We explore the distribution, morphology and structure of zirconium hydrides formed using different cooling rates through the solid state Zr+[H]  Zr + hydride transus, in fine and blocky alpha Zircaloy-4. We observe that cooling rate and grain size control the phase and distribution of hydrides. The blocky alpha (coarse grain, > 200 µm) Zircaloy-4, has a smaller grain boundary area to grain volume ratio and this significantly affects nucleation and growth of hydrides as compared to fine grain size (~11 µm) material. Main BodyZirconium alloys are used in the nuclear industry as fuel cladding, as it has a good strength to neutron absorption cross section ratio and reasonable corrosion resistance. One concerns when using zirconium alloys in high temperature water reactors is corrosion, as during service it can react with high temperature water to generate an oxide scale and pick up hydrogen [1]. For service conditions (~350°C) this hydrogen may exist in solution where it is highly mobile [2,3]. The hydrogen travels from hot to cold regions and from low to high hydrostatic stress. As the solubility of hydrogen in zirconium has a steep incline against temperature, and so excess hydrogen or changes in temperature may result in the precipitation of hydrides.

Zircaloy-4 is used extensively in the nuclear industry. It has an allotropic phase change from hexagonal close packed (HCP) alpha phase at room temperature to body centred cubic (BCC) beta phase at high temperature. This phase transformation results in a 'memory' of the microstructure when it is subject to thermomechanical processing and introduces special interfaces via the Burgers orientation relationship.

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