Grain Boundary Character and Microstructural Evolution During Hot Deformation of a High Nb Containing TiAl Alloy

Grain boundary character and microstructural evolution during hot deformation of a high Nb-containing TiAl alloy with the composition of Ti-45Al-8.5Nb-(W, B, Y) was investigated. The plasma arc-melted alloy exhibits nearly lamellar microstructures with β/B2 phase precipitates along colony boundaries. The retained β/B2 phase rich in Ti, Nb and W and poor in Al is formed by the β→α transformation during nonequilibrium solidification. This β/B2 precipitation is caused by the high cooling rate, low atom diffusion rate of β stabilizers and different partition coefficients of the elements. B and Y elements exist mainly as borides and Y₂O₃, respectively. The morphology, size, composition and stability of β/B2 phase are determined by hot deformation. The high temperature and press enhance the elements diffusion, and thus the variation of the composition of β/B2 phase in deformed samples is obviously observed. When the samples are deformed in (α + γ) phase region, the transformation β/B2→α₂ takes place. Considering the different crystal structures of β/B2 and α₂ phases, there is a tendency for β/B2 phase to transform to a more closed packed structure of α₂ during deformation. The β/B2→ α₂ transformation is promoted by the hot deformation.