Fatigue properties of spray formed hypereutectic aluminium silicon alloy DISPAL® S232 at high and very high numbers of cycles

The fatigue properties of the spray formed hypereutectic aluminium silicon alloy DISPAL® S232 were studied at constant load amplitude and load ratio R = −1 using the 20 kHz ultrasonic testing technique. S–N curves were measured at 20 °C and 150 °C with peak aged material heat treated for improved ductility, named S232 T6x. Increasing testing temperature shifts the S–N curve to approximately 80% of the stress amplitude measured at 20 °C. Comparison with measurements with conventional frequency equipment shows that a frequency effect does not exist at both temperatures. Correlation of fracture surface features with stress amplitudes and corresponding fatigue lifetimes reveals insight into the crack initiation mechanisms. In the HCF regime partly Al2O3 particles besides several other kinds of defects serve as crack starters at the surface or in the interior of a specimen. With decreasing load amplitude and increasing fatigue lifetimes in the VHCF regime, internal fracturing becomes more probable. Crack initiation at Al2O3 particles is observed less frequently, and initiation at porous areas or at areas with higher Si content is observed more often.