Influence de la température et de la vitesse de chargement sur les instabilités plastiques du type PLC dans l'alliage 5182 O

Abstract : In certain material containing solutes diffusing towards dislocations, one observes a heterogeneous macroscopic deformation during tensile tests for certain loading rates and temperatures, this macroscopic heterogeneity of the deformation is called ?Portevin ? Le Chatelier? effect ; it results in the presence of serrations or stairs on the traction diagrams. Each serration of stair corresponds to the propagation of deformation bands located along the tensile specimen. The microscopic interpretation of PLC effect is often associated with a dynamic interaction between the clouds of impurities or solute atoms and mobile dislocations. These instabilities which develop during forming process lead to a fall of ductility of material as well as in a bad surface quality of the finished product. This work consists in studying the influence of the temperature and loading rate on plastic instabilities of type Portevin-Le Chatelier in an aluminium alloy of 5000 series in the case of uniaxial tensile tests carried out using hard and soft tensile machines respectively at imposed strain and stress rates. The domain of occurrence of PLC effect in log [varepsilon]. - 1/T, then log [sigma]. - 1/T diagram is closed at high temperatures and loading rates. This result, envisaged by theoretical approaches, is confirmed by the experimental results of this stuudy. This result, envisaged by theoretical approaches, is confirmed by the experimental results of this study. Experimental measurements of the critical strain [varepsilon]c for the oneset of repeated yielding show that [varepsilon]c presents a normal behaviour for high loading rates. However, an inverse behavior is observed at low loading rates. The stress drop [delta][sigma] and reloading times tR increase with strain and generally tend to a maximum value of saturation. Further, the average value of saturation presents a decrease when strain rate increases with two values of slopes rather close to those envisaged by the theory of the diffusion. The results obtained starting from tests carried out on soft machine enabled us to investigate different characteristics of localised deformation bands and to discuss their evolution during the deformation as function of stres rate and variopus test temperatures. The variations of band strain [delta][varepsilon]b present a similarity with those of the critical strain [varepsilon]c : one observes for [delta][varepsilon]b the same transition between inverse and normal behaviours. Band propagation velocities Vb, which present the same variations with strain that the bandwidths Wb, decrease when the stress rate [sigma]. increases. Certain experimental results were compared with those envisaged by the theoretical models of Hähner, Kubin-Estrin and Brechet-Estrin
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Mohammed Abbadi. Influence de la température et de la vitesse de chargement sur les instabilités plastiques du type PLC dans l'alliage 5182 O. Autre. Université Paul Verlaine - Metz, 2000. Français. ⟨NNT : 2000METZ030S⟩. ⟨tel-01748943⟩

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