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Mécanismes élémentaires de la migration de joints de grains couplée au cisaillement

Abstract : The shear coupled grain boundary migration of aluminum bicrystals has been investigated at the microscopic level using a double approach combining atomistic simulations of molecular dynamic and in-situ straining experiments in the transmission electron microscope (TEM). With the simulations we have characterized the effect of the introduction of a sessile disconnections dipole in a SIGMA41[001](450) symmetric tilt grain boundary (GB). The minimum energy path as well as the microscopic mechanism of the GB migration has been revealed using the Nudged Elastic Band method. The description of the migration mechanism through disconnection composition / decomposition and propagation has proven fruitful. In particular it shows that the disconnections dipole acts as a favored site for the nucleation of mobile disconnections that carry the GB migration. Despite the fact that only a tiny effect on the yield stress of the GB migration is calculated it appears that the activation energy is significantly lowered by a factor that can be of the order of 0,65 comparing to the value of the ideal GB migration case. Moreover it is also shown that a preexisting disconnection gliding in the GB experience a friction force from the disconnection dipole emphasizing the ambivalent role of this defect concerning the GB dynamic. In-situ TEM straining experiments have been performed using thin foils extracted from two different bicrystals : the SIGMA3[110](111) (known as a twin boundary) and the SIGMA41[001](450). The samples were deformed under tensile stress in the microscope at temperatures ranging from 280°C to 480°C. Globally the SIGMA3[110](111) GB showed a very low mobility, however some macro-steps associated with intergranular dislocations were seen to occasionally move in the GB. Yet an intense activity of several disconnections families has been observed among which the 1/6 < 211 > type family that has been identified using the asymmetry of their TEM contrast under several diffraction vectors (g). These results are compatible with the presence in the GB of a combination of steps of different height and sign. The straining of the SIGMA41[001](450) GB sometimes results in its shear coupled migration with a measured coupling factor close to a predicted value of the disconnection model. In both bicrystals it appears that the GB dynamic and its disconnection distribution are correlated to the interactions with the neighboring defects like sub-grain boundaries and intragranular dislocations. [...]
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Melvyn Larranaga. Mécanismes élémentaires de la migration de joints de grains couplée au cisaillement. Science des matériaux [cond-mat.mtrl-sci]. Université Paul Sabatier - Toulouse III, 2020. Français. ⟨NNT : 2020TOU30202⟩. ⟨tel-03209992⟩

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