Dynamic Recrystallization Behaviours in Metals and Alloys

International audienc

Influence of Boundary Migration Induced Softening on the Steady State of Discontinuous Dynamic Recrystallization

During discontinuous dynamic recrystallization (DDRX), new dislocation-free grains progressively replace the initially strain-hardened grains. Furthermore, the grain boundary migration associated with dislocation elimination partially opposes strain hardening, thus adding up to dynamic recovery. This effect, referred to as boundary migration induced softening (BMIS) is generally not accounted for by DDRX models, in particular by “mean-field” approaches. In this paper, BMIS is first defined and then analyzed in detail. The basic equations of a grain scale DDRX model, involving the classical Yoshie–Laasraoui–Jonas equation for strain hardening and dynamic recovery and including BMIS are described. A steady state condition equation is then used to derive the average dislocation density and the average grain size. It is then possible to assess the respective influences of BMIS and dynamic recovery on the strain rate sensitivity, the apparent activation energy, and the relationship between flow stress and average grain size (“Derby exponent”) of the material during steady state DDRX. Finally, the possible influence of BMIS on the estimation of grain boundary mobility and nucleation rate from experimental data is addressed

Claude Rossard, pionnier français de d'étude des traitements thermomécaniques

Les principales contributions de Claude Rossard à l’étude de la mise en forme à chaud des alliages métalliques sont brièvement évoquées : développement des essais de compression et de torsion, analyse des mécanismes de recristallisation dynamique et des interactions recristallisation-précipitation, caractérisation de la forgeabilité. Dans de nombreux domaines, Claude Rossard apparaît comme un précurseur, et ses publications pour la plupart en langue française méritent d’être lues ou relues (cf. liste complète en Annexe). Ses travaux concernent essentiellement les aciers (austénitiques et ferritiques), mais pas exclusivement. Bien que directement motivés par des applications industrielles, ils visent toujours à la compréhension profonde des phénomènes

Some Facts We Can Learn from Analytical Modeling of DDRX in Pure Metals and Solid Solutions

Modeling and simulation of discontinuous dynamic recrystallization (DDRX) are now commonly carried out by numerical methods, most often finite element computation. It is also possible to use simple analytical approaches on the grain scale to get relevant information about the basic mechanisms involved in DDRX, in particular regarding the large strain steady state behaviour. This is illustrated in the present paper on the basis of a model previously proposed by the author and co-workers, which is first briefly presented. The macroscopic constitutive parameters associated with DDRX are then derived and three distinct “Derby exponents” are introduced for describing the relationship between steady state grain size and flow stress. Finally, it is shown, with the example of grain sizes, that not only can average quantities be predicted analytically, but also their distributions

Etude expérimentale et simulation de soudures obtenues par procédé de frottement-malaxage (FSW) sur des alliages d'aluminium de la série 2000

ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Traitements thermomecaniques de l'alliage de titane Ti-17. Etude expérimentale et modélisation de recristallisation de la phase Beta.

Au cours de leur mise en forme, les alliages de titane destinés à la fabrication de pièces tournantes pour moteurs d'avions suivent un schéma thermomécanique complexe constitué généralement de plusieurs opérations de forgeage. L'étape de forgeage dans le domaine monophasé b joue un rôle important, car elle conditionne la transformation de phases qui s'opère au cours du refroidissement ou du traitement thermique consécutif dans le domaine biphasé a-b, et donc les propriétés mécaniques finales de la pièce. La maîtrise de l'évolution de la microstructure et de la texture cristallographique au cours du forgeage dans le domaine b est donc indispensable pour optimiser l'ensemble du procédé de mise en forme. Pour cela, il est important de bien comprendre les mécanismes de recristallisation dynamique (continue et géométrique) se produisant dans ce type de matériau.Dans cette étude, les effets de la température, de la déformation et de la vitesse de déformation ont été étudiés au moyen d'essais de compression uniaxiale et de torsion à chaud. Les microstructures et textures de déformation ont été caractérisées par diffraction des électrons rétrodiffusés (EBSD). Ces approches expérimentales sont associées à un travail de modélisation, pour lequel le schéma de recristallisation dynamique "continue" (ou CDRX), consistant en une fragmentation progressive des grains b initiaux par la formation de nouveaux joints de grains, a été adapté à partir d'études antérieures. Les paramètres caractérisant les mécanismes élémentaires de la CDRX (écrouissage, restauration dynamique, migration des joints) ont été déterminés par ajustement sur les données expérimentales.Ces travaux ont pour objectif l'élaboration d'un post-processeur métallurgique dédié au forgeage des alliages de titane en phase b.Titanium alloys used for the manufacture of rotating parts of aircraft engines undergo complex thermomechanical processing schedules, involving usually several forging operations. The forging step in the single -phase domain plays an important role, because it governs the phase transformation that occurs during cooling or subsequent heat treatment in the two-phase a+b range, and therefore the final mechanical properties of the workpiece. Controlling the microstructural evolutions during forging in the b-domain is of prime interest to optimize the entire process of disks manufacture. A full understanding of the (continuous and geometric) dynamic recrystallization mechanisms occurring in this type of material is therefore required.In this study, the effects of temperature, strain and strain rate were investigated by hot uniaxial compression and torsion tests. The microstructures and textures were characterized using backscattering electron diffraction (EBSD). These experimental approaches were associated with the development of a physical model. In the latter, the mechanism of "continuous" dynamic recrystallization (CDRX), consisting of a progressive initial fragmentation of the grains leading to the formation of new grain boundaries, was adapted from former studies. The parameters characterizing the elementary mechanisms of CDRX (strain hardening, dynamic recovery, grain boundary migration) were determined by fitting the experimental data.The final objective of this work is to develop a metallurgical post-processor dedicated to forging operations of titanium alloys in the -phase range.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Influence des orientations cristallines sur la localisation en bande de cisaillement dans des alliages A1-Mg soumis à compression plane

ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Why Do Grains Remain Roughly Equiaxed during Steady State Discontinuous Dynamic Recrystallization?

International audienceThe combination of advection and migration of grain boundaries is analyzed on the basis of a simple mesoscale model, where parallelepipedic grains are considered under uniaxial compression straining. Strain hardening and dynamic recovery are described by the classical Yoshie-Laasraoui-Jonas equation. Grain-boundary migration is driven by the difference in dislocation densities between one representative grain and the average over the material. Finally, nucleation is assumed to occur at grain boundaries. Special attention is paid to the aspect ratio, which starts from unity (infinitely small cubic nucleus) and tends to zero when the grain disappears. In spite of the role of migration, the average shape of the grains is determined as a first approximation by their lifetimes

RECRISTALLISATION DYNAMIQUE D'ACIERS INOXYDABLES AUSTENITIQUES DE HAUTE PURETE

GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Derivation of the stress concentrations associated with non classical inclusion configurations

A semi-analytical method is proposed for deriving the strain and stress fields associated with “non classical” inclusion configurations in an elastic isotropic matrix. Three cases are addressed, viz: two interacting spheroidal inhomogeneities, duplex spheroidal inclusions, and axisymmetric inclusions of “rounded polygonal” section. The method, based on the three function approach, is shown to provide accurate predictions, provided that sufficiently many terms are used in the series expansions