Contrôle du maclage thermique et de la taille de grains par traitements thermomécaniques dans deux superalliages base de Ni

Abstract : This work has investigated the microstructure evolution of nickel-base superalloys during thermomechanical processing emphasis being placed on grain size and annealing twining. The aim is to assess the possibility of optimizing the properties of these materials through grain boundary engineering principle. We studied grades belonging to precipitation-hardened superalloys: the alloy PER®72 processed by ingot casting and, less thoroughly, the alloy N19 processed through powder metallurgy. Two deformation modes have been tested: torsion and compression. We pointed out that annealing twinning is promoted by high velocity grain boundary migration, which can be induced by (1) the grain boundary curvature that decreases when grain size increases (2) dislocation density gradients that are produced by low deformation steps applied at high strain rates and can sustain high velocity grain boundary migration even when grain size increases. The driving force related to curvature depends also on grain size distributions, which have been identified as an important microstructure parameter. When grain boundaries migrate as a result of the driving force related to dislocation density gradients they can pass the primary precipitates despite the zener drag these precipitates exert on grain boundaries in the two-phase microstructures: those that have not undergone any supersolvus heat treatment. The dynamic recrystallization occurrence or nonoccurrence during deformation steps has also an important impact on microstructure evolution during subsequent sub- or supersolvus heat treatments.
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Submitted on : Friday, March 23, 2012 - 2:21:50 PM
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  • HAL Id : pastel-00682084, version 1

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Nadia Souai. Contrôle du maclage thermique et de la taille de grains par traitements thermomécaniques dans deux superalliages base de Ni. Matériaux. École Nationale Supérieure des Mines de Paris, 2011. Français. ⟨NNT : 2011ENMP0077⟩. ⟨pastel-00682084⟩

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