A transmission electron microscopy study of molybdenum irradiated with helium ions

Abstract

Abstract The paper describes the application of transmission electron microscopy to investigate the structure developed in the surface layers of molybdenum samples during irradiations with helium ions in the range 18 to 60 keV. Specimen temperatures were varied over a wide range from 20° C to above 1000°C and both dynamic and post-irradiation analytical techniques were utilised to provide information on the separate contributions of interstitials, vacancies and the helium gas to the formation of the damage structures. The interstitials formed dislocation loops, with subsequent growth to a dislocation network, while the combination of vacancies and gas resulted in a very high density (≈10 19 cm −3 ) of small helium bubbles over a wide temperature range (20–700°C). The helium bubbles were frequently found to be aligned in a body-centred-cubic lattice which produced extra reflections in electron diffraction patterns. Measured [110] bubble lattice spacings varied between 32 A and 50 A but showed no significant temperature dependence up to 700°C. At high helium doses (≈5 × 10 17 ions cm −2 ) blisters were clearly seen, together with the small bubbles, thus indicating the rapid growth of blisters rather than the slow coalescence and growth of the bubbles. At high temperatures larger helium bubbles were present. The insensitivity of the bubble parameters to vacancy mobility, the role of interstitials in creating surface stresses, and other aspects, are discussed particularly in relation to mechanisms of blistering. Although no definite conclusions are reached, it is clear that TEM evidence is important and that any proposed mechanism of blister formation must take this evidence into account.