Role of Si particle damage on fatigue characteristics of cast Al-Si alloys

In the present study, initiation and evolution of damage at eutectic and primary Si particles during monotonic and cyclic loadings were investigated utilising in situ studies. Al-7%Si and Al-20%Si binary alloys were produced as model materials. Their damage behaviour was analysed in terms of composite theory. In the tensile tests, particles were found to crack perpendicular to a loading axis, being triggered by a far-field uniaxial stress. The damage is accumulated gradually with an increase in applied strain. The in situ strengths of the Si particles are 500-900 MPa for the small eutectic Si particles, whereas it is as low as 200 MPa for the coarse primary Si particles. During the cyclic loading, gradual accumulation of damage at the Si particles was observed with the increase in number of cycles. Propagating through such a weakened zone, a fatigue crack seems to receive some acceleration by the existence of the cracked particles ahead of it. The degree of acceleration was quantitatively evaluated by experiments.