Mechanism of collective interstitial ordering in Fe–C alloys

Abstract

Collective interstitial ordering is at the core of martensite formation in Fe–C-based alloys, laying the foundation for high-strength steels. Even though this ordering has been studied extensively for more than a century, some fundamental mechanisms remain elusive. Here, we show the unexpected effects of two correlated phenomena on the ordering mechanism: anharmonicity and segregation. The local anharmonicity in the strain fields induced by interstitials substantially reduces the critical concentration for interstitial ordering, up to a factor of three. Further, the competition between interstitial ordering and segregation results in an effective decrease of interstitial segregation into extended defects for high interstitial concentrations. The mechanism and corresponding impact on interstitial ordering identified here enrich the theory of phase transitions in materials and constitute a crucial step in the design of ultra-high-performance alloys.

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Fig. 1: Disorder–order transition in Fe–C alloys.
Fig. 2: Strain-induced interaction.
Fig. 3: Impact of local anharmonicity on the disorder–order transition.
Fig. 4: Competition between C ordering and segregation.
Fig. 5: Comparison between theory and experiment.

Data availability

The datasets generated during the current study are available from the corresponding authors on request.

Code availability

The computer code developed during the current study is available from the corresponding authors on request.

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Acknowledgements

We gratefully acknowledge the financial support from the German Research Foundation (DFG) under grant HI 1300/15-1 within the DFG-ANR project C-TRAM. H.W. thanks J. Westraadt at the Nelson Mandela University for help with measuring the TEM foil thickness by EELS. X.Z. thanks W. Wang for fruitful discussions.

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X.Z., T.H. and J.N. designed the project. X.Z. performed all atomistic calculations under the supervision of T.H., J.R. and J.N. H.W. and Y.L. did the HRTEM characterization and APT analysis. All authors discussed the results and contributed to writing the manuscript.

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Correspondence to Xie Zhang or Hongcai Wang.

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Supplementary Information

Supplementary Notes 1–5, Figs. 1–10 and Table 1.

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Zhang, X., Wang, H., Hickel, T. et al. Mechanism of collective interstitial ordering in Fe–C alloys. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-0677-9

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