Formation of a tetragonal phase hydride in Ti–Mo–H system

The structural relationship between the hydride phases in Ti–Mo–H solid solution system (Mo content up to 15 at% in the alloy) during dehydrogenation process under annealing has been studied by conventional and in situ X-ray powder diffraction and transmission electron microscopy (TEM) analysis. During dehydrogenation, the saturated hydrides of the Ti–Mo alloys with fcc δ-phase structure transfer into bcc β-phase at higher temperatures. An associated hydrogen concentration reduction for the δ-phase hydride is observed in the process. However, as the hydrogen concentrations decrease to certain values (H/M  1.1–1.7), the unsaturated δ-phase formed at high temperature would become unstable at lower temperature, and transfer into a tetragonal phase (denoted the -phase here). Unlike that of the -phase in Ti–H system, the phase transition does not occur for the saturated δ-phase with hydrogen concentration close to the stoichiometric limit. The hydrogen concentration of this -phase hydride is in between that of the tetragonal γ and -phase in Ti–H system, but more close to the γ-phase. The occurrence region of this -phase expands along with the increase of the Mo content in the alloys. The phase has a lattice similar to that of the -phase in Ti–H system with corresponding fct unit-cell c/a < 1.