Superplasticity in a large-grained TiAl alloy

Abstract

The superplastic behaviour was systematically investigated in a large-grained Ti–47Al–2Mn–2Nb–B alloy having nearly equiaxed γ-phase with grain size of 95 μm, in which a small amount of fine particles of α 2 distribute uniformly. Superplastic deformation was examined at a temperature range of 1025–1100 °C and strain rates range of 4×10 −5–1.28×10 −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 °C and an initial strain rate of 4×10 −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 μm 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 15–30° 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 Fe 3Al alloys was proposed as the superplastic deformation mechanism in the large-grained TiAl alloy.