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27 Février 2014 - Isaac Toda-Caraballo

Understanding grain boundary migration from atomistic simulations. Mobility, migration mechanisms and grain boundary stiffness

par LABGPM - publié le

Understanding grain boundary migration from atomistic simulations.
Mobility, migration mechanisms and grain boundary stiffness, Jeudi 27 Février à 10h00 en Salle de Conférences du GPM

présenté par

Isaac TODA-CARABALLO

Department of Materials Science and Metallurgy, University of Cambridge, Royaume-Uni

Grain boundary migration plays a central role in annealing processes of alloys. The equation v = M Γ / R, governs grain growth kinetics, and relates the boundary velocity to grain boundary mobility M, stiffness Γ and boundary curvature. Extensive experimental work provides evidences for this, and some measurements of M and estimations of Γ can be found in the literature. Nevertheless, the complete understanding of the nature of this two parameters and how they are related to misorientation and inclination angle has not been achieved.
Atomistic simulations on curvature driven migration of tilt boundaries in a BCC Fe–20Cr alloy are performed using molecular dynamics to investigate the relationship between the atomic migration mechanism and mobility. The boundaries studied include low angle, high angle and singular boundaries. Results indicate that low energy singular boundaries exhibit high mobility.
The steady-state boundary shape and curvature are compared with a simple analytical model which incorporates the dependence of absolute mobility and free energy on boundary inclination. The comparison supports the atomistic simulations, showing that low energy does not necessarily mean low mobility.

The stiffness is a difficult quantity to calculate from first principles since in general it is a second rank tensor that depends on the direction of boundary curvature as well as boundary orientation. Often the second derivative term is neglected although recent analysis of boundary fluctuations has shown it to be numerically significant. Molecular dynamics calculations on several hundred tilt boundaries have been performed, corresponding to the boundaries along the curvature of the previous migration simulations, and that span a large region of orientation and inclination space. According to the results, the rule is that tilt boundaries generally facet and that these facets correspond to a certain set of tilt boundaries.