Grain size effects on He bubbles distribution and evolution

•SMAT treated T91 and conventional T91 were implanted by 200keV He2+ to 1×1021 Hem−2 at room temperature and annealed at 450°C for 3.5h.•He bubbles in nanometer-size-grained T91 are smaller in as-implanted case.•The bubbles in the matrix of nanograins were hard to detect and those along the nanograin boundaries coalesced and filled with the grain boundaries after annealing.•Brownian motion and coalescence and Ostwald ripening process might lead to bubbles morphology presented in the nanometer-size-grained T91 after annealing.