Mechanism of O phase induced property variation in Ti2AlNb alloy via laser powder bed fusion over a wide temperature range

Laser powder bed fused Ti2AlNb alloy exhibits excellent strength and ductility at room temperature but suffers from severe brittle fracture at 650 °C. To uncover the underlying mechanism, this study systematically investigates the microstructure evolution and mechanical responses at 450 °C, 550 °C, and 650 °C. Results show that the B2→O phase transformation occurs above 380 °C, with the O phase precipitating as nanoparticles at 450 °C, growing into acicular morphology at 550 °C, and coarsening dramatically at 650 °C. The interface between O and B2 phases induces local stress concentration. At 550 °C, deformation twinning is activated, alleviating stress and leading to an anomalous work-hardening effect. However, at 650 °C, the overgrown O phase suppresses twinning, resulting in catastrophic brittle fracture. The morphological evolution of the O phase is thus identified as the key factor governing the high-temperature property degradation.