Kink-Helium Interactions in Tungsten: Increased Dislocation Mobility in the Infinitely Dilute Regime

Point defects such as interstitial atoms are known to be attracted to screw dislocations. Understanding these interaction mechanisms is key to predicting the plasticity of real materials. Using a new machine learning interatomic potential derived from ab initio calculations of helium in tungsten, we investigate the binding of small helium clusters ($He_{n}$, ${n}=1-3$) to screw dislocation kinks. We find that helium binds significantly more strongly to kinks than to straight dislocation segments. For a single helium atom, the preference reduces the kink pair nucleation energy from 1.58 eV in pure tungsten to 0.48 eV when helium binds to the vacancy kink, indicative of increased dislocation mobility (material softening). In the case of ${n}=2$, kink binding stabilises the kink pair configuration as the ground state, while the straight dislocation is metastable; the two are separated by a 0.60 eV barrier that is the rate determining step for dislocation motion. The energy difference between the ground state kink pair and the metastable straight dislocation increases for ${n}=3$, connected by a 1.00 eV barrier, indicating a progressive loss of the softening effect with increasing cluster size. Molecular dynamics simulations at 900 K support the proposed existence of helium-induced softening in this extremely dilute regime.