Extruded Mg based hybrid composite alloys studied by longitudinal impression creep

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been examined in the longitudinal direction, i.e., the plane containing random fibre orientation was parallel to the loading direction, in the temperature range of 175-300 C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. At 175 C, normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at all the stresses employed. At 240 C, normal creep behaviour is observed up to 80 MPa and reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and again decreasing, is observed above that stress. At 300 C, reverse creep behaviour is observed at all the stresses employed. This pattern remains the same for all the composites. The reverse creep behaviour is found to be associated with the fibre breakage. The stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the stress exponent varies from 3.9 to 7.0, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Qc was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres at all the temperatures and stress levels investigated.