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

Abstract

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.

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Fig. 1: The Ti–Mo dual-phase alloy.
Fig. 2: In situ TEM and STEM characterization at ~650 °C showing the non-classical nucleation process.
Fig. 3: In situ STEM characterization and DFT modelling.
Fig. 4: DFT modelling of the phase transformation.

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Data availability

All data generated or analysed during this study are included in the published article and Supplementary Information, and are available from the corresponding authors upon reasonable request.

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Acknowledgements

We acknowledge Y.-Z. Wang and J. Xu for useful discussions. Q.Y. acknowledges support by the Natural Science Foundation of China (51671168 and 51871197), the National 111 Project under grant no. B16042 and the State Key Program for Basic Research in China under grant no. 2017YFA0208200. W.Z. acknowledges support by the National 111 Project 2.0 under grant no. BP2018008, the Xi’an Jiaotong University high-performance computing platform and the International Joint Laboratory for Micro/Nano Manufacturing and Measurement Technologies at Xi’an Jiaotong University. E.M. acknowledges Xi’an Jiaotong University for hosting his work. L.-q.C. acknowledges the generous support from the Hamer Foundation through the Hamer Professorship in the Department of Materials Science and Engineering at Penn State. We thank beamline BL14B1 of the Shanghai Synchrotron Radiation Facility for providing the beamtime. The financial support is from the National Science Foundation of China (NSFC grant U1932201).

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Contributions

Q.Y. and W.Z. designed the research. X.F., Q.Y., B.Z., Q.Z. and L.G. performed STEM and in situ experiments. Y.Z. and W.-Z.Z. synthesized alloys. X.-D.W., W.Z., S.S. and J.-J.W. performed theoretical modelling and ab initio simulations. W.W., Q.Y. and X.F. performed synchrotron X-ray diffraction experiments. Z.Z., Q.Y., W.Z., E.M. and L.-q.C. contributed to data analysis and discussions. Q.Y., W.Z. and E.M. wrote the paper, with input from their coauthors and especially L.-q.C.

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Correspondence to Wei Zhang, Long-qing Chen, Qian Yu or En Ma.

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Supplementary Information

Supplementary Figs. 1–10 and caption for Supplementary Video 1.

Supplementary Video 1

The in situ HRTEM observation of the transformation. The blue dashed line marks the position of the pre-existing phase boundary. The red arrows point to the sites of nucleation.

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Fu, X., Wang, XD., Zhao, B. et al. Atomic-scale observation of non-classical nucleation-mediated phase transformation in a titanium alloy. Nat. Mater. 21, 290–296 (2022). https://doi.org/10.1038/s41563-021-01144-7

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