Application of microLaue diffraction using a 3D energy-dispersive detector to study the evolution of fatigue damage in polycrystalline structural materialsy

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Engineering structural alloys are usually polycrystalline and multiphase alloys, i.e. they con-sist of many grains, which differ in crystallographic orientation and lattice structure. Many material properties, such as the quasistatic strength and the fatigue lifetime, depend directly on the interaction between the grains. Grain and phase boundaries represent obstacles for the dislocation motion, determine the propagation behaviour of microstructural short fatigue cracks, and may lead to stress concentrations and eventually to crack initiation as a conse-quence of anisotropic elastic and crystal-plastic deformation. Microscopic techniques, such as scanning electron microscopy, allow visualization of damage at the surface resulting for example from fatigue loading. Experiments applying high-energy synchrotron beam sources enable to determine the stress condition in the grain volume both at the surface and the interior by the evaluation of few reflexes. A new innovative and promising technique, which is intended to be developed further in the framework of this project for the characterisation of the fatigue damage evolution, is the white beam Laue diffraction in combination with the use of an energy-dispersive semiconductor area detector. Since all Bragg angles and reflex energies can be determined simultaneously, all reflexes can be indexed immediately. If the lattice structure is know, it is additionally possible to allocate the reflexes to the reflecting grain. However, thus far it is not possible to determine the spatial relationship of the indexed grains. In damaged grains, the Laue reflexes are expanded. By means of the energy-dispersive detector, the energy dependence of the streaks can be evaluated and assigned to specific defect patterns (dislocation clusters, internal stresses, microcracks, etc.). As soon as some methodical problems (grain-grain neighbourhood relationships, allocation of energy dependence of Laue streaks, correlation between microscopic and roentgenographic defect identification, etc.) have been solved, the white beam Laue diffraction method will be applied to engineering materials. In particular, the grain-grain interaction of singe-phase and multiphase polycrystalline materials during cyclic loading will be studied. From the results obtained, new insights on the efficiency of microstructural barriers will be gained enabling an unerring application of this knowledge to the increase of the fatigue resistance of modern structural materials.