Ab initio study of competitive hydride formation in zirconium alloys

We present a detailed atomic-scale investigation of coherent hydrides in Zr alloys. While previous experimental and theoretical studies emphasized the γ′ (ZrH) and ζ (Zr2H) phases as potential precursors in the hydride precipitation sequence, they led to contradictory conclusions justifying further analysis of the nucleation of these two types of precipitates. To this aim, we take proper account of the three energy contributions (bulk chemical, interfacial and elastic) involved in Classical Nucleation Theory by incorporating original features such as (i) ζ bulk off-stoichiometry, (ii) anisotropic matrix/hydride interfacial energies and (iii) shape factors of platelike nuclei. These effects turn out to be essential to provide a realistic description of the conditions of temperature and H content leading to preferential γ′ or ζ formation. Our work provides a unifying picture of the previous, apparently diverging, works on coherent zirconium hydrides.