Effects of subtransus triplex heat treatments on the microstructure and mechanical behaviors of a Ti-5Al-5Mo-5V-1Cr-1Fe near β titanium alloy

The effects of subtransus triplex heat treatments on the microstructure and mechanical behaviors of a Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) near beta titanium alloy were systematically investigated in this study. The tensile testing results indicated that the yield stress and total elongation can be optimized from-1040 MPa, 3% of the as-cast alloy to-1140 MPa, 16% and-1195 MPa,12% of the heat-treated samples, respectively. The as -cast alloy exhibited a basket-weave structure containing mostly lamellar alpha phase. After triplex heat treatment, the morphology of alpha phase changed to a rod shape, and some nano-sized secondary alpha phase precipitates formed. The effects of heat treatment on microstructures and mechanical properties were analyzed as follows: (i) The first step (hot rolling at 830 degrees C + heating to 830 degrees C, holding for 0.5 h and furnace cooling) was conducted for recrystallization and homogenization to reduce the casting defects; (ii) The second step (heating to 750 degrees C, holding for 2 h and air cooling) was performed to adjust the morphology of alpha phase to a rod shape to improve the ductility; (iii) The third step (heating to 450 degrees C, holding for 8 h/16 h and air cooling) was performed to promote precipitation of secondary alpha and growth of primary alpha, to further enhance the strength. Dislocation slip was in dominance in the soft beta phase, while both dislocation slip and {1011} twinning occurred in the alpha phase, during tensile deformation. The fracture behavior changed from brittle to ductile after heat treatment. Our work pro-vided a convenient and feasible way to optimize the mechanical properties of the Ti-55511 alloy.