Effects of heat treatment on microstructure evolution and mechanical properties of Ti-22Al-24Nb-0.5Mo alloy

In this study, microstructure evolution, tensile and fracture behaviors of Ti-22Al-24Nb-0.5Mo alloy processed under different heat treatment processing routes were investigated systematically. The results revealed the ex-istence of bimodal-size microstructures of the primary O lamellae and the secondary acicular O phase. The strength and ductility of alloy specimens were optimized by accommodating the volume fractions and mor-phologies of the precipitates. With the increase of the solution temperature, more acicular O phase precipitated out, which led to the increase of the yield strength and ultimate tensile strength. It was observed that the acicular O phase led to the increase of the strength at elevated temperature without sacrificing plasticity. Owing to the improvement of microscopic strain homogeneity under coupled actions of stress and high temperature, the tensile fracture mode changed from quasi-cleavage to ductile transgranular creep fracture. The fracture tough-ness of this alloy was determined by the B2 phase and the acicular O phase. It was evaluated that the fracture toughness reduced with a higher volume fraction of the acicular O phase, which in turn affected the contiguity of the B2 matrix. Considering this, balanced strength and toughness were achieved by tailoring the relative contents of the B2 and acicular O phase.