Evolution of the secondary α phase morphologies during isothermal heat treatment in Ti-7333 alloy

Evolution of the secondary α phase morphologies during isothermal heat treatment of a near β titanium alloy Ti-7Mo-3Al-3Nb-3Cr (Ti-7333) has been studied by using SEM and TEM. The results indicated that the morphologies of α phase widely varied by changing the isothermal temperature and time. The needle-like secondary α precipitate morphology formed after solution treatment above β transus followed with aging treatment. When the aging temperature reduced, the length and width of α phase reduced while the volume fraction increased. This increasing in the volume fraction of α phase resulted in the strengthening of the micro-hardness of the alloy. In addition, the results showed that there was almost no precipitation after 1 min isothermal holding time at 650 °C. After 2 min, α phase presented a dot shape precipitation with a nonuniform distribution. Increasing the holding time to 5 min, the α precipitation demonstrated a needle-like shape. At last, the coarse α precipitation formed after aging 3 h. The volume fraction of α increased from 4.6% to 44% while the aging time increasing from 2 min to 3 h. The effect of the alloy elements on governing the microstructure is discussed.