OBSERVATIONS OF BUBBLE-VOID TRANSITION EFFECTS IN NICKEL-ALLOYS

High-nickel alloys based on the Nimonic PE16 composition have been injected at temperatures of 525° and 625°C with 1OOOppm helium to produce a high gas-bubble concentration and subsequently irradiated with 46 MeV nickel ions. Extensive heterogeneous nucleation of bubbles is observed on faulted interstitial loops and dislocations. Evidence is found in standard PE16 alloy for bimodal bubble plus void distributions which persist during nickel-ion irradiation to 30 and 60dpa at 625°C and result in a low void volume swelling of ~1%. The observations can be correlated with the critical bubble/void transition radius which is calculated from theory to be ~4.4nm. Pre-injection of helium into a 'matrix' PE16 (low Si, Ti and Al) alloy produced an initial bubble population whose average size was above the calculated transition radius such that all bubbles eventually grew as voids during subsequent nickel-ion irradiation up to 60dpa at 625°C where the void volume swelling reached ~12%. The observations are discussed briefly and related to theoretical predictions of the bubble/void transition radius. © 1979.