Fatigue damage in metallic materials investigated by µLaue diffraction using a 3D energy-dispersive detector
Cyclic plastic deformation can lead to sudden failure as final stage of damage evolution resulting from material fatigue. Therefore, the degree of component fatigue needs to be monitored reliably. With increasing fatigue of a metal or an alloy, the dislocation density approaches a quasi-steady-state condition and characteristic dislocation arrangements are formed. The type of dislocation arrangement depends on the slip character of the metallic material studied and the magnitude of stress amplitude applied. Wavy slip character and high cyclic plastic deformation result in a cell structure. In contrast, a bundle and vein structure appears at lower cyclic loads, and persistent slip bands might form. Because of the change in the dislocation density and arrangement the internal stress distribution within the grains and from grain to grain changes. In the study presented white synchrotron radiation and an energy-dispersive pnCCD detector is used to analyse the dislocation arrangement. White radiation consists of various frequencies and so the X-ray beam is diffracted by different lattice plane distances. By using polycrystalline alloys many different Laue spots appear on an energy-dispersive detector. This kind of detector allows to determine simultaneously both, the diffraction energy and the diffraction angle. This information can be used to determine the internal stress state in these grains. A solely annealed sample shows Laue spots which are points. With increasing cyclic plastic deformation and increasing stress within the grains the Laue reflections become streaked. Hence, the diffraction pattern can be considered as a fingerprint representing the dislocation arrangement and the fatigue damage state. The intention of the investigation is to correlate the diffraction pattern with the dislocation arrangement. For this purpose, the dislocation arrangements were selectively created by fatigue in the low cycle fatigue (LCF), high cycle fatigue (HCF) and very high cycle fatigue (VHCF) range. As an example of a structural material exhibiting wavy-slip-type behaviour, nickel is used. The specific defect structure in the individual grains was examined and analyzed by means of transmission electron microscopy (TEM). Using this information, it is possible to correlate the defect types and arrangements with the X-ray diffraction images, if the reflections can be allocated to the individual grains. Selected examples will be shown and discussed in the presentation.