Microvoids in electrochemically hydrogenated titanium-based alloys

In a recent study, it was observed that electrochemical charging with hydrogen produces microvoids both in wrought and in additively manufactured, electron beam melted (EBM) Ti–6Al–4V alloys. This result is surprising since titanium forms stable hydrides and has an exothermic heat of hydrogen solution. By comparison, hydrogen bubble formation is typically observed only in metals and alloys with an endothermic heat of hydrogen solution that do not form hydrides. Here, we evaluate possible mechanisms for the formation of microvoids and bubbles in Ti-based alloys. Additional experimental work confirms that voids do not form in electrochemically hydrogenated, single-phase, pure wrought Ti, whereas they do form in the wrought Ti–6Al–4V alloy hydrogenated under the same conditions. In commercially pure Ti (CP–Ti), hydride is formed from the surface inward, and the surface is brittle and heavily cracked and disintegrated. By contrast, in the two-phase alloy, hydrides are formed also deeper in the bulk, and microvoids are evident both adjacent to the surface and along interphase boundaries. Alongside the forming hydride, the surface integrity is maintained, although some cracks are formed due to microvoid coalescence. While the incorporation of hydrogen into the alloy causes a large increase in its volume, we note that the precipitation of hydride from a supersaturated solution causes a net contraction. We suggest that the mechanism that best reflects the experimental evidence of microvoids formation is a manifestation of the contraction that results from hydride precipitation from a hydrogen-supersaturated alloy.