Precipitation kinetics of Al3Zr and Al3Sc in aluminum alloys modeled with cluster dynamics

Precipitation kinetics of Al3Zr and Al3Sc in aluminum supersaturated solid solutions is studied using cluster dynamics, a mesoscopic modeling technique which describes the various stages of homogeneous precipitation by a single set of rate equations. The only parameters needed are the interface free energy and the diffusion coefficient which are deduced from an atomic model previously developed to study the same alloys. A comparison with kinetic Monte Carlo simulations based on the vacancy diffusion mechanism shows that cluster dynamics correctly predicts the precipitation kinetics provided a size dependent interface free energy is used. It also manages to reproduce reasonably well existing experimental data.Comment: Acta Mater. (2005), in pres

Complex Precipitation Pathways in Multi-Component Alloys

One usual way to strengthen a metal is to add alloying elements and to control the size and the density of the precipitates obtained. However, precipitation in multicomponent alloys can take complex pathways depending on the relative diffusivity of solute atoms and on the relative driving forces involved. In Al-Zr-Sc alloys, atomic simulations based on first-principle calculations combined with various complementary experimental approaches working at different scales reveal a strongly inhomogeneous structure of the precipitates: owing to the much faster diffusivity of Sc compared with Zr in the solid solution, and to the absence of Zr and Sc diffusion inside the precipitates, the precipitate core is mostly Sc-rich, whereas the external shell is Zr-rich. This explains previous observations of an enhanced nucleation rate in Al-Zr-Sc alloys compared with binary Al-Sc alloys, along with much higher resistance to Ostwald ripening, two features of the utmost importance in the field of light high-strength materials

Etude de la précipitation en dynamique d'amas dans les alliages d'aluminium et dans les aciers

Jury: P. Fratzl, A. Barbu, G. Inden, C. Sigli, P. Maugis, P. Guyot et J. Lépinoux. Thèse réalisée dans le cadre du Contrat de Programme de Recherche "Précipitation" en collaboration avec Alcan, Arcelor, le CNRS et le CEA.The prediction of precipitation kinetics in alloys is a key issue in metallurgical industry. This study focuses on the modelling of homogeneous precipitation using cluster dynamics in an attempt to improve the understanding of the controlling phenomena. In the first stage, an agreement has been found between this technique and methods based on classical nucleation theory. The influence of the precision of the calculation of cluster free energy on the nucleation kinetics has been studied by comparing cluster dynamics and Monte Carlo kinetics simulations on alloy systems iron-copper, aluminium-zirconium and aluminium-scandium. The agreement found between the two models has enabled the development of a cluster dynamics model that accounts for the non-stoichiometry of L12 Al3ZrXSc1-X precipitates in the ternary system AlZrSc. The results of this simulation compare well to Kinetic Monte Carlo simulations. The ternary system has also been characterised experimentally using small angle X-ray scattering and transmission electron microscopy. The evolution of the precipitate volume fraction, average radius and composition, were followed over the initial stages of precipitation. The experimental and theoretical works on the ternary system both highlight the strong competition between the preferential thermodynamic pathways and kinetics, as well as showing the strong effect of segregation of zirconium to the precipitate surface on the coarsening regime.La prédiction des cinétiques de précipitation dans les alliages métalliques est un enjeu industriel majeur. Cette étude, dont le sujet principal est la modélisation de la précipitation homogène en dynamique d'amas, traduit la volonté d'aboutir à une meilleure compréhension des phénomènes mis en jeu. Dans un premier temps, un domaine d'accord a pu être trouvé entre cette technique et les outils basés sur la théorie classique de la germination. L'influence de la précision du calcul de l'énergie libre des amas sur les cinétiques de germination a ensuite été étudiée lors de comparaisons entre la dynamique d'amas et Monte Carlo Cinétique, sur les systèmes binaires fer-cuivre, aluminium-zirconium et aluminium-scandium. L'accord obtenu a permis le développement d'un modèle de dynamique d'amas prenant en compte l'absence partielle de stoechiométrie des précipités L12 Al3ZrXSc1-X dans les alliages ternaires AlZrSc dont les résultats de simulation ont été confrontés à des simulations Monte Carlo Cinétique. Ce système a également fait l'objet d'une caractérisation expérimentale en diffusion centrale des rayons X et en microscopie électronique en transmission. Elle a permis de suivre l'évolution, durant les premiers stades de la précipitation, de la fraction précipitée, du rayon moyen des particules ainsi sur les alliages AlZrSc que de leur composition. Les études tant théoriques qu'expérimentales ont mis en évidence la forte compétition entre les chemins thermodynamiques préférentiels et cinétiques ainsi que l'effet de la ségrégation du zirconium à la surface des précipités sur les cinétiques de coalescence