A first principles investigation of the hydrogen-strain synergy on the formation and phase transition of hydrides in zirconium

Abstract

The precipitation of hydrides phases is known to be a source of embrittlement in zirconium and its alloys. In the present work, the effects of hydrogen (H) in the bulk of Zirconium (Zr) systems has been investigated using density functional theory (DFT). A model for the nucleation of hydrides in terms of the thermodynamic and strain conditions has been determined from the examination of interstitial H configurations in hcp-Zr (α-Zr). We found that several coherent hcp structures for Zr2H have similar formation energies and structural properties, and thus are relatively easy to form. A number of Zr2H hydrides with hcp structures were found to be dynamically unstable, and were found to promote a phase transformation to the fcc lattice. Based on the hydrogen distribution, the orientation relationship and the strain conditions, the transition of Zr2H hydrides from the hexagonal to cubic phases have been determined. Our results suggest that hydrogen plays a fundamental role, since a substantial rearrangement is required to promote the hcp→fcc transformation of hydrides phases. We determined that a hcp→fcc transformation of the Zr host lattice is assisted by the lower barrier for phase transition in Zr2H compared to that of pure α-Zr. Thus, leading to the formation of the brittle δ hydride phase. The reported formation mechanisms of hydrides provide the basis to understand the precipitation of hydrides observed experimentally.