HYDRIDE EMBRITTLEMENT IN ZIRCALOY-4 PLATE .1. INFLUENCE OF MICROSTRUCTURE ON THE HYDRIDE EMBRITTLEMENT IN ZIRCALOY-4 AT 20-DEGREES-C AND 350-DEGREES-C

The hydride embrittlement in ZIRCALOY-4 was studied at room temperature and 350-degrees-C. Sheet tensile specimens of two fabrication routes in the stress-relieved, recrystallized, and beta-treated states were hydrided with or without tensile stress. It was found generally that the effect on strength of increasing hydrogen content was not important. However, for the tensile tests at room temperature, there is a ductile-brittle transition when the hydrogen content is higher than a certain threshold. The prior thermomechanical treatment shifts this transition considerably. In situ scanning electron microscopy (SEM) tests, fractography, and fracture profile observations were carried out to determine the fracture micromechanisms and the microscopic processes. At 20-degrees-C, the fracture surfaces are characterized by voids and secondary cracks for low and medium hydrogen contents and by intergranular cracks and decohesion through the continuous hydride network for high hydrogen contents. This phenomenon disappears at 350-degrees-C, and the hydrogen seems to exert no more influence on the fracture micromechanism even for very high hydrogen contents (up to 1500 wt ppm). A full-coverage model is proposed to estimate the critical hydrogen content that makes ZIRCALOY-4 totally brittle. The effect of microstructure on hydride embrittlement in different metallurgical states is thus explained according to the modeling. Special attention is devoted to relating the micromechanisms and the modeling in order to propose the possible measures needed to limit the hydride embrittlement effect.