Abstract: K418C Ni-based superalloy casting ingots for engine turbochargers fractured during casting. Macrostructure and microstructure were studied by optical microscopy, scanning electrical microscopy (SEM), electrical probe micro-analysis (EPMA) and transmission electron microscopy (TEM), and the fracture mechanism was discussed. The results show that there were an amount of abnormal black phases near the fracture of K418C casting ingot, which distributed on the matrix, companied with transgranular crack. The electron diffraction result indicated that the black phase is β-NiAl, which is brittle and resulted from local segregation of Al during solidification. The cracks between β-NiAl and matrix were induced by heat stress during rapid heating and propagated rapidly across the whole ingot. Finally, the casting ingot fractured instantly.