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 β-Zr. A single hydride phase, termed γ′ was found to be present in β-Zr over a wide range of H concentrations up to 1559 wppm. γ′-hydride had an orthorhombic crystal structure with the composition ZrH0.4 ± 0.2, and was found to be stable during heating to 450 °C; it can therefore be considered the equilibrium hydride in β-Zr. Accompanying hydride nucleation is a volumetric strain of 10.4% that is accommodated elastically and plastically by the β-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)β 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, ∼130 wppm and ∼290 wppm respectively. The room temperature solubility difference between body-centre cubic Nb and β-Zr can be attributed to their electron configurations and the reduction in energy associated with the metallic Zr/Nb–H bonding.