Atomic-scale observation of non-classical nucleation-mediated phase transformation in a titanium alloy

Two-phase titanium-based alloys are widely used in aerospace and biomedical applications, and they are obtained through phase transformations between a low-temperature hexagonal closed-packed alpha-phase and a high-temperature body-centred cubic beta-phase. Understanding how a new phase evolves from its parent phase is critical to controlling the transforming microstructures and thus material properties. Here, we report time-resolved experimental evidence, at sub-angstrom resolution, of a non-classically nucleated metastable phase that bridges the alpha-phase and the beta-phase, in a technologically important titanium-molybdenum alloy. We observed a nanosized and chemically ordered superstructure in the alpha-phase matrix; its composition, chemical order and crystal structure are all found to be different from both the parent and the product phases, but instigating a vanishingly low energy barrier for the transformation into the beta-phase. This latter phase transition can proceed instantly via vibrational switching when the molybdenum concentration in the superstructure exceeds a critical value. We expect that such a non-classical phase evolution mechanism is much more common than previously believed for solid-state transformations.