Effects of solid solution and aging treatment on the microstructure evolution of Ti2AlNb alloy

The effects of solid solution temperature, aging temperature and aging cooling rate on the microstructure evolution of forged Ti-22.5Al-24.7Nb (at. %) alloy were investigated. With the increasing solid solution temperature, the number of equiaxed microstructure first increases and then decreases, reaching the maximum at 900 degrees C. Lamellar microstructure solubilizes in the matrix or gradually becomes coarser and shorter, and transforms into equiaxed microstructure. When the solution temperature is 1000 degrees C, equiaxed alpha 2 phase gradually changes to be lath-shaped and concave. During the aging process, some alpha 2 phase transforms into O phase, and the aging temperature affects the shape of equiaxed microstructure and the spacing between the needle-like microstructure. With the increasing aging temperature, the number of equiaxed microstructure increases linearly, and the proportion of fine secondary O phase decreases while the thickness increases. The lamellar morphology of O phase can be retained at a faster cooling rate, while a portion of lamellar O phase is converted to the B2 phase after furnace cooling. Through heat treatment, the volume fraction and the size of lamellar O phase could be well controlled.