Surface features and plasticity induced by tension-tension fatigue of Inconel 718

The accumulation of fatigue damage in Inconel 718 has been investigated with a combination of X-ray diffractiOn techniques and scanning electron microscopy. X-ray line broadening analyses and computer -aided rocking curve measurements have been conducted for solutionsewed and age-hardened specimens, cycled in tension-tension. The line profile breadths were found to be very sensitive to the magnitude of the constant stress amplitude employed in the fatigue testing. After increasing sharply during the initial 15% of the fatigue life to a value which depends on the maximum stress level, the breadths are relatively invariant during the reminder of the life. The rocking curve breadths obtained using double crystal diffractometry, on the other hand, increase progressively with continued cycling from about 20% of the life to failure and are nearly independent of the stress amplitude used in the fatigue testing. These techniques can be applied effectively in combination to determine prior cyclic history by providing information on both the stress level and expended fraction of life. Therefore, application of the X-ray diffraction analyses to spectrum fatigue tests has been initiated, beginning with sequences involving regular decrements in the stress level. In order to evaluate differences in the response of the surface layer and the bulk material during fatigue, depth studies have also been carried out The development of topographical features, such as slip bands, and their relationship to crack initiation and propagation mechanisms have been examined by scanning electron microscopy and correlated to the X-ray diffraction data.