A threshold density of helium bubbles induces a ductile-to-brittle transition at a grain boundary in nickel

Abstract

This paper describes molecular dynamics simulations of fracture along a grain boundary (GB) in nickel (Ni) decorated with helium (He)-filled bubbles. We find that the quantity governing embrittlement of the GB is its areal coverage by He bubbles. Indeed, there is a threshold coverage—approximately 18%, for the GB studied here—above which the GB undergoes brittle fracture. During brittle fracture, intergranular cracks advance by the growth and coalescence of He-filled cavities ahead of the crack front. This process is enabled by plastic deformation of inter-bubble ligaments. In models with lower He bubble coverage, dislocation activity initiates in the adjoining grains prior to cavity growth and coalescence, relaxing crack tip stresses and averting brittle crack propagation. We conclude that the ductile-to-brittle transition observed in this study is governed by a competition between dislocation emission ahead of the crack tip and localized deformation confined to the inter-cavity ligaments.