Towards resolving a long existing phase stability controversy in the Zr-H, Ti-H systems

In the binary Zr-H, Ti-H systems, there are three common hydride phases, namely γ-, δ-, and ε-hydride, with different ratios of H to Zr/Ti atoms. Among them, the relative stability of γ-hydride remains controversial, although it has been observed in many studies. In this study, we demonstrate that γ-hydride is the only stable hydride phase in high purity Zr and commercial purity Ti, using electrolytically loaded hydrogen and ex-situ synchrotron x-ray diffraction. Further, in-situ synchrotron heating/cooling experiments were conducted with hydrided pure Zr sample demonstrating the co-existence of γ- and δ-hydride phase, from a furnace cooling heat treatment followed by three days room temperature aging. It is shown that during heating γ-hydride dissolved first, while δ-hydride remained stable at temperatures up to 280°C, followed by dissolution of δ-hydride until all hydrogen was in solution. Under subsequent slow cooling, δ-hydride precipitated first, followed by a slow formation of γ-hydride when temperatures were below 210°C, which continued during a subsequent 60 hours aging at room temperature. The phase stability when hydride forms as precipitates in Zr is therefore determined to be δ-hydride as the high temperature phase, and γ-hydride as the low temperature phase. The microstructural characteristics of γ-hydride in Zr and Ti were studied by electron microscopy. The observed co-existence of γ- and δ-hydride as well as change from δ to γ during long term room temperature aging in the TEM confirmed the synchrotron data.