TEM study of bubble-evolution related to the absence of a second generation of blisters in low energy He bombarded Nb

Abstract

Results are presented of work using transmission electron microscopy to examine the evolution of the structure related to the absence of new blisters after high fluence He bombardment of Nb. At 500°C, the mean bubble diameter increases from 3.2 nm at 1.9 × 10¹⁷ He cm⁻² (fluence smaller than the blistering threshold) to 9.7 nm at 9.4 × 10¹⁸ He cm⁻² (fluence larger than the fluence necessary for the sputtering of the blisters). This increase is done at the expense of the bubble density which decreases from 5.5 × 10¹² cm⁻² to 2.8 × 10¹¹ cm⁻². However it is remarkable that the total volume of bubbles remains constant. Due to He saturation in the implanted layer the pressure is constant. It is shown that the diameter increase and density decrease mean a lowering of the stresses in the membranes between bubbles, thus preventing the formation of new blisters. This suggests that large bubbles can continue to grow, without becoming overpressurized, by absorbing mostly radiation-produced vacancies whereas small bubbles shrink by absorbing mostly interstials and releasing He through radiation assisted diffusion.