Secondary phase induced cracking initiation of high-speed railway gearbox

Abstract High-speed trains have gained a significant influence over the way people live and travel. However, the gearboxes used in high-speed railway vehicles inevitably experience cracking during normal operation, resulting in leakage of the lubrication oil, reduction of the service life, and serious safety hazards. This study investigated the role of secondary-phase particles in crack initiation and propagation in a high-speed railway cast Al gearbox during operation by combining the conventional post-mortem sectional examination and the advanced synchrotron X-ray computed tomography based on in-situ tensile tests. Shrinkage/gas pores increase the stress and act as stress concentrators, and the presence of secondary-phase particles, such as eutectic Si, Fe-rich intermetallic phase, and Mg2Si particles, causes inhomogeneously distributed micro-stress/strain. The soft α-Al matrix accommodates a higher strain, while hard secondary-phase particles accommodate a higher stress. The average volume fraction of eutectic Si is 15.96% at dendritic cell boundaries, which was increased to approximately 24.43% by the presence of blocky secondary-phase particles (Fe-rich intermetallic phase and Mg2Si). The corresponding bulk strain required to fracture the secondary-phase particles at dendritic cell boundaries decreased to 2.93% from 3.42% due to this increase in secondary-phase particle volume fraction. A 16% volume fraction of secondary-phase particles at dendritic cell boundaries is the turning point below which their tendency to fracture during tensile tests decreases significantly.