SUSTAINED PEAK LOW-CYCLE FATIGUE IN SINGLE CRYSTALS WITH EQUILIBRIUM γ-γ' COATINGS

摘要

Damage due to fatigue with compressive holds developed during cycling of two single crystal superalloys, Rene N5 and TMS-138A, with equilibrium (EQ) γ-γ' bond coatings has been examined. Isothermal strain-controlled fatigue tests with a two minute compressive hold were performed at 1093°C with a total strain range of 0.35%. Oxide-assisted fatigue cracks initiate at the bond coat surface before penetrating into the single crystal superalloy. Crack depths and spacing were characterized in interrupted and failed specimens to investigate crack initiation and propagation behavior. The EQ coatings were found to enhance crack penetration rates compared to previous studies on uncoated superalloys and β-based bond coats. Electron backscatter diffraction and energy dispersive spectroscopy were used to characterize the microstructure of the EQ coating and oxides on the cracks. The EQ coating was found to have a coarse two-phase structure and a small (3-4 μm) grain size. The implications of coating microstructure, strength, and thickness are discussed with regard to fatigue lifetime.