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Mechanism of plasticity : when dislocations participate to the movement of grain boundaries.

Published in Physical Review Materials

par Evelyne Prevots - publié le , mis à jour le

When a metal deforms plastically, several families of mechanisms at the atomic scale operate. Among these, grain boundary migration is an efficient plastic mechanism in the absence of dislocation activity. This migration occurs through the nucleation and displacement of defects called disconnections. Atomic scale simulations have shown that a dislocation absorbed in a grain boundary could operate as a source of disconnections and thus facilitate the migration of a grain boundary.

JPEG - 65 ko
La disconnection sessile δ opère comme une source de disconnections glissile μ<110>

When a stress is imposed on a metal, this latter deforms first reversibly, the elastic regime and then irreversibly, the plastic regime. Many applications (mechanical piece of a motor for example) require a high value of the critical stress above which one reaches the plastic regime. Thus, a fine understanding of the mechanisms of plasticity is necessary to improve the properties of materials.

The defects usually responsible of plasticity are called dislocations. The knowledge acquired over the past 70 years on the motion of dislocations has made it possible to design materials in which plasticity by movement of dislocations is practically inhibited and which are therefore extremely resistant : special steels, super alloys, nanocrystalline alloys. In these situations, it has been known for fifteen years that other mechanisms of plasticity can be activated. Among them, the migration of grain boundaries is currently the subject of intense research activity. For some years, the migration of the joints has been known to result from the nucleation and the displacement of specific defects called disconnections, which combine a character of dislocation (elemental shear) and of step (lack of flatness). Using numerical simulation, the nucleation of these disconnections was studied on perfectly flat and defectless grain boundaries, which only rarely exist in real materials. Real joints contain vacancies, impurities, dislocations, steps, and of course disconnections. All these imperfections of the joint can potentially alter its migration mechanism by potentially creating some sources of inhomogeneous nucleation of disconnections.

By conducting molecular dynamics simulations at the atomic scale, a CEMES team proposed for the first time an inhomogeneous nucleation mechanism of disconnection from an imperfection of the joint created by the absorption of a dislocation. It has been shown that the presence of this imperfection significantly decreases the energy barrier necessary for the nucleation of the mobile disconnections.

This work was published in Physical Review Material in June 2019.

 

Reference

Combe, N. ; Mompiou, F. & Legros, M. 
Heterogeneous disconnection nucleation mechanisms during grain boundary migration
Phys. Rev. Materials, 2019, 3, 060601 (R)

 

 

Contact

Dr. Nicolas COMBE