Role of secondary phase particles in fatigue behavior of high-speed railway gearbox material

The cast Al gearbox widely applied into Chinese high-speed railway vehicles ever exhibits fatigue cracks during normal operation, which induces the lubrication oil leak and a serious safety hazard. In this paper, microstructural features and its influences on fatigue resistance of cast Al gearbox have been investigated by combining the conventional post-mortem sectional examination and the advanced synchrotron X-ray computed tomography based on in situ fatigue loading on sampled material. The results show that microstructure of cast Al gearbox is mainly composed of α-Al, eutectic Si, Fe-rich phase and Mg2Si phase. The presence of secondary phases significantly deteriorates the fatigue properties of casting Al gearbox because the brittle secondary phases are prone to failure particularly when subjected to external fatigued stress. Furthermore, the coalescence of micro-cracks initiated from the fractured secondary phases is clearly found and can accelerate the initiation and propagation of the fatigue cracks. Methodologies to improve the fatigue life of cast Al gearboxes have been proposed but are not tested.