The Influence of Cyclic Hardening and Microstructure on the Fatigue of an Al–Si Alloy

An investigation has been made into the influence of cyclic hardening and microstructure on the fatigue of an Al–l % Si alloy in reversed bending over a life range of 10⁴–10⁷ cycles. It is found that in fatigue the solution-treated structure is stronger than the aged structures, contrary to the behaviour in tensile testing. At high stress amplitudes, the solid solution hardens very considerably because the density of dislocation loops becomes high as a result of strain-ageing. In particular, this greatly extends the crack-growth stage. At low stress amplitudes, the crack-initiation stage is very long in the solution-treated alloy but in the aged alloy microcracks nucleate rapidly at grain-boundary areas. The empirical Manson relation is valid approximately for plastic strain amplitudes and the observed variation in the exponent is shown to be a result of differences in hardening and in crack initiation and propagation rates among the structures tested.