Formation of diverse B2+O structure and hardness of Mo-modified Ti-22Al-25Nb alloys upon cooling

Mo-modified Ti-22Al-25Nb alloys are sintered at 900 °C, 1000 °C, 1070 °C, and 1150 °C, followed by quenching and furnace cooling, respectively. We made comparisons of phase composition and microstructure between quenched and furnace-cooled alloys to interpret the phase transformation and microstructure evolution during cooling. The phase transformation was B2→O for the alloy cooled from B2+O region, and was B2→B2+O for those cooled from α₂+B2+O, α₂+B2, and B2 phase regions. Moreover, the morphology was B2+O colonies for those cooled from B2+O and α₂+B2+O phase regions, and was Widmanstätten structure for those cooled from α₂+B2, and B2 phase regions. Kinetics analysis indicated that the activation energy for the phase transformation of α₂→B2 was 170 kJ mol⁻¹ for Ti-22Al-25Nb, while it was 330 kJ mol⁻¹ for Mo-modified Ti-22Al-25Nb. The retarded phase transformation in Mo-modified Ti-22Al-25Nb further led to curvy Widmanstätten structure when the alloy was furnace-cooled from single B2 phase region. The hardness of the Mo-modified Ti-22Al-25Nb alloys is up to 488 HV, while that of the Ti-22Al-25Nb alloys without Mo addition is less than 400 HV.