Microstructural stability of a two-phase (O + B2) alloy of the Ti-25Al-25Nb system (at.%) during thermal cycling in a hydrogen atmosphere

In this work, the stability of the microstructure of the experimentally obtained two-phase (O + B2) alloy of the Ti–25Al–25Nb (at.%) system were studied during thermal cycling in a hydrogen atmosphere. It was found that the two-phase structure (O + B2) of the alloy of the Ti–Al–Nb system shows high thermodynamic stability. In this case, phase transformations of secondary phases (α2, AlNb₂) are observed in the microstructure of the alloy, the volumetric content of which at all stages of testing does not exceed 2%. Thus, after the first cycle of high-temperature exposure, single inclusions of the α2 phase precipitate, while in the areas enriched in Ti and Al, due to the redistribution of Nb, a new colony of the α2 phase is observed. After five test cycles, it was found that large accumulations of the α2 colony, due to the α2 → B2 phase transformations, form new micron-sized grains of the B2 phase. A volumetric accumulation of nanosized precipitates of the AlNb₂ phase was found near the triple joints of the grain boundaries of the B2 phase after 10 cycles of thermal exposure, which is caused by the supersaturation of B2 grains with niobium.