Dynamic behavior of helium bubbles in Fe9Cr1.5W0.4Si F/M steel at elevated temperature

The degradation of mechanical properties caused by transmutation/deuterium-tritium reaction produced helium (He) is directly related to the behavior of He bubbles in nuclear materials. Here, the dynamic behavior of He bubbles was in-situ studied in an annealed Fe9Cr1.5W0.4Si ferrite-martensite (F/M) steel using transmission electron microscopy. The critical temperature at which He bubbles started to appear obvious Brownian motion was similar to 1013 K. The mobile spherical bubbles gradually evolved into immobile polyhedral bubbles through migration and coalescence, and the critical size between mobile spherical and immobile polyhedral was similar to 4 nm. The macroscopic diffusion coefficients of bubbles at 1023 K were obtained as a function of bubble size by quantifying the bubble motion in the two-dimensional plane. Meanwhile, the motion characteristics of bubbles away from or close to dislocation were compared and analyzed in detail. Due to the attraction of dislocations, the migration path of He bubbles exhibited a similar 1D Brownian motion. This was attributed to the short-range elastic interaction between He bubbles and dislocations. These results provide direct evidence and potential insights for understanding the dynamic behavior of He bubbles in metals.