Crystallographic fatigue crack initiation in nickel-based superalloy Rene 88DT at elevated temperature

The fatigue behavior of a polycrystalline nickel-based superalloy Rene 88DT was examined in the lifetime regime of 10(5)-10(9) cycles at 593 degrees C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Sigma 3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Sigma 3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.