Prediction of Fatigue Endurance of Ni-Base Superalloy,IN738LC with Protective Coatings
Okazaki Masakazu; Sadasue Teruki; Mutoh Yoshiharu
Abstract:The effect of protective coatings on the high cycle fatigue strength of Ni base superalloy, IN738LC, was first modeled, summarizing the previous studies which covered the fatigue failure of the IN738LC specimens with three kinds of protective coatings: the CoCrAlY alloy overlay coated IN738LC, the CoNiCrAlY alloy overlay coated IN738LC, and the combined coatings in which the aluminide coating was conducted with different thickness on the CoNiCrAlY overlay coated IN738LC. The effect was found to be condensed into the following roles, depending on the mechanical properties of the coating material and substrate: the role as a fatigue crack initiator, the role to make a geometrical change of fatigue crack, and the role to induce a stress concentration. A new semi empirical method to predict the fatigue endurance of the coated material was proposed, by applying the concept of latent, or fictitious crack, which has been originally introduced by Haddad et al. to explain the dependence of crack length on threshold of fatigue cracks in monolithic materials. In the proposed method the coating and the incidental diffusion zone were treated as a part of latent crack interacting with the substrate, depending on the mechanical properties of coating material. It was shown that the proposed method gave a good estimation corresponding well with the experimental results. Key Words:MCrAlY, aluminide coating, Ni-base superalloy, fatigue, prediction of fatigue endurance, latent crack, threshold, interaction between substrate and coating