**Homogeneous hydride formation path in alpha-Zr: Molecular dynamics
simulations with the charge-optimized many-body potential**

YF Zhang and XM Bai and JG Yu and MR Tonks and MJ Noordhoek and SR Phillpot, ACTA MATERIALIA, 111, 357-365 (2016).

DOI: 10.1016/j.actamat.2016.03.079

A path for homogeneous gamma hydride formation in hcp alpha-Zr, from solid solution to the zeta and then the gamma hydride, was demonstrated using molecular static calculations and molecular dynamic simulations with the charge-optimized many-body (COMB) potential. Hydrogen has limited solubility in alpha-Zr. Once the solubility limit is exceeded, the stability of solid solution gives way to that of coherent hydride phases such as the zeta hydride by planar precipitation of hydrogen. At finite temperatures, the zeta hydride goes through a partial hcp-fcc transformation via 1/3 < 1 (1) over bar 00 > slip on the basal plane, and transforms into a mixture of gamma hydride and alpha-Zr. In the zeta hydride, slip on the basal plane is favored thermodynamically with negligible barrier, and is therefore feasible at finite temperatures without mechanical loading. The transformation process involves slips of three equivalent shear partials, in contrast to that proposed in the literature where only a single shear partial was involved. The adoption of multiple slip partials minimizes the macroscopic shape change of embedded hydride clusters and the shear strain accumulation in the matrix, and thus reduces the overall barrier needed for homogeneous gamma hydride formation. This formation path requires finite temperatures for hydrogen diffusion without mechanical loading. Therefore, it should be effective at the cladding operating conditions. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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