The fatigue fracture characteristics of the bond zone of aluminum matrix composites (Al-12Si/ABOw) with Al-12Si alloys

The strain controlled low-cycle fatigue (LCF) test of the bond zone of aluminum matrix composites (Al-12Si/ABOw) with Al-12Si alloys was carried out with the test temperature range from 25 °C to 350 °C. The cyclic mechanical response characteristics of material at different test temperatures were analyzed. The microstructural evolution process and fracture mechanism of three characteristic fracture models were studied through SEM observation. The influence of temperatures and load magnitudes on fatigue behaviors were also discussed. The results showed that, the cyclic stress responses of the material exhibited cyclic hardening under low-cycle fatigue loading at 25 °C, and the crack occurred at the casting defects in the composite part first, resulting in a flat fracture surface with both composite part and Al-12Si alloy part. When the temperature raised to 200 °C, the cyclic stress response of the material would show out slight cyclic hardening at beginning stage and distinct cyclic softening during later stage. The casting defects in the composite would no longer be the prior crack source, and the crack would initiate at the large-sized primary Si particles nearby the bond zone which led to a fracture surface with the bond zone shape under low-cycle fatigue loading at 200 °C. With the continued increasing of temperature to 350 °C, the cyclic stress responses of the material showed the characteristics of continuous softening, and the fatigue crack generated at the primary Si phases in the Al-12Si alloy part, which caused the specimens fractured at the unreinforced Al-12Si alloy parts with a fracture surface like common aluminum alloy fracture under high temperature. There was one point had to be mentioned that when the magnitude of cyclic strain loadings (△ε/2) was larger than 0.003, the failure mode would be the fracture of the unreinforced Ai-12Si alloy part at any temperature due to the prior fracture of Al-12Si matrix alloy under relatively high loadings. Microstructure observation indicated that the fatigue fracture features of the material transformed from hard-brittle to soft-ductile gradually with the change of test temperatures from 25 °C to 350 °C.