Maîtrise et optimisation des microstructures des superalliages à base de nickel

This work was undertaken to enhance the current knowledge about the hot deformation of nickel-based superalloys. The main objective was to establish the influence of niobium in solid solution on the behavior of hot working of binary alloys of high purity nickel. For this purpose, different model materials (i.e. Ni-0, 01, 0.1, 1, 2, 5 and 10% Nb + pure Ni) were prepared and hot deformed by torsion test at different temperatures (800, 900 and 1000°C) and strain rates (0,03, 0,1 and 0,3 s-1). This wide range of experimental conditions were found able to determine, in a first step, the basic rheological characteristics and the influence of niobium addition in high purity nickel on the behavior of dynamic recrystallization during hot deformation. The parameters relating to the strain rate sensitivity (m) and the apparent activation energy (Q) were derived from the stress-strain curves. Moreover, the parameters h and r, which describe the hardening and dynamic recovery, were also deduced either directly from the experimental curves using an analytical adjustment method that integrates two types of the classical laws (Law Yoshi-Lasraoui and Jonas [YLJ ] and power law [PW]), or from a procedure based on a direct passage without thereby using this adjustment. The relevance of this procedure lies in its contribution in modeling discontinuous dynamic recrystallization and determining the mobility of grain boundaries. A second step of the experimental program, equally important, relates to the micro-structural/textural characterization through conventional techniques (metallography (OM) and backscattered electron (SEM-EBSD)). Based on the obtained results (microstructures and EBSD cartographies), there were a procession of three different mechanisms operating during the DRX: Continuous-Discontinuous-Geometric (DDRX-CDRX-GDRX). In a third and final step, a formulation for the passage between the two abovementioned laws, was introduced and the concept of the size quantification of the microstructural components (i.e. grains and sub-grains) was also introduced in the DDRX model by comparing the two classical methods of measurement that are the Intercept and the Equivalent Diameter.Ce travail de thèse a été entrepris dans le but d’améliorer les connaissances actuelles sur la déformation à chaud des superalliages à base de nickel. L'objectif principal est d'établir l'influence de la teneur en niobium en solution solide sur le comportement de déformation à chaud d’alliages binaires de haute pureté à base de nickel. Pour ce faire, différents matériaux modèles Ni-Nb (Ni-0,01, 0,1, 1, 2, 5 et 10%Nb, + Ni pur), ont été élaborés et déformés en torsion à chaud à différentes températures (800, 900 et 1000°C) et vitesses de déformation (0.03, 0.1 et 0.3 s-1). Cette large gamme de conditions expérimentales est en mesure de déterminer, dans une première étape, les caractéristiques rhéologiques essentielles et l’influence de l’addition de niobium sur le comportement de la recristallisation dynamique. Les paramètres relatifs à la sensibilité à la vitesse de déformation (m) et à l'énergie d'activation apparente (Q) ont été dérivés à partir des courbes contrainte-déformation. En outre, les paramètres h et r, quantifiant l'écrouissage et la restauration dynamique, ont pu être également déduits soit directement à partir des courbes expérimentales en utilisant une méthode analytique d'ajustement qui intègre deux types de lois classiques (loi Yoshi-Lasraoui et Jonas [YLJ], et loi puissance [PW]), soit à partir d'une procédure de passage direct sans pour autant passer par cet ajustement. La pertinence de cette procédure réside dans sa contribution à modéliser la recristallisation dynamique discontinue et à déterminer la mobilité des joints de grains. Une deuxième étape du programme expérimental, non moins importante, se rapporte à la caractérisation microstructurale/texturale des ces matériaux moyennant les techniques classiques de la métallographie (MO) et celles des électrons rétrodiffusés (MEB-EBSD). Sur la base des résultats obtenus (microstructures et cartographies EBSD), une procession de trois différents mécanismes opérant au cours de la DRX a été observée : Continue-Discontinue-Géométrique (DDRX-CDRX-GDRX). Dans une troisième et finale étape, une formulation permettant le passage entre les deux lois citées précédemment a été introduite et le concept de la quantification de la taille des composantes microstructurales (grains, sous-joints) a été également initié dans le modèle DDRX par la comparaison de deux méthodes classiques de mesure qui sont l’Intercept moyen et le diamètre équivalen

Influence of niobium solutes on the mechanical behavior of nickel during hot working

International audienceAn experimental program was performed to determine the rheology and influence of niobium additions to high-purity nickel on dynamic-recrystallization behavior during hot working. Various high-purity alloys were prepared (pure Ni and Ni-0.01, 0.1, 1, 2, 5 and 10 wt% Nb) and deformed to high strains by hot torsion to characterize the mechanical behavior within the temperature range from 800 to 1000 degrees C at (von Mises equivalent) strain rates of 0.03, 0.1 and 03 s(-1). A simple analytical method was proposed for predicting the strain-hardening and dynamic-recovery parameters in the classical Yoshie-Laasraoui-Jonas equation. By the means, the effect of niobium solutes on plastic flow was determined, thus enabling a reasonable fit for the flow curves for the entire range of solid solution Ni-Nb alloys. (C) 2013 Elsevier B.V. All rights reserved

A Critical Assessment of Three Usual Equations for Strain Hardening and Dynamic Recovery

International audienceThe Laasraoui-Jonas (LJ), Kocks-Mecking (KM), and power law (PW) stress-strain equations pertaining to hot working of metals within the range of moderate strains (i.e., before the occurrence of dynamic recrystallization) are compared. It is shown that it is not possible to select the "best" one to fit a given experimental flow curve, neither in the sigma - epsilon nor in the diagram. Noting that each of the three laws depends on two constitutive parameters, transformation formulae are then derived allowing the parameters of one law to be derived from the parameters of any of the two others. The fit of a given LJ equation by a PW law is then discussed. Finally, the transformation formulae are used to estimate the current rate of dynamic recovery when the flow rule is known in the form a PW law. The above theoretical derivations are illustrated by the specific case of a Fe-C alloy in the ferritic phase domain. However, they suggest that the conclusions are widely applicable to hot working of metals and alloys