Formation of blisters in molybdenum bombarded with helium

Abstract

THE formation of surface blisters on metals bombarded with low energy inert gas ions was first observed by Primak¹ but only recently, when the potential importance of blistering in the first wall of a fusion reactor became recognised, has much further attention been paid to the phenomenon^2,3. Most of this work has used scanning electron microscopy to examine the blistered surface although considerable information has also been obtained from gas release measurements both during bombardment and in post-bombardment anneals. Transmission electron microscopy has been used to a lesser extent but nevertheless has yielded important information on the substructure within the layer where the gas atoms came to rest—normally some hundreds of Ångstroms from the metal surface. For example, several workers have shown that gas bubbles form in thin foils irradiated with low energy (<300 keV) helium or argon ions^4–6. One interesting fact (D. J. Mazey, unpublished) is that over a wide temperature range from 600 °C down to 20 °C, 36 keV helium bombardment of molybdenum produces a fine distribution of helium bubbles crystallising on to a body-centred-cubic (b.c.c.) lattice analogous to the void lattice^4,7. Here I describe an extension of transmission microscopy to view the blisters themselves and suggest some details of the blister formation mechanism.