Scanning electron microscopic observations of samples, which had been annealed after recovery from high-temperature heat treatment in the hydride phase, revealed the presence of numerous holes which were 20 to 200nm in size. On the basis of various features of the holes, they were identified as being voids that were formed via the agglomeration of supersaturated vacancies (5at%) which had diffused from the surface to the interior during heat treatment.

H.Osono, T.Kino, Y.Kurokawa, Y.Fukai: Journal of Alloys and Compounds, 1995, 231, 41-5