Thermal decomposition of titanium hydrides in electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V alloys using in situ high-temperature X-Ray diffraction.

Thermal decomposition of titanium hydrides in electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V alloys containing 6 wt% β is compared. Differential scanning calorimetry (DSC) is used to identify phase transitions. High-temperature X-ray diffraction (HTXRD) is used to identify phases and determine their contents and crystallographic parameters. Both alloys are found to contain α H (hcp) and β H (bcc) solid solutions, as well as δ a (fcc) and δ b (fcc) hydrides after hydrogenation. δ a is found to decompose between room temperature and 350 °C to α H (in both alloys) plus either β H and δ b (wrought alloy) or δ b only (EBM alloy). δ b fully decomposes at either 450 °C (wrought alloy) or 600 °C (EBM alloy) to α H plus H 2 desorption (which starts at 300 and 350 °C in the wrought and EBM alloys, respectively). In the case of the wrought alloy, β H is also formed in this decomposition reaction due to faster diffusion of hydrogen. The non-continuous, finer needle-like morphology of the β-phase in the as-printed EBM alloy combined with its smaller lattice constants seem to inhibit hydrogen diffusion into the bulk alloy through the β-phase, thus triggering δ a dissociation into δ b (rather than to β H +δ b) and δ b decomposition into α H (rather than to α H + β H). Hydrogen incorporation in the α H phase results in its expansion in the c direction in both alloys. HTXRD allows to conclude that both δ a and δ b hydrides decompose up to 600 °C. Hydrogen peaks measured at higher temperatures are due to hydrogen desorption from the hydride that is decomposed from the sample's bulk and/or hydrogen desorption from β H and/or α H during heating. These findings indicate that the EBM Ti–6Al–4V alloy might be more prone to hydrogen damage at elevated temperatures than its wrought counterpart when both have a similar β-phase content. • Dehydrogenation of hydrogenated EBM and wrought Ti–6Al–4V alloys is compared. • The kinetics and products of hydride decomposition are different in the two alloys. • The differences result from different microstructures of the origin alloys. • Decomposition of δ a to δ b is reported here for the first time. • The EBM alloy may be more prone to hydrogen damage at elevated temperatures. [ABSTRACT FROM AUTHOR]