Abstract
The mobility of helium bubbles in iron and niobium have been measured by two different techniques. The mobility of helium bubbles in iron was investigated by applying a temperature gradient driving force to the bubbles and measurements of the migration distance were made using optical microscopy and the bubble size was estimated using the scanning electron microscope. The mobility of bubbles in niobium was investigated by measuring the rate at which the average bubble size increased with time and temperature as a result of random migration and coalescence, the measurements being taken from transmission electron micrographs. The temperature gradient migration measurements for iron revealed that the bubbles travel up the temperature gradient, thus indicating a positive effective heat of transport. The rate of migration was compatible with a surface diffusion mechanism with a heat of transport approximately equal to the activation energy for surface self-diffusion. It is suggested that the surface transport entity is the adatom. The calculated migration rates for bubbles moving by volume diffusion or vapour transport mechanisms are considerably lower than those measured in this investigation. The migration and coalescence results for niobium gave a log (bubble radius) versus log (time) plot with a slope of 1/5 which indicates that the bubbles move by a surface diffusion mechanism. The high values obtained for the activation energy for surface self-diffusion and the pre-exponential constant suggest that the surface transport entity is also the adatom. It has been found that the distribution of bubble sizes does not follow the simple skew distribution that would be expected from random migration and coalescence and it is suggested that interaction of bubble stress fields may be responsible.