  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.
