CRACK INITIATION AT LONG RADIAL HYDRIDES IN ZR-2.5NB PRESSURE TUBE MATERIAL AT ELEVATED-TEMPERATURES

Crack initiation at hydrides in smooth tensile specimens of Zr-2.5Nb pressure tube material was investigated at elevated temperatures up to 300-degrees-C using an acoustic emission (AE) technique. The test specimens contained long, radial hydride platelets. These hydrides have their plate normals oriented in the applied stress direction. Below approximately 100-degrees-C, widespread hydride cracking was initiated at stresses close to the yield stress. An estimate of the hydride's fracture strength from this data yielded a value of approximately 520 MPa at 100-degrees-C. Metallography showed that up to this temperature, cracking occurred along the length of the hydrides. However, at higher temperatures, there was no clear evidence of lengthwise cracking of hydrides, and fewer of the total hydride population fractured during deformation, as indicated by the AE record and the metallography. Moreover, the hydrides showed significant plasticity by being able to flow along with the matrix material and align themselves parallel to the applied stress direction without fracturing. Near the fracture surface of the specimen, transverse cracking of the flow-reoriented hydrides had occurred at various points along the lengths of the hydrides. These fractures appear to be the result of stresses produced by large plastic strains imposed by the surrounding matrix on the less ductile hydrides.