Development of new high strength Al-Si eutectic alloys and investigation of their fatigue properties in extra-long life range.

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To find the optimum conditions for the material development, the relationship between Si-phase distribution and mechanical properties of Al-Si eutectic alloys in terms of production factors was systematically investigated.Tensile test specimens were prepared from two materials produced by two different processes, continuous casting and extrusion.The obtaind results are : (1) Heat treatment influences the matrix hardness without changing Si-phase distribution. (2) Ultimate tensile strength siguma v is proportional to Vickers hardness H_v of Al matrix. (3) The size(*area_<max>) of the maximum Si-phase has a most crucial effect on ductility epsilon_f.In the next step, to investigate the effect of Si-phase on fatigue properties of Al-Si eutectic alloys in long fatigue life range (N*10^7) and extra-long fatigue life range (N>10^8), tension-compression fatigue tests were carried out. The obtained results are : (1) In the case of extrusion, the fatigue fracture mechanism in extra-long life was shear type and different from that in long life in which the fracture origin was at Si-phase. (2) In the case of continuous casting, all the fatigue fractures were caused by shear type cracking.(3) In the case of extrusion, the fatigue limit can be predicted by the *area parameter model, i.e. with two parameters, the *area_<max> of Si-phase and H_v.