Rapid decomposition of lamellar microstructure and enhanced hot workability of an as-cast triphase Ti-45Al-6Nb-1Mo alloy via one-step alpha-extrusion & annealing

This study proposed a new alpha-extrusion & annealing route to rapidly break down the coarse as-cast lamellar microstructure of a Nb-rich beta-solidifying gamma-TiAl alloy. It was found that one-step alpha-extrusion could induce widespread decomposition of lamellar structures via a precipitation reaction L(alpha/gamma)-* gamma + beta internally and along primary lamellar-colony boundaries, leading to formation of a streamlined microstructure consisting of alternately-arranged elongated lamellar structures and bands of beta(o) + gamma grains. Large-scale dynamic recrystallization of gamma grains was motivated during alpha-extrusion and resulted in bands of fine gamma-grains between elongated lamellar structures. A post-extrusion annealing at temperature far below T alpha thoroughly broke down the residual lamellar structures and gave rise to a higher content of globularized gamma and beta(o) grains. Finally, the resultant finegrained and high-angle misoriented alpha(2) + gamma + beta(o) triphase microstructure rendered an excellent superplastic deformability at 950 degrees C and especially a highly-promising one-step hot-die forming ability. The related micro structural evolution, phase transformation and recrystallization behaviors during one-step alpha-extrusion & annealing were investigated systematically.

Automated summary

This study proposed a new alpha-extrusion & annealing route to rapidly break down the coarse microstructure of a Nb-rich beta-solidifying gamma-TiAl alloy, enhancing its superplastic deformability. The one-step alpha-extrusion induced widespread decomposition and dynamic recrystallization of lamellar structures, resulting in a streamlined microstructure. Post-extrusion annealing further broke down residual structures and improved the content of globularized grains, leading to excellent superplasticity at high temperatures.