Fine grained fully lamellar structure acquisition and microstructure characteristics of Ti-44Al-4Nb-4V-0.3Mo-Y alloy

Abstract In this paper, the microstructure characteristic of a β solidifying TiAl alloy Ti-44Al-4Nb-4V-0.3Mo-Y (at. %) was investigated systematically. Cylindrical blanks were isothermal forged at 1250 °C within (α + β) two-phase-field and 1200 °C within (β + γ) two-phase-field under the strain rate of 0.02 s −1 and 0.03 s −1 , with a total reduction of 75%. The microstructure of the alloy forged in α + β two-phase-field consists of lamellar colonies (only represents α 2 /γ lamellar colonies), mixture structure of β/B2 phase and γ phase as well as β/γ lamellae resulted from cellular reaction. However, a large amount of equiaxed γ grains appear with no β/γ lamellae available under forging in β + γ two-phase-field, and the average size of γ grain is about 10 μm. By further annealed at 900 °C for about 0.5 h, the structure of hot forging at 1250 °C is composed of β/B2 phase and equiaxed γ grains with the mean size about 5 μm. After heat treated at 1200 °C for 2 h, the microstructure of this annealed samples is characterized by fine grained fully lamellar (FFL) structure with size of 20 μm. A specially designed hot forging process is proposed, which prevents the existence of inhomogeneous lamellar structure.