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 α 2 +B2+O, α 2 +B2, and B2 phase regions. Moreover, the morphology was B2+O colonies for those cooled from B2+O and α 2 +B2+O phase regions, and was Widmanstätten structure for those cooled from α 2 +B2, and B2 phase regions. Kinetics analysis indicated that the activation energy for the phase transformation of α 2 →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. • Diverse B2+O structures are observed in Mo-modified Ti 2 AlNb alloys. • Mo-rich kinky Widmanstätten structure corresponds to high Vickers hardness. • Kinetic analysis shows Mo addition inhibits the phase transformation of α 2.→B2. • The phase transformation during different cooling routes is reported. [ABSTRACT FROM AUTHOR]