To investigate the effect of Si-phase on fatigue properties of Al-Si eutectic alloys in the long life fatigue range (N≅107) and extra-long life fatigue range (N>108), tension-compression fatigue tests were carried out. Fatigue test specimens were prepared from two materials produced by two different processes, continuous casting and extrusion.
The results obtained are:
(1) In the case of continuous casting, all the fatigue fractures were caused by shear type crack, resulting shorter fatigue life.
(2) In the case of extrusion, the fatigue fracture origin in the long life fatigue range was at Si-phase, while in the extra-long life fatigue range it was at the matrix.
(3) The fatigue limit defined by the cycle N=107 can be predicted by the √area parameter model, i.e. with two parameters, the maximum size of Si-phase √area max and the Vickers hardness Hv.
(4) In the case of Al-Si alloy, a crack leading to fracture initiates even after 108 cycles under very low stress. Thus, extra-long life fatigue behaviour must be taken into account to both material and structural designs.
