An atomistic study of γ → γ0 → α'' martensitic transformation in uranium-6wt % niobium

YF Huang and SL Yao and HJ Chen and P Li and K Wang and J Chen and HQ Deng and WY Hu, JOURNAL OF NUCLEAR MATERIALS, 606, 155617 (2025).

DOI: 10.1016/j.jnucmat.2025.155617

A machine-learning interatomic potential based on the moment tensor framework (moment tensor potential, MTP) for uranium and niobium binary system is developed to investigate the gamma - gamma 0 - alpha" martensitic transformation of U-6 wt % Nb alloy (U-6Nb) under rapid cooling. The training of the MTP utilized an extensive reference database generated from density functional theory calculations. The final MTP could accurately predict the fundamental properties of U, Nb and gamma-U-6Nb. During the rapid cooling from high temperature, the MTP predicts a continuous martensitic transformation from gamma - gamma 0 - alpha", with the final product being a 130 twinned alpha" phase. In contrast to the traditional understanding of the phase transition mechanism from the orientationrelationship-based inference, the transition path is directly observed at atom scale. The body-centered atoms in adjacent gamma-cell shifts approximately 0.12c along opposite 001 direction, with a slight contraction along the caxis to form the gamma 0 phase. With the decreasing of the temperature, alternative 110 planes of the gamma 0 phase further slides, followed by a shearing along the (111) direction or its opposite on the 112 planes, resulting in the formation of alpha" phase. A unique "monoclinic-bcc" nested lattice structure existing in the alpha" phase is first revealed, which accounts for the emerging of the metastable alpha" phase upon the rapid cooling. This peculiarity of alpha" phase in U-6Nb would provide new clues for developing high-fidelity models and understanding the structureproperty relationships.

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