Precipitation characteristics and distributions of subsurface hydrides in zirconium

Hydride precipitates that develop in Zr after absorbing hydrogen can impart significant material hardening and embrittlement. Here, we use electrolytic hydrogen charging to synthesize Zr samples with different subsurface hydride aging times and transmission electron microscopy to understand the mechanisms underlying hydride precipitation and their distributions across the microstructure of pure Zr. Subsurface hydride formation involves volumetric swelling, causing the sample surface to produce bumps. Analysis of the morphologies and spatial distributions of these bumps can reveal several important characteristics of hydrides below. The analysis indicates that subsurface hydrides first form by isolated nucleation followed by coalescence. The shape of the hydride bump is found to be determined by the angle between the basal plane of the subsurface hydride and the sample surface. We reveal that, at room temperature, the hydride phase transition sequence follows γ-ZrH→δ-ZrH_1.66→ε-ZrH₂. The two main α-Zr/hydride orientation relationships are 1) (0001)_α||(111¯)_δ with [211¯0]_α||[011]_δ or 2) (0001)_α||(001)_γ/_δ with [12¯10]_α||[110]_γ/δ. Last, we show that the orientation of the basal plane plays a decisive role in the formation of both intragranular and intergranular subsurface hydrides. Statistical analysis of several hundred grain boundaries reveals that grain boundaries with c-axis misorientation of <15°, =55°-60° and >85° are preferential sites for subsurface hydride formation, while those with c-axis misorientations higher than 15° and one grain's basal plane nearly perpendicular to the grain boundary plane resist hydride precipitation. These findings can guide grain boundary engineering efforts for controlling hydrogen damage in Zr.