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 α-phase and a high-temperature body-centred cubic ß-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 ß-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 ß-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.

Brittle-to-ductile transition in Ti-Pt intermetallic compounds.

Phase transformation changes numerous properties of materials. Ti-Pt alloys have received much interest because of high martensitic transformation temperature. However, the intrinsic brittleness of these intermetallic compounds with low crystal symmetry and complicated phase structure limit their applications, especially when composition deviates from stoichiometry ratio. By performing in situ heating high-resolution scanning transmission electron microscopy experiment and micro-mechanical testing on Ti-35 at% Pt that contained majorly Ti3Pt and αTiPt phases, it was found that precipitating herringbone twinned αTiPt islands within Ti3Pt could occur upon heating, significantly refining mixed-phase structure. The refinement of multi-intermetallic mixed-phase structure endowed brittle material with remarkable capacity for plastic deformation and strain hardening. The plastic deformation mechanisms include phase transformation upon yielding and dislocation slips during hardening, which rarely occurs in intermetallic compounds with low symmetry. The strong interaction between different deformation modes even caused nano-crystallization along slip bands. The results demonstrate that brittle-to-ductile transition in intermetallic compounds can be achieved by tuning mixed-phase structure through phase transformations.

Temperature Effect on Stacking Fault Energy and Deformation Mechanisms in Titanium and Titanium-aluminium Alloy.

Alloying elements have great influence on mechanical properties of metals. Combining dislocation characterization and in-situ transmission electron microscope straining at ambient and liquid-nitrogen temperature in high-purity titanium and Ti-5at%Al, we investigated the modulation of Al on dislocation behaviours as temperature changed. It reveals that segregation of Al at edge dislocation cores in Ti-5at%Al generates strong obstacles, promoting room temperature cross-slips. However, the effect of Al on reducing stacking-fault energy (SFE) as decreasing temperature is significant. Consequently, the lower SFE in Ti-5at%Al results in ordinary planar dislocation slip while massive dislocation cross-slips occurred in Ti at liquid-nitrogen temperature.