Effect of W addition on phase transformation and microstructure of powder metallurgic Ti-22Al-25Nb alloys during quenching and furnace cooling

Powder metallurgic Ti ₂ AlNb alloys with W addition are sintered at 900, 1000, 1070 °C, and 1150 °C (i.e., in the O + B2, α ₂ + B2 + O, α ₂ + B2, and single B2 phase regions, respectively) for 12 h, followed by water quenching and furnace cooling. Comparisons of phase and microstructure between quenched and furnace-cooled W-modified alloys are carried out to illustrate the phase transformation and microstructure evolution during the cooling process. Furthermore, a comparison is also made between W-modified and W-free alloys, to reveal the function of the W alloying. W addition accelerates the solutions of α ₂ and O phases during the high-temperature holding, and a Widmannstätten B2 + O structure, which contributes to the properties, is induced by furnace cooling from all the phase regions. The Widmannstätten structure includes a B2 matrix, primary O, and secondary O precipitates. However, W alloying refines the Widmannstätten structure only when the alloys are solution-treated and then cooled from the single B2 phase. Although the hardness of the W-modified alloys is lower than that of the W-free alloys sintered in the same phase region, an enhancement of hardness, 489 ± 18 HV, is obtained in the alloy solution-treated in the single B2 phase region for only 0.5 h.