Fatigue fracture behavior of Ni base superalloy IN738LC coated with MCrAlY alloys at high temperatures
Sadasue Teruki; Okazaki Masakazu; Mutoh Yoshiharu; Itoh Yoshiyasu; Saitoh Masahiro
Abstract:The fatigue fracture of a Ni base superalloy, IN738LC, coated with so called MCrAlY alloys by low pressure plasma spraying was studied at 400 deg. and 800 deg. in vacuum condition. Most of the protective coatings exhibited the fatigue strength comparable to that of IN738LC substrate, when fatigue fracture was controlled by the propagation of fatigue cracks which initiated from the casting defects in the superalloy substrate. However, when the fatigue crack initiation occurred from the coated layer, the fatigue strength was significantly reduced depending on the MCrAlY alloy systems, compared with that of the substrate alloy. The fatigue crack propagation process was also elaborately investigated at 4 deg. by employing the special equipment with a cyclic loading stage which fits within a scanning electron microscope. Special focus was paid on the fatigue crack growth behavior in the coated layer, as well as near the interface. The fatigue threshold level significantly depended on the MCrAlY alloy systems used. The fatigue crack grew in a yew complicated manner near the interface in all of the protective coatings. Based on the measurements of the residual stress and the mechanical properties of the MCrAlY bulk coating alloys, the factors relevant to the specific feature of the crack propagation in protective coatings were discussed from the viewpoint of fracture mechanics. Key Words:MCrAY alloys, Ni-base superalloy, protective coatings, fatigue, fatigue crack propagation, residual stress, interface