Low cycle fatigue behaviour at 300 °C and microstructure of Al-Si-Mg casting alloys with Zr and Hf additions

Abstract The strain-controlled low cycle fatigue (LCF) behaviour at 300 °C and the microstructure of Al-7Si-0.3Mg casting alloys with additions of Zr and/or Hf are studied. Microstructural characteristics are investigated in detail by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the Zr- and/or Hf-rich primary particles and precipitates form in the alloys and have a significant effect on the fatigue properties. The cyclic softening characteristics are mainly attributed to the combination of the effects of the interaction of dislocations with nanobelt-like precipitates and an over-ageing phenomenon in the Mg 2 Si phase during the LCF test at 300 °C. The dislocations climbed over the nanobelt-like precipitates and tangled around the rectangle-like precipitates. Fatigue cracks initiated from the specimen surface, and crack propagation was characterized by dimple-like features coupled with fatigue striations. The best fatigue resistance of Al-7Si-0.3Mg-0.14Zr-0.44Hf casting alloy is related to the combination of morphological changes of the primary phase, the pinning effect of the Zr + Hf-rich precipitates and the different fracture characteristics.