Hydrogen-induced phase separation in amorphous Cu0.5Ti0.5 alloys. II. Thermal evolution.

Abstract

The thermal evolution of ${\mathrm{Cu}}_{0.5}$${\mathrm{Ti}}_{0.5}$ amorphous hydrides has been studied by both calorimetry and neutron-diffraction experiments for different hydrogen contents. At high hydrogen contents, real-time neutron experiments performed during heating at a constant rate show that long-range crystallization into copper and titanium hydride takes place at a well-defined temperature, as confirmed by the exothermic peak in calorimetry. The onset of desorption of hydrogen at high temperature leads to the formation of metastable Cu-rich and then stable \ensuremath{\gamma}-CuTi phases. At smaller hydrogen contents, the evolution is much more gradual. Cu and ${\mathrm{TiH}}_{\mathrm{x}}$ diffraction lines progressively appear in the diffraction pattern. In the meantime the small-angle maximum shifts towards the origin. Crystallization into \ensuremath{\gamma}-CuTi proceeds in two stages. In the first one, unusual variation of the lattice parameters is observed, whereas the size of the crystallites is constant. The second one coincides with the decomposition of titanium hydride upon desorption of hydrogen. Such variations are compared to those extracted from small-angle neutron scattering experiments.