Browsing by Author "Kumar, Atul"

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Open Access
Diffraction stress analysis of thin films - investigating elastic grain interaction
(2005) Kumar, Atul; Mittemeijer, Eric J. (Prof. Dr. Ir.)
The components of the macroscopic, mechanical stress tensor of a polycrystal can be determined by X-ray diffraction stress analysis. Traditional diffraction stress analyses generally presuppose that (i) the specimen is macroscopically elastically isotropic (ii) no depth gradients (of, for example, the stress state) occur. These conditions are often not fulfilled for thin films or surface regions of bulk polycrystals. As a consequence, erroneous results can be obtained. This work is dedicated to the investigation of specimens exhibiting anisotropic microstructures (and thus macroscopic elastic anisotropy) and/or inhomogeneous microstructures, as met near surfaces and in textured materials. The following aspects are covered: (i) Analysis of specimens with direction-dependent (anisotropic) elastic grain-interaction. Elastic grain-interaction determines the distribution of stresses and strains over the (crystallographically) differently oriented grains of a mechanically stressed polycrystal and the mechanical and diffraction (X-ray) elastic constants (relating (diffraction) lattice strains to mechanical stresses). Grain interaction models that allow for anisotropic, direction-dependent grain interaction have been developed very recently. The notion 'direction-dependent' grain-interaction signifies that different grain-interaction constraints prevail along different directions in a specimen. Practical examples of direction-dependent grain interaction are the occurrence of surface anisotropy in thin films and the surface regions of bulk polycrystals and the occurrence of grain-shape (morphological) texture. In this work, for the first time, stress analyses of thin films have been performed on the basis of these newly developed grain-interaction models and it could be shown that curvature observed in so-called sin2ψ plots (i.e. plots of the lattice strain versus sin2ψ, where ψ is the specimen tilt angle), which is incompatible with traditional grain-interaction models, can be fully explained on this basis. It has also been demonstrated that the identification of the (dominant) source of direction-dependent grain interaction is possible. The results for the grain interaction have been discussed in the light of microstructural investigations of the specimens by microscopic techniques. (ii) Analysis of specimens with depth gradients: Diffraction stress analysis can be hindered if gradients of the stress state, the composition or the microstructure occur in the specimen under investigation, as the so-called information depth varies in the course of a traditional stress measurement: Ambiguous results are thus generally obtained. In this work, a strategy for stress measurements at fixed information depths has been developed and from such stress measurements at fixed information depths employing a laboratory diffractometer and a diffractometer at a synchrotron source, the stress gradients and gradients in the elastic grain-interaction constraints of Nickel layers (layer thicknesses 2 micron and 4 micron) have been successfully deduced. Thereby the first evidence ever for the depth-dependence of the so-called surface anisotropy has been obtained.