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arXiv · 2609.23564

Dual Role of Mobile Interstitials in Defect Kinetics: From Retardation to Acceleration

Shihao Zhang·Shihao Zhu·Junping Du·Shuhei Shinzato·Ju Li·Shigenobu Ogata
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Abstract

Mobile interstitial atoms redistribute while defects migrate, complicating prediction of defect kinetics and leading to contradictory reports of acceleration and retardation. We formulate defect activation in a grand-canonical ensemble at fixed interstitial chemical potential and define a pathway free-energy landscape $ΔG(Λ;T,μ)$ and the corresponding activation free energy $ΔG^{\ddagger}$. We compute these quantities using both hyperplane-constrained thermodynamic integration via a mean force relation and a two-state chemical-potential integration, denoted TSCPI. The latter requires sampling only the initial and transition states; a single chemical-potential integration then yields $ΔG^{\ddagger}(T,μ)$ across a wide $μ$ range, enabling rapid mapping over temperature and chemical potential. Applied to vacancy diffusion in FCC, BCC, and HCP metals with H (including in plane and cross plane diffusion in Zr) and in BCC W with He, both free energy routes agree with diffusion coefficients from molecular dynamics mean square displacements. The resulting maps reveal regimes of suppression and enhancement, including a crossover from suppression to enhancement with increasing hydrogen concentration. A site occupancy analysis links barrier shifts to state dependent site spectrum changes and transferable interstitial interaction terms.

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Shihao Zhang, Shihao Zhu, Junping Du, Shuhei Shinzato, Ju Li, Shigenobu Ogata. 2026-09-20. Dual Role of Mobile Interstitials in Defect Kinetics: From Retardation to Acceleration. https://arxiv.org/abs/2609.23564

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