Helium bubble growth in 316 stainless steel

Abstract
A systematic TEM investigation has been made of helium bubble growth in type 316 stainless steel. Commercial stainless steel samples have been vacuum annealed following room temperature helium implantation to a concentration of 5 × 1026 He m−3. The bubble growth kinetics have been determined by measuring the mean bubble radius at annealing times in the range 1 to 200 h for temperatures of 873, 923 and 1023 K. At the lower two temperatures the bubble growth mechanism is believed to be migration and coalescence, with the migration limited by volume diffusion of the metal atoms. Four additional anneals for 2 h at temperatures in the range 923-973 K have been made to determine the activation energy for diffusion. This is found to be 4.9 eV (assuming the migration is limited by volume diffusion). At 1023 K and for annealing times of less than 8 h, bubble growth appears to occur mainly by the acquisition of thermal vacancies from the sample surface. At longer annealing times the bubble migration becomes limited by the nucleation of atomic ledges on the bubble facet and the rate of bubble growth is reduced. The ledge energy is estimated to be 1 × 10-11 J-m−1.

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