Mechanical and microstructural differences between uniaxial and multiaxial creep loading of short fibre reinforced aluminium alloys

The creep behaviour of a short fibre reinforced (SFR) aluminium alloy (Al Zn11 Mg0.2) with 8 vol.-% Saffil fibres was investigated. The composite was produced by squeeze casting. Creep experiments under uniaxial (tension and compression) and biaxial (shear) loading conditions were performed in the temperature range from 548 K to 623 K. The creep behaviour was analysed in terms of the shape of individual creep curves and in terms of the stress, strain and the temperature dependence of the creep rate. The SFR materials studied in this work show a characteristic fibre texture (preferential random planar orientation of fibres) and the effect of the different orientations of. the fibres on the creep behaviour (compression and shear) was evaluated. The mean fibre length in the as processed material was 110 mum. Fibre reinforcing results in a decrease of creep rate as compared to the unreinforced matrix material under all loading conditions. The presence of fibres strongly influences the overall stress and temperature dependence of creep as compared to the fibre free material. The uniaxial creep behaviour of the SFR material was found to be anisotropic due to the preferential orientation of fibres. However, no significant anisotropic effect was found in the biaxial tests. The changes in the microstructure after creep were analysed focusing on fibre length distributions. After sufficient creep deformations fibre breakage resulted in a decrease of the mean fibre length down to 45 mum. This value represents a critical mean fibre length below which no further fracture occurs; this was observed for all loading conditions within the ranges of stress and temperatures considered in the present study.