Fatigue damage process in M-Cr-Al-Y alloy protective coatings at high temperatures

摘要

The fatigue fracture of the Ni-base superlaloy, IN 738LC, coated with the so-called M-Cr-Al-Y alloys by low pressure plasma spraying was studied at high temperature in vacuum condition. Most of the protective coatings exhibited the fatigue strength comparable to the IN 738LC substrate, when the fatigue fracture was controlled by the propagation of the fatigue crack which initiated from the casting defects in the superalloy substrate. However, when the fatigue crack initiation occurred from the coated layer, the fatigue strength of the coatings was significantly reduced depending on the M-Cr-Al-Y alloy systems, compared with that of the substrate alloy, In 738LC. The fatigue crack propagation process was also elaborately investigated by employing the special equipment with cyclic loading stages which fit within the 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 M-Cr-Al-Y alloy systems. The fatigue crack grew in a very 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 M-Cr-Al-Y bulk coating alloy, the factors which controlled the specific crack propagation behavior in the protective coatings were discussed from the viewpoint of fracture mechanics.