation of grain boundaries with misorientation of 15-30degrees was found, which was evolved from subboundaries. The evidence of subboundary formation and dislocation glide in the interior of grains was revealed by TEM observation. A continuous recovery and recrystallization process similar to that in FeAl and Fe3Al alloys was proposed as the superplastic deformation mechanism in the large-grained TiAl alloy. (C) 2004 Elsevier Ltd. All rights reserved.