Comportement viscoplastique des alliages austénitiques pendant la recristallisation sous faibles contraintes

Abstract : An obvious increase in strain rate is observed during recrystallisation when an alloy is subjected to low stress levels. This phenomenon, known as “Recrystallisation-Induced Plasticity” (RIP), is observed in a number of hot-forming processes. The dominant underlying mechanisms are not yet clear and no constitutive viscoplastic material formulation is available to describe the associated microstructural evolution and mechanical behaviour. This thesis work aims at understanding this phenomenon and developing physics-based models to describe and predict it. In this study, the deformation evolution during recrystallisation is investigated on hot and cold worked Fe-Ni alloys subjected to different relevant temperature-stress conditions. The microstructural characteristics of partially and fully recrystallised materials are first identified through EBSD and TEM observations. The experimental results reveal that deformation accelerations during recrystallisation are mainly associated with the viscoplastic behaviour of recrystallised grains and their evolving volume fraction. Two sets of constitutive equations have been developed to model the RIP phenomenon: (i) one where the material is assumed to be composed of two “phases” (i.e. recrystallised grains and cold-worked grains) and the flow behaviour of the alloy is obtained by homogenising that of each “phase”, and (ii) another where the material is treated as an homogeneous one and where its flow behaviour is assumed to depend on the microstructural “state” described by two internal state variables, namely those associated with strong obstacles and with the recrystallisation volume fraction. The results of this work confirm that a considerable contribution to the deformation enhancement associated with the RIP mechanism arises from viscoplastic deformation by dislocation activity, coupled with a reduction in the dislocation density by recovery processes. This contribution accounts for approximately 70% of the total deformation. It will be shown that this work provides fundamental knowledge to the understanding of the RIP phenomenon which should, in turn, contribute towards improving the control of product dimensioning in hot-forming processes.
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Minghao Zhang. Comportement viscoplastique des alliages austénitiques pendant la recristallisation sous faibles contraintes. Science des matériaux [cond-mat.mtrl-sci]. Ecole Nationale Supérieure des Mines de Paris, 2014. Français. ⟨NNT : 2014ENMP0060⟩. ⟨tel-01144443⟩

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