Abstract:The size, morphology and distribution of the secondary α phase were observed by EBSD, SEM and TEM. Tensile properties at room temperature of aged alloys were measured. The effect of secondary α phase on strength and ductility of novel near β titanium alloy, Ti-6Mo-5V-3Al-2Fe-2Zr, has been studied. The results show that with the increase of aging temperature from 520℃ to 560℃, the size of the intragranular αi phase changes little but the spacing decrease. With further increase of aging temperature, the width and spacing of the αi phase increase. As the increase of aging time from 4h to 8h, the width of the αi phase increases but the spacing changes little. When the aging time is 12h, there is no significant change in the αi phase. A relationship equation between secondary α phase and alloy strength has been established, which shows a good agreement with the calculated values of strength change induced by secondary α phase. It can be indicated that the spacing of the αi phase determines the alloy strength. The spacing of αi phase is the smallest and the ultimate tensile strength is the highest 1502MPa after aged at 560℃ for 8h. The alloy ductility is affected by both intragranular α phase and grain boundaries α phase. The continuous αGB phase and αi phase with smallest spacing form after aged at 560℃ for 8h, resulting in poor alloy ductility. The αWGB phase, which is distributed along the grain boundaries and grows parallel into the grains, as well as discontinuous αGB phase and αi phase with largest spacing, form after aged at 680℃ for 8h, resulting in significant improvement of alloy ductility.