Thermal-mechanical fatigue behaviour of cast aluminium alloys for cylinder heads reinforced with 15 vol.% discontinuous_{2O_{3 (Saffil) fibers

Thermal-mechanical fatigue (TMF) tests were done on the aluminium alloys AlSi10Mg0.3, AlSi10Mg0.6 and AlSi12CuMgNi unreinforced and reinforced with 15 vol.% discontinuous 2O3 (Saffil) fibres. The tests were carried out with total strain constraint, triangular shaped temperature cycles, a minimum temperature of 50°C and maximum temperatures from 200°C up to 400°C. The lifetimes of the unreinforced alloys lie within a relatively broad common scatterband. At all Tmax, the fibre reinforcement distinctly increases the lifetimes of AlSi10Mg0.3, whereas for AlSi10Mg0.6 and AlSi12CuMgNi this is only the case at Tmax≥ 300°C. The lifetime behaviour is discussed on the background of the cyclic deformation behaviour, the influence of the fibre reinforcement on the microstructure and the damage mechanisms occurring in the fibre reinforced and the unreinforced materials, respectively. All materials show cyclic softening due to overageing of the initially age hardened matrix alloys. The fibre reinforcement leads to an enrichment of Mg near the fibre-matrix interfaces and therefore reduces the hardenability of AlSi10Mg0.3, whereas the hardness of the matrix alloys with higher Mg contents remains unaffected. Generally, a very good fibre-matrix bonding is observed. The preferred crack initiation sites are fibre-matrix interfaces in the compound materials and eutectic Si-particles in the unreinforced alloys. The damage parameter of Ostergren is able to roughly correlate the numbers of cycles to fracture of the compound materials with each other and even with the values measured at the respective unreinforced matrix materials.