Brittle fracture of gas turbine blade caused by the formation of primary β-NiAl phase in Ni-base superalloy

During the manufacturing process of a cast Ni-base superalloy K403 turbine blade, the blades from some mold assembly broke down into small pieces at the thin section trailing edge in the shell removal process. The fractured blade was sampled and its fracture characterization and microstructure were investigated by chemical analysis, optical metallography (OM), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and microhardness measurement in order to explore the major cause of failure. The results indicated that the fracture of K403 blade was cause by the abnormal large amount of primary β-NiAl phase formation. The primary β phase possesses a Vickers microhardness of 5.78 GPa, which is a brittle phase. The primary β phase cracked extensively in turbine blades at low impact load during the shell removal process. A “river pattern” structure is obvious in β-NiAl phase on the fracture surface, which is the main feature of the cleavage fracture. The formation of the primary β phase was caused by the heavy macrosegregation of the Al and Ti elements in the master alloy ingot.