High temperature compressive flow behavior and associated microstructural development in a β-stabilized high Nb-containing γ-TiAl based alloy

The characteristics of hot deformation behavior and associated microstructural development of a cast γ-TiAl based Ti-45Al-8Nb-2Cr-0.2B (at.%) alloy have been investigated in the temperature range of 1273–1473 K and the strain rate range of 0.5–0.005 s−1 using a Gleeble thermo-mechanical simulator. The as-cast pancake shows a uniform distribution of fine (α2+γ) and (γ+γ) lamellar colonies with intermittent retained β(B2) and γ grains (average volume fraction: γ = 0.89, α2 = 0.03, and β(B2) = 0.08). Hot deformation behavior has been characterized by detailed analyses of processing maps, Arrhenius type constitutive models, and microstructural evolution. The results indicate that the apparent activation energy (Q = 429 kJ/mol) measured through hyperbolic-sinusoidal relationship between flow stress, temperature and strain rate could be correlated to the dynamic recrystallization (DRX) and presence of lamellar (α2+γ) microstructure. Further, the morphological alignment of lamellae boundaries in conjunction with deformation temperature plays a crucial role in determining the DRX mechanism and controls the microstructural evolution. Dynamic recrystallization occurs preferentially in the grain boundary β-phases, and thereby, improves the workability of the alloy. It has been established that the β-phase assumes a “supplementary” role during hot deformation of TiAl alloys. A systematic variation in β-phase volume fraction indicates that the amount of retained β-phase along with its morphology plays an important role as it suppresses the crack initiation and/or propagation by absorbing the stress concentration arising during deformation of lamellar colonies in high Nb containing γ TiAl alloys.