Caractérisation et modélisation thermomécaniques multi-échelles des mécanismes de déformation et d'écrouissage d'aciers austénitiques à haute teneur en manganèse : application à l'effet TWIP

Abstract : The high manganese austenitic steels have a low stacking fault energy (SFE). Their plastic deformation is achieved by slip, but also by twinning or by [epsilon] martensitic transformation. Our thermochemical modeling including the Néel magnetic transition determines the value of the SFE and the deformation mechanisms activated as a function of the temperature and the composition. Tensile tests performed on Fe22MnO.6C and 1.OC grades between 77 K and 673 K show that the homogeneous elongation is controlled by the work hardening rate through Considère's criterion. The best compromise between elongation and tensile strength is obtained at 298 K when twinning is activated (TWIP effect). At high temperature, the mere latent hardening leads to a low elongation and tensile strength. At low temperature, twinning is replaced by the [epsilon] martensitic transformation, gliding is thermally activated and the tensile strength is maximal. At 298 K, the TEM study reveals that twinning occurs by formation of microtwins gathered into stacks which are strong obstacles for dislocation gliding. Their thickness is determined by a 2D simulation at the scale of the dislocations. At the scale of the grains, two secant twinning systems are sequentially activated along with strain. Only the second twinning system, which appears at about 15 % strain, contributes efficiently to work hardening by reducing the mean free path of mobile dislocations. A crystal plasticity framework which is based on simple scale transition laws bas been developed. The viscoplastic behavior of each grain depends on the dislocation densities stored on each slip system. The activation of two twinning systems is triggered by a Schmid law and leads to a rapid decrease of the mean free path for the secant slip systems. The model well reproduces the link between the twinning microstructure and the mechanical properties.
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Sébastien Allain. Caractérisation et modélisation thermomécaniques multi-échelles des mécanismes de déformation et d'écrouissage d'aciers austénitiques à haute teneur en manganèse : application à l'effet TWIP. Autre [cond-mat.other]. Institut National Polytechnique de Lorraine, 2004. Français. ⟨NNT : 2004INPL006N⟩. ⟨tel-01749952⟩

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