Very high cycle fatigue rupture mode at high temperatures of Ni-based superalloys coated with a slurry aluminide

The failure mode in the very high cycle fatigue (VHCF) regime of four different nickel-based single-crystal (SX) superalloys coated with a slurry aluminide has been investigated at high temperature (1,000 degrees C) and under fullyreversed conditions (R epsilon = -1). The 3rd generation superalloy CMSX-4 Plus presented a lifetime one to two order (s) of magnitude higher than AM1, CMSX-4 and Rene N5 for the same applied alternating stress (180 MPa). The crack initiation zones were distinct. CMSX-4 Plus samples failed from the interdiffusion zone (IDZ) while AM1, CMSX-4 and Rene N5 samples failed from casting pores close to the IDZ. The interaction between the casting pore, the IDZ and the rough zone is believed to control the premature crack initiation. A new crack initiation mode in VHCF for coated specimens is proposed in this article and the competition between crack initiation from a casting pore or from the surface is considered and discussed.