A study of the effect of helium concentration and displacement damage on the microstructure of helium ion irradiated tungsten

Transmission electron microscopy (TEM) with in situ He ion irradiation has been used to examine the damage microstructure of W when varying the helium concentration to displacement damage ratio, irradiation temperature and total dose. Irradiations employed 15, 60 or 85 keV He+ ions, at temperatures between 500 and 1000°C up to doses of ~3.0 DPA. Once nucleated and grown to an observable size in the TEM, bubble diameter as a function of irradiation dose did not measurably increase at irradiation temperatures of 500°C between 1.0 and 3.0 DPA; this is attributed to the low mobility of vacancies and He/vacancy complexes at these temperatures. Bubble diameter increased slightly for irradiation temperatures of 750°C and rapidly increased when irradiated at 1000°C. Dislocation loops were observed for at irradiation temperature of 500 and 750°C and no loops were observed at 1000°C. Burgers vectors of the dislocations were determined to be b = ±1/2<111> type only and both vacancy and interstitial loops were observed. The proportion of interstitial loops increased with He-appm/DPA ratio and this is attributed to the concomitant increase in bubble areal density, which reduces the vacancy flux for both the growth of vacancy-type loops and the annihilation of interstitial clusters.