Creep anisotropy of aluminium alloy-base short fibre reinforced MMC

Creep specimens were cut out of the metal matrix alloy Al-7Si-3Cu-1Mg (in wt%) reinforced with 15vol% of Al2O3 short Saffile fibres. Saffile fibre preforms used in the squeeze casting processing route exhibited a preferential orientation of fibres characterized as a random planar fibre texture (RPFT). The orientation of the metal matrix composite (MMC) specimen axis was selected as either normal to or parallel with the fibre texture plane. These specimens were subjected to tensile and compression creep at 623K and in the applied stress range 20-70 MPa. Results of the creep study strongly suggest that the minimum creep rate changes considerably with the orientation of the specimen axis and also with the loading mode applied (tension vers. compression). A careful metallographic analysis showed that the damage accumulation kinetics in terms of fibre breakage is sensitively dependent on both, the loading mode and the direction of the specimen axis with respect to the plane describing the preferential orientation of fibres. The observed anisotropy effects are interpreted on the basis of a micromechanical model which links processes of hardening, recovery and damage accumulation in MMCs driven by different stress states associated with a particular experimental situation.