MICROSTRUCTURAL ASPECTS OF FATIGUE CRACK INITIATION AND SHORT CRACK GROWTH IN RENE 88DT

摘要

The effect of local microstructure on fatigue crack initiation and propagation in polycrystalline Rene 88DT superalloy specimens has been examined at room temperature. Miniaturized samples were fatigue tested in reverse cycle bending in a novel resonance micro-bending fatigue setup. Concomitant analyses of the surface microstructure with orientation mapping, scanning electron microscopy, and confocal microscopy facilitated direct experimental measurements of the specific microstructural locations associated with: slip and slip band formation, microcrack initiation, and short-crack propagation. The underlying mechanisms were observed to be: slip on {111} planes with the highest resolved shear stress, subsequent initiation of microcracks preferentially along, but not at, twin boundaries in large grains oriented for high shear and experiencing elastic incompatibility, and successive propagation of cracks on {111} planes with high resolved shear stresses in the neighboring grains. Analyses of many short non-propagating cracks revealed crack arrest at high-angle grain boundaries.