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

Abstract

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 αH (hcp) and βH (bcc) solid solutions, as well as δa (fcc) and δb (fcc) hydrides after hydrogenation. δa is found to decompose between room temperature and 350 °C to αH (in both alloys) plus either βH and δb (wrought alloy) or δb only (EBM alloy). δb fully decomposes at either 450 °C (wrought alloy) or 600 °C (EBM alloy) to αH plus H2 desorption (which starts at 300 and 350 °C in the wrought and EBM alloys, respectively). In the case of the wrought alloy, βH 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 δa dissociation into δb (rather than to βH+δb) and δb decomposition into αH (rather than to αH + βH). Hydrogen incorporation in the αH phase results in its expansion in the c direction in both alloys. HTXRD allows to conclude that both δa and δb 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 βH and/or αH 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.