Very high cycle fatigue behavior of nickel-based superalloy Rene 88DT at 593 ℃

摘要

The very high cycle fatigue behavior of a polycrystalline nickel-based superalloy, Rene 88 DT, was studied in the lifetime regime of 10~5 ～ 10~9 cycles at 593 ℃ in laboratory air using an ultrasonic fatigue testing apparatus operating at 20 kHz. Fatigue failures were observed at lifetimes beyond 10~8 cycles, and the variability in fatigue life increased with decreasing stress levels. Fractographic analysis indicated that all fatigue cracks initiated internally. Most crack initiation sites consisted of large crystallographic facets and no inclusions were observed. These large crystallographic facets were formed by fatigue crack initiation and early small crack growth in large grains with dimensions much larger than the average grain size. Quantitative fracture surface analysis showed that the crystallographic facets at the crack initiation sites are oriented close to 45 degrees to the loading axis and correspond to maximum shear stress planes. Crystallographic orientations of grains near internal crack initiation sites were revealed using EBSD combined with metallographic serial sectioning. Cyclic strain localization within favorably oriented large grains and early crack growth in similarly oriented neighboring grains is the likely mechanism controlling the VHCF lifetime in this alloy.