Microstructure Evolution, B2 Grain Size Uniformity, and Performance of a Powder Metallurgy Ti-22Al-25Nb Alloy during Solution Treatment

The powder metallurgy (P/M) Ti-22Al-25Nb alloy obtained by spark plasma sintering (SPS) of Ti-22Al-25Nb prealloyed powders at 950 °C/80 MPa/10 min (α2 + B2 + O three-phase region) was adopted as the starting material to systematically investigate the evolution of microstructure, phase transition, B2 grain size uniformity and microhardness with solution temperature and holding time. The solution treatment was conducted at temperatures from 940 to 1100 °C (α2 + B2 + O, α2 + B2 and B2 phase regions) and holding times range of 10-120 min (10, 30, 60, and 120 min). After solution treatment, the microhardness of the alloy was measured. The results showed that the microstructure and microhardness of the alloy were closely related to the solution temperature and holding time. The B2 phase grain size and its uniformity increased with the increase in the solution temperature and holding time, and the microhardness decreased with the growth of the grains. The P/M Ti-22Al-25Nb alloy tested at 940 °C/10 min had the best performance, exhibiting three phases of α2 + B2 + O and a grain size of 43.57 μm. The microhardness was 359.52 ± 3.73 HV. In addition, the grain growth activation energy and grain growth index of different solution temperatures and holding times were obtained by linear fitting, and the relationship between the B2 grain size and the solution temperature and holding time was further established.