Hot corrosion and low cycle fatigue of a Cr2AlC-coated superalloy

Low temperature Type II hot corrosion is a serious problem for low cycle fatigue (LCF) failure of advanced turbine disk alloys operating at increased temperatures. Accordingly, the present effort studied 15-20 µm corrosion resistant Cr2AlC sputter coatings on Low Solvus High Refractory (LSHR) disk alloy LCF test specimens. These were cycled to failure at 840/−430 MPa and 0.33 Hz, after 500 h oxidation and 50 h of Mg-Na2SO4 hot salt corrosion, all at 760 °C. The coating successfully prevented hot corrosion pitting that was responsible for a 90% decrease in uncoated LCF specimens. However, fractography identified unintentional 15-30 µm deep defects produced by grit blast surface preparation of coated samples. These acted as failure origins and introduced anomalous life reduction for some coated test specimens. Furthermore, the presence and growth of an undesirable Cr7C3 second phase diminished protectiveness by promoting internal oxidation and embrittlement of the coating.