Shrinkage and coalescence behavior of helium bubbles in Fe9Cr1.5W0.4Si F/M steel under the coupling effect of compression stress and temperature

As important irradiation defects, helium bubbles will threaten the stability and reliability of reactor structural materials. Extensive research has been conducted on the dynamic response of helium bubbles under thermal annealing, irradiation, or tension, but the behavior and mechanisms of helium bubble evolution under compression remain to be understood. Here, by adopting in-situ cyclic nanocompression testing on Fe9Cr1.5W0.4Si F/M steel containing helium bubbles at 673 K, we present direct experimental evidence for the shrinkage and coalescence behavior of helium bubbles under the coupling effect of compression stress and temperature for the first time. Unlike the stress-free condition where the bubble size shows an increasing trend, compressive stress causes the bubble to shrink. Moreover, the large-sized bubbles show a greater change in size than small-sized bubbles. At 673 K, there is no thermal motion of the bubbles in F/M steel, and it is the deformation and mutual squeezing of bubbles under compression that causes the coalescence of tightly arranged bubbles. Furthermore, helium bubbles can act as dislocation sources to emit dislocations under compression, and the emitted dislocations can in turn envelope around the helium bubbles, thus hindering bubble coalescence and leading to the formation of deformed bubbles.