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Comportement et transformations martensitiques de deux aciers inoxydables austénitiques : effets de la température, de la vitesse et du chargement

Abstract : During recent years, a growing interest has been focused on the potential use of austenitic stainless steels as crash-relevant structures of automobiles due to their high energy absorbing capacity and excellent formability. This properties improvement is due to the transformation induced plasticity (TRIP) which occurs during the gamma → alpha' transition in unstable grades. The mechanical behaviour of two grades AISI 301LN and AISI 201 were studied under tension over a wide temperature range [-150°C; +150°C] and at various stain rates [3.10-4 s-1; 200 s-1] in order to cover forming and crash conditions. The evolution of the alpha' martensite amount formed was followed by magnetic measurements with a dedicated device. A thermodynamical law were used to model the gamma → alpha' transformation kinetics. Deformation microstructures have been identified via X-ray diffraction and TEM. The resulting transformation and deformation modes indicate that high strain rate favoured planar defects formation and promote gamma → alpha' transformation when temperature is around Md. The materials tensile behaviour has been model by a mixture law. Triaxiality ratio impact has been studied by testing the materials under simple shear, plane strain state and balanced biaxial tension. The local strain state appears to be a significant parameter regarding the martensite volume fraction measured. The loading path influence was explored by changing for a given specimen the loading orientation. The variation observed on the stress-strain curves and the alpha' volume fraction measured indicates that the martensitic transformation should be considered at the scale of the implicated slip systems. The main results of the present study are: i) 301LN and 201 grade present different deformation modes but their comparable mechanical properties allow the materials to be substitute each other for automotive applications. ii) The increase of strain rate promotes the martensites (alpha' and epsilon) and twins formation. iii) A good representation of transformation kinetics is subjected to crystal scale description. iv) The softening effect of the transformation cannot be described using the classical homogenization laws for composite. During recent years, a growing interest has been focused on the potential use of austenitic stainless steels as crash-relevant structures of automobiles due to their high energy absorbing capacity and excellent formability. This properties improvement is due to the transformation induced plasticity (TRIP) which occurs during the gamma → alpha' transition in unstable grades. The mechanical behaviour of two grades AISI 301LN and AISI 201 were studied under tension over a wide temperature range [-150°C; +150°C] and at various stain rates [3.10-4 s-1; 200 s-1] in order to cover forming and crash conditions. The evolution of the alpha' martensite amount formed was followed by magnetic measurements with a dedicated device. A thermodynamical law were used to model the gamma → alpha' transformation kinetics. Deformation microstructures have been identified via X-ray diffraction and TEM. The resulting transformation and deformation modes indicate that high strain rate favours planar defects formation and promotes gamma → alpha' transformation when temperature is around Md. The materials tensile behaviour has been model by a mixture law. Triaxiality ratio impact has been studied by testing the materials under simple shear, plane strain state and balanced biaxial tension. The local strain state appears to be a significant parameter regarding the martensite volume fraction measured. The loading path influence was explored by changing for a given specimen the loading orientation. The variation observed on the stress-strain curves and the alpha' volume fraction measured indicates that the martensitic transformation should be considered at the scale of the implicated slip systems. The main results of the present study are: i) 301LN and 201 grade present different deformation modes but their comparable mechanical properties allow the materials to be substitute each other for automotive applications. ii) The increase of strain rate promotes the martensites (alpha' and epsilon) and twins formation. iii) A good representation of transformation kinetics is subjected to crystal scale description. iv) The softening effect of the transformation cannot be described using the classical homogenization laws for composite.
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https://pastel.archives-ouvertes.fr/pastel-00577815
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Submitted on : Thursday, March 17, 2011 - 3:19:32 PM
Last modification on : Thursday, September 24, 2020 - 6:30:07 PM
Long-term archiving on: : Saturday, June 18, 2011 - 2:44:42 AM

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  • HAL Id : pastel-00577815, version 1

Citation

Stéphanie Nanga-Nyongha. Comportement et transformations martensitiques de deux aciers inoxydables austénitiques : effets de la température, de la vitesse et du chargement. Mécanique [physics.med-ph]. École Nationale Supérieure des Mines de Paris, 2008. Français. ⟨pastel-00577815⟩

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