Insight into the loop-punching mechanism for He bubble growth: The plastic deformation modes and their competition

Loop-punching is the manner a helium (He) bubble enlarges itself through emitting a dislocation loop during its growth in metals. It has been demonstrated that there are two types of loop-punching: the two-stage loop -punching for small bubbles and the direct loop-punching for large bubbles. In this work, via the molecular dynamics (MD) simulations of helium bubble growth in tungsten (W), we revealed the crucial process from the bubble-attached self-interstitial atoms (SIAs) to the escapable dislocation loop in the two-stage loop-punching. Different from the conventional theory that the loop is merely the collection of the sequentially kicked-out SIAs, it is formed by the synergistic action of the atomic plane on the bubble surface being pushed into interstitial sites and the attached SIA cluster slipping. Building on this, two plastic deformation (PD) modes, the tangential PD and the normal PD, are extracted due to the generated SIAs' directions tangent or normal to the bubble surface. The SIA-accumulation stage and the loop-emission stage of the two-stage loop-punching correspond to the tangential PD and the normal PD, respectively, while the direct loop-punching merely consists of the normal PDs. Therefore, the direct loop-punching is only a special form of the two-stage loop-punching that contains zero previous tangential PDs. Further, it is found the temperature plays an important role in the PD mode switch, suggesting the modes compete on the energy barriers, which accounts for the detachment of the attracted SIA cluster from the bubble.