Effect of hydride distribution and oxide film on the ductility of Zircaloy-4

In the nuclear reactors, structural components built in Zr alloys suffer corrosion. This process facilitates the continuous pick up of hydrogen. In low thickness components, and after several years in service, the terminal solid solubility at operation temperature is overcome. Under these conditions, hydride particles are precipitated with a nearly uniform distribution. This phenomenon can produce alloy embrittlement when a critical concentration of hydrogen is attained. In order to asses that critical value, it is necessary to obtain in the laboratory hydride distributions similar to that one produced in the reactor. This was accomplished when specimens were hydrided in autoclave with LiOH solution. The critical concentration measured in these specimens was 1500-1600 ppm. Other samples, coated with a copper film, were hydrided by gas phase charging. Two different hydride distributions were obtained and the samples were free of superficial oxide. It was verified that with a quite uniform hydride distribution the critical concentration rises to a level of 2900 - 3100 ppm. In the presence of hydride clusters the critical concentration is about 1000 ppm lower. Other test must be done in order to elucidate whether the loss of ductility in autoclaved samples is due to the presence of oxide or to the non-quite uniform hydride distribution. (author)