Dendritic growth of voids in irradiated nickel--aluminum alloys

Void morphology suggestive of dendritic growth observed in ion irradiated Ni-Al alloys is described. Observations of dendritic voids provide rather strong, although indirect, evidence for interface-reaction controlled void growth, at least during the early stages of swelling. This is because the shape instability that changes a polyhedron into a dendrite is consistent with the theoretically expected, and experimentally observed, transition from interface-reaction controlled to diffusion-controlled growth. The interface-reaction in this case is the lateral spreading process. Al additions lower the steady-state vacancy concentration, c/sub v/, at a given temperature and dose rate promoting the onset of dendritic growth. There is evidence that this occurs because of the observation that void growth rates in Ni-Al alloys decrease as the Al content increases; lowering c/sub v/ is one way to account for this. Consequently, a reduction in c/sub v/ simultaneously lowers the void growth rate and the critical size for the onset of dendritic growth. Whereas the simplicity of this explanation is appealing, it is believed that the conditions governing the onset of morphological instability are complicated by the effects of irradiation-induced solute segregation, which have been observed in several other alloys