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 α-phase and a high-temperature body-centred cubic β-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-ångström resolution, of a non-classically nucleated metastable phase that bridges the α-phase and the β-phase, in a technologically important titanium–molybdenum alloy. We observed a nanosized and chemically ordered superstructure in the α-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 β-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.

双相钛基合金广泛用于航空航天和生物医学应用，它们是通过低温六方密排α相和高温体心立方β相之间的相变获得的。了解新相如何从其母相演化对于控制转变微观结构和材料性能至关重要。在这里，我们报告了时间分辨的实验证据，在亚埃分辨率下，在技术上重要的钛钼合金中，非经典成核亚稳态桥接 α 相和 β 相。我们在 α 相矩阵中观察到纳米尺寸和化学有序的超结构；它的组成、化学顺序和晶体结构都被发现不同于母相和产物相，但为转变为 β 相激发了一个几乎消失的低能垒。当上部结构中的钼浓度超过临界值时，后一种相变可以通过振动切换立即进行。我们预计这种非经典的相演化机制比以前认为的固态转换更为普遍。