1991 Volume 77 Issue 12 Pages 2139-2146
Effect of cooling rates after β treatment on microstructures in an α titanium alloy, Ti-5Al-2.5Sn, is examined.
At the cooling rate of 300°C/s, no grain boundary α phase is observed and lath type of plate structure is formed throughout the specimen. In the lath plates, highly dense dislocations are included, and Fe-enriched β phase is retained between lath plates, which indicates that both shear and diffusional components of transformation operate during cooling. At the lower cooling rate (≤4°C/s), structural dislocations other than β phase are observed between α plates. It is considered to be due to the coalescence of two adjacent α plates which have slightly different crystallographical orientations each other. At the cooling rate of 0.1°C/s, ring-like diffuse streaks and extra spots which are related to ω phase are detected in the diffraction pattern of Fe-enriched β phase.
Furthermore, aging treatment at 550°C for 24 h after cooled from β region at the cooling rate of 300°C/s produces the refined equiaxed structure containing sub-grains, recrystallized grains and β phase at the triple points of α (sub-) grain boundaries.
