Annealing cracking in Zr and a Zr-alloy with low hydrogen concentration

Hydrogen embrittlement, also known as hydrogen-induced cracking, is a longstanding concern for zirconium (Zr) and its alloys. The affinity for hydrogen in zirconium results in the formation of brittle hydrides. Hydrides are conventionally thought to facilitate crack propagation, and the prevailing recommendation to hinder hydride embrittlement involves reducing the hydrogen concentration in the material and applying slow cooling to reduce thermal stresses. Here, in Zr with a maximum hydrogen concentration of 43 wppm, we show that cracks initiate unexpectedly after annealing, a step that involves slow cooling and no external stress. With a suite of high-resolution microscopy, we show that annealing cracks initiate at prismatic δ-hydrides, whose presence is unexpected. The microscopic annealing crack paths are consequently crystallographic, following the habit plane of these prismatic δ-hydrides. The commonly reported γ-hydride and δ-hydrides that develop on the basal plane are found not to initiate cracks. We rationalize that prismatic δ hydrides result from the intergranular thermal residual stresses that develop during slow cooling. This finding of uncommon annealing cracking further advances the mechanistic understanding of hydride embrittlement in zirconium and can lead to new strategies for design and fabrication of advanced zirconium alloys for harsh environments.