The Core/Shell Diffusion-Based Microalloying Process: Mathematical Modelling

Abstract

In an effort to produce low cost metal matrix composites for use in automobiles, inexpensive and naturally occurring minerals were used as a microalloying source. To maximize the extent of mineral dissociation prior to microalloying, a novel technique known as core/shell processing, was developed using the thermodynamic software package F*A*C*T* by Thompson et al. This powder metallurgy processing technique consists of blending, cold isostatic pressing and sintering. After the production of the core component, the process is repeated and the core is encased in a shell containing the diffusing elements. Sintered samples were examined by means of mercury densitometry, optical/scanning electron microscopy, electron microprobe micro-analysis and chemical analysis. Microprobe analysis of sintered core/shell samples was used to determine the composition of diffusing elements in the core region. Experimental and calculated diffusion coefficients were used along with thermodynamic data to develop an equilibrium mass balance model using the thermodynamic software package F*A*C*T (Facility for the Analysis of Chemical Thermodynamics) by Thompson et al. The model-predicted values for the diffusion of Ag were found to agree favourably with the experimentally determined values. The effect of potential chemical reactions among the components and the attainment or not of equilibrium to the model predictions are discussed with a view to assessing the viability of using the model and technique to selectively enhance the mechanical properties of automotive aluminum alloys.

In an effort to produce low cost metal matrix composites for use in automobiles, inexpensive and naturally occurring minerals were used as a microalloying source. To maximize the extent of mineral dissociation prior to microalloying, a novel technique known as core/shell processing, was developed using the thermodynamic software package F*A*C*T* by Thompson et al. This powder metallurgy processing technique consists of blending, cold isostatic pressing and sintering. After the production of the core component, the process is repeated and the core is encased in a shell containing the diffusing elements. Sintered samples were examined by means of mercury densitometry, optical/scanning electron microscopy, electron microprobe micro-analysis and chemical analysis. Microprobe analysis of sintered core/shell samples was used to determine the composition of diffusing elements in the core region. Experimental and calculated diffusion coefficients were used along with thermodynamic data to develop an equilibrium mass balance model using the thermodynamic software package F*A*C*T (Facility for the Analysis of Chemical Thermodynamics) by Thompson et al. The model-predicted values for the diffusion of Ag were found to agree favourably with the experimentally determined values. The effect of potential chemical reactions among the components and the attainment or not of equilibrium to the model predictions are discussed with a view to assessing the viability of using the model and technique to selectively enhance the mechanical properties of automotive aluminum alloys.

Dans un effort pour produire à bas prix des composites à matrice métallique pour utilisation dans les automobiles, on a utilisé des minéraux naturels bon marché comme source de micro alliage. Afin de maximiser le degré de dissociation minérale avant le micro alliage, on a développé une nouvelle technique connue sous le nom de traitement de noyau/enveloppe, en utilisant le logiciel de thermodynamique F*A*C*T* de Thompson et al. Cette technique de traitement de la métallurgie des poudres comprend le mélange, la compression isostatique à froid et le frittage. Après la production de la composante centrale, le procédé est répété et le noyau est encastré dans une enveloppe contenant les éléments pour la diffusion. On a examiné les échantillons frittés au moyen de la densitométrie à mercure, de la microscopie optique/électronique à balayage, de la microanalyse par microsonde électronique et de l'analyse chimique. On a utilisé l'analyse par microsonde des échantillons frittés noyau/enveloppe pour déterminer la composition des éléments de diffusion dans la région du noyau. On a utilisé les coefficients de diffusion expérimentaux et calculés ainsi que les données thermodynamiques pour développer un modèle de balance de masse d'équilibre en utilisant le logiciel de thermodynamique F*A*C*T* (Dispositif pour l'analyse de la thermodynamique chimique) de Thompson et al. On a trouvé que les valeurs prédites par le modèle pour la diffusion de Ag étaient en accord favorable avec les valeurs déterminées expérimentalement. On discute de l'effet de réactions chimiques potentielles parmi les composantes et l'obtention ou non de l'équilibre des prédictions du modèle. On évalue également la viabilité d'utiliser le modèle et la technique pour améliorer sélectivement les propriétés mécaniques des alliages d'aluminium pour l'industrie automobile.