The migration and coalescence of inert gas bubbles in metals

Abstract

Thin copper films containing 0* I atomic percent of helium , introduced by bombardment with 38 MeV a-particles, have been pulse-heated at ~ 800 °C in the electron microscope and the small helium bubbles which form ed were observed and photographed after successive pulses. The bubbles could be seen to move through the copper with velocities ~ 1000 A/s, exploding when they met the surface of the film, and coalescing with other bubbles which lay in their path . Measurements of the bubble radii before and after coalescence suggested that the gas in the larger bubbles obeyed the perfect gas laws and that its pressure almost exactly balanced the surface pressure of th e bubble. Since no bubble was observed to reduce in size during its m igration, it was concluded th a t helium (the sm allest inert gas atom ) wTas unable to dissolve in copper, even at high temperature. In these thin films the bubbles all moved in th e same general direction, and it is thought that they migrated under th e influence of the temperature gradient. The coalescence of the inert gas bubbles reduces their number, and enlarges them , and in consequence causes th e metal to increase its overall volume. The observation of inert gas bubbles attached to precipitates in aluminium , beryllium , and uranium suggests that the resultant lowering of free energy when a bubble collides with a precipitate tends to anchor the bubble and restrict the volume increase.