Helium bubble formation in ultrafine and nanocrystalline tungsten under different extreme conditions

Bright-field TEM micrographs of UFG and NC tungsten irradiated with 2keV helium ions (flux of 3.3×1016ionsm−2s−1) at 1223K demonstrating: (a) overview of sample with bubbles decorating grain boundaries at a fluence of 3.6×1019ionsm−2; (b) nanocrystalline grain with large facetted bubbles/voids on grain boundaries and few bubbles in the grain matrix at a fluence of 3.6×1019ionsm−2; and (c) grain boundary and (d) grain boundary triple-junction decorated by facetted bubbles with different sizes inside ultrafine grains at fluence of 4.0×1020ionsm−2. [Display omitted] •Helium ion irradiation was performed on ultrafine grained and nanocrystalline tungsten.•Irradiations were performed at different extreme conditions.•Bubble formation and evolution were performed via ex-situ and in-situ TEM.•Preferential bubble formation on grain boundaries occurred at displacement energies and high temperatures.•Vacancy formation and migration is important for preferential bubble formation on grain boundaries. We have investigated the effects of helium ion irradiation energy and sample temperature on the performance of grain boundaries as helium sinks in ultrafine grained and nanocrystalline tungsten. Irradiations were performed at displacement and non-displacement energies and at temperatures above and below that required for vacancy migration. Microstructural investigations were performed using Transmission Electron Microscopy (TEM) combined with either in-situ or ex-situ ion irradiation. Under helium irradiation at an energy which does not cause atomic displacements in tungsten (70eV), regardless of temperature and thus vacancy migration conditions, bubbles were uniformly distributed with no preferential bubble formation on grain boundaries. At energies that can cause displacements, bubbles were observed to be preferentially formed on the grain boundaries only at high temperatures where vacancy migration occurs. Under these conditions, the decoration of grain boundaries with large facetted bubbles occurred on nanocrystalline grains with dimensions less than 60nm. We discuss the importance of vacancy supply and the formation and migration of radiation-induced defects on the performance of grain boundaries as helium sinks and the resulting irradiation tolerance of ultrafine grained and nanocrystalline tungsten to bubble formation.