Deformation Behavior and Microstructure of 20% Al2O3 Particle Reinforced 2618 and 6061 Composites

Deformation and microstructural behaviours of 20% AI2O3 particle reinforced 2618 and 6061 Al matrix composites are studied. The torsion test temperature range is from 25 to 500 °C with strain rates of 0.1, 1.0 and 5.0 s. The torsion tests show that both composites have much greater strain hardening behaviour than those of the matrix alloys and marked decrease with rising temperature. The 2618 Al composite shows slightly higher strength in comparison to the 6061 Al one, possibly due to its much higher alloy content. Logarithmic stress versus reciprocal temperature plots present two slopes within 25 to 250 °C and 250 to 500 °C ranges for both composites, which indicates that different softening mechanisms occur during deformation. The 2618 Al composite shows slightly lower ductility above 300 °C compared to the 6061 Al composite. TEM investigations show that large amount of precipitates and dispersoids in the 2618 Al matrix which coalesce above 300 °C and lead to lower ductility. The increase in strength of the composites arises from very small subgrain size and very high dislocation density in the matrices which form by reinforcement deformation constraint and ÄCTE dislocations. As a cause of softening, dynamic recovery and local dynamic recrystallization have been found in both composites. This agrees with the previous observations on the particle reinforced metal matrix composites. *Present address: Department of Materials Science and Engineering, The University of Michigan, 2126 H.H. Dow Building, 2300 Hayward Street, Ann Arbor, MI48109-2136, U.S.A.. Vol. 5, Nos. 3-4, 1996 Deformation Behavior and Microstructure of 20% A1203 Particle Reinforced 2618 and 6061 Composites INTRODUCTION Particle and whisker reinforced metal matrix composites (MMCs) are good candidates for automotive applications because of their much higher strength and stiffness in comparison to the monolithic alloys; moreover, they have isotropic properties and rather low processing cost compared to the fiber reinforced metal matrix composites [1-5]. The stress, strain, strain rate and temperature effects on the composite processing and mechanical properties have been studied extensively. The particle reinforced MMCs bring much greater strength as a result of the very high dislocation density and particle deformation constraint at cold and warm forming temperatures [610], but much lower ductility in comparison to the monolithic alloys. A large amount of the research has been carried out during the last thirty years for improving the composite forming and mechanical properties [1-11]. Dynamic recovery (DRV) and recrystallization (DRX) behaviours have been observed at various temperature and strain rate conditions which show that MMCs may achieve better formability under a certain combination of temperature and strain rate [12,13]. EXPERIMENTAL PROCEDURES The 20 % AI2O3 particle reinforced 2618 and 6061 Al composites were supplied by Alcan International Limited, Kingston R & D Center and Duralcan, San Diego. The composites were fabricated by liquid metal mixing and DC casting. The chemical compositions of the matrix alloys are listed in Table 1.