Effects of Secondary α Phase on Strength and Ductility of Ti-6Mo-5V-3Al-2Fe-2Zr Alloy

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