Cohesive stress heterogeneities and the transition from intrinsic ductility to brittleness

• Published in: Physical review. B, Condensed matter and materials physics Vol. 96
• Main Author: Tanguy, Döme
• Format: Journal Article
• Language: English
• Published: American Physical Society 27-11-2017
• Subjects: Condensed Matter; Engineering Sciences; Materials; Materials Science; Mechanics; Mechanics of materials; Physics
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.96.174115

The influence of nanoscale cavities on the fracture of the Σ33{554}[110] symmetrical tilt grain boundary is studied by atomistic simulations. The crack crystallography is chosen such that dislo-cation emission is easy. A transition from a ductile behavior of the tip to a brittle one is obtained for a dense (coverage beyond 15% and inter-cavity spacing smaller than 4 nm) distribution of small cavities (sizes in between 1 and 2 nm). The results are in good agreement with recent experiments from the literature. Even at the highest coverage, the character of the crack is highly sensitive to the initial position of the tip and a mixture of ductile and brittle responses is found. This complexity is beyond the usual criterion based on the drop of the work of separation with the amount of damage in the structure. It is shown that a heterogeneous cohesive zone model, with parameters extracted from the simulations and enriched with a criterion for plasticity, can explain the simulations and reproduce the transition. Additional simulations show that outside this range of small sizes and dense packing, which gives essentially a two dimensional response (either crack opening or infinite straigt dislocation emission), dislocation half loops appear for inter-cavity spacing starting at about 4 nm. They constitute, together with regions of low covage/small cavities, efficient obstacles to brittle cracking. These results could be guidelines to designing interfaces more resistant to solute embrittlement, in general. The cohesive zone model is generic. Furthermore, the {554} single crystal was used to determine to which extent the results depend on the details of the core structure vs. the cavity distribution. These elements show that the conclusions reached have a generic character.