The migration and coalescence of tritium-induced helium bubbles on grain boundaries in nickel

The migration and coalescence of sub-critical helium bubbles on grain boundaries during creep in nickel has been studied. Helium was introduced without radiation damage by tritium decay following gas phase charging. An initial population of helium bubbles was then stabilized by suitable heat treatment. Interrupted creep tests, followed by sectioning and quantitative metallography, revealed that the initial fine scale distribution of small (2-4 nm) bubbles quickly coarsened during primary creep until the critical size for growth by vacancy absorption was attained, resulting in a fixed distribution of cavity nuclei. The characteristics of that distribution, including average spacing and stress dependence, strongly supported the Beere migration and coalescence model. Thin film transmission microscopy of interrupted creep specimens provided additional supporting evidence. (orig.)