The Zr20Nb-H phase diagram and the characterisation of hydrides in β-Zr

In this work a combination of synchrotron X-ray diffraction, transmission electron microscopy and differential scanning calorimetry have been used to characterise the Zr20Nb-H phase diagram and hydrides in beta-Zr. A single hydride phase, termed gamma' was found to be present in beta-Zr over a wide range of H concentrations up to 1559 wppm. gamma'-hydride had an orthorhombic crystal structure with the composition ZrH0.4 +/- 0.2, and was found to be stable during heating to 450 degrees C; it can therefore be considered the equilibrium hydride in beta-Zr. Accompanying hydride nucleation is a volumetric strain of 10.4% that is accommodated elastically and plastically by the beta-Zr. The body-centre cubic to orthorhombic martensitic transformation is analogous to the Au-Cd system where the basal plane in the hydride is constructed from the (0 1 1)(beta) to give the Bain correspondence. There are strong similarities between the Zr20Nb-H and Nb-H phase diagrams with the former having a lower solubility for H at room temperature, similar to 130 wppm and similar to 290 wppm respectively. The room temperature solubility difference between body-centre cubic Nb and beta-Zr can be attributed to their electron configurations and the reduction in energy associated with the metallic Zr/Nb-H bonding. (C) 2013 Elsevier B.V. All rights reserved.