Effect of Temperature and Secondary Orientation on Notch Fatigue Resistance of Cmsx4

A turbine blade root contains notches that locate into a “fir-tree” root fixing in the turbine disc. Fatigue initiation in these stress concentrating features is of some concern. When a turbine blade is cast, the primary orientation along the blade is controlled (usually in the <001> direction) to within certain limits, however the secondary orientation (i.e. the orientation of any notches) is not generally controlled. The notch fatigue crack initiation and growth behaviour of CMSX-4 under low cycle fatigue conditions has been investigated at 650°C and 725°C. Two secondary orientations have been investigated and a separate oxidation study has been conducted. Heavy oxidation is observed in the notch root after a short amount of time at 650°C. At high temperatures, crack initiation occurs at subsurface pores.