HYDRIDE PRECIPITATION IN alpha / beta ZIRCONIUM ALLOYS.

The precipitation of delta -hydride (ZrH//1//. //5) in fine-grained alpha / beta zirconium alloys has been studied. Both interphase and transgranular hydrides are observed, the mechanism of growth being dependent on the nature of the alpha / beta interface. Macroscopic hydrides observed in the optical microscope are always composed of stacks of smaller hydride plates with habit planes close to the basal plane. The apparent habit plane of the macroscopic hydride plate is determined by the geometry of the stack. The stability of different hydride stacks is considered and the role of the texture of the alloy and of a stress (external or internal) acting on the basal planes in determining the orientation of the stacks is explained.