Atomistic study on helium-to-vacancy ratio of neutron irradiation induced helium bubbles during nucleation and growth in α-Fe

•The equilibrium He/V ratio of small helium bubbles indicated by MD simulation is around 1.4 ± 0.15.•Molecular statics and Metropolis Monte Carlo algorithm were combined to investigate the energetics of HenVm clusters.•The He/V ratio of the most stable helium bubbles indicated by energetics analysis is around 1.5 ± 0.1.•The He/V ratio obtained here is indirectly proved by an experiment result on neutron-irradiated F/M steels. Helium bubbles produced by neutron irradiation can induce property deterioration in ferritic/martensitic (F/M) steels serving in nuclear reactor. To investigate helium-to-vacancy (He/V) ratio of helium bubbles during nucleation and growth in α-Fe, molecular dynamics (MD) method was used to study the dynamic evolution of small helium bubbles under different concentrations of Frenkel pairs (0 ~ 6000 appm) and fixed helium atoms concentration of 2000 appm. Molecular statics (MS) method and Metropolis Monte Carlo (MMC) algorithm were combined to investigate the energetics of HenVm clusters with different He/V ratios and sizes to analyze underlying evolutionary mechanisms. The MD simulation results show that the He/V ratios increase firstly to about 1.4 and then fluctuate within an interval of about ± 0.15 with the increasing of bubble size, corresponding to the main peak of He/V ratio distribution of helium bubbles. And the dynamic evolution results suggest that the equilibrium He/V ratios decrease as Frenkel pairs concentrations increase. The energetics mechanism analysis also shows that the He/V ratio of the most stable helium bubbles is around 1.5 ± 0.1, the crossover of binding energy curve of self-interstitial atoms (SIA) and vacancy when the SIA and vacancy have identical ability to bind with the bubbles under neutron irradiation. Moreover, the simulation results of the He/V ratio of small helium bubbles are consistent with a previous experiment result on F/M steels irradiated in the spallation neutron source.