New opportunities in the texture and stress field by the whole pattern analysis

Normally the texture of a polycrystal is determined by the measurement and analysis of some pole figures and the stress field is obtained from the trend of the 2θ peak position of a selected reflection (hkl) varying the angle between the scattering vector and the sample reference plane. Some more physically sophisticated methods take into account the texture in determining the elastic properties of the polycrystal and use more than one peak to determine the strain field. However the complete task of analysing stress and texture is time consuming and requires several interconnected steps. Using the whole diffraction spectra collected at different sample orientations we propose two procedures to solve the problem in a more elegant way that can work both for materials with high and low crystal symmetries. The first procedure makes use of a modified Rietveld method incorporating an orientation distribution function (ODF) described by the harmonic approximation and a micromechanical model for the elastic properties and strain field computation. The fitting of the spectra gives, in a single step, both the texture and the stress field in the sample. The technique is well suited for high symmetric samples due to the simple harmonic description of texture in this case. For low symmetries the above procedure cannot be used because the number of well separated peaks is to small. We propose to perform the texture and stress analysis in two directly linked steps. In the first step texture depending weight factors of peaks were extracted from the spectra using the Rietveld and Le Bail method [1], assuming the knowledge of the crystal structure. Then in a second step these weights were used to obtain the ODF by the WIMV analysis [2]. At this point the two steps can be repeated using, as initial input in the Le Bail methodology, the weights computed from the last ODF obtained. It can also be used to resolve the superposition of peaks and to fit the strain field in the sample. At this stage we can show some experimental results of the first procedure on thin film samples while for the second procedure we have completed the testing stage. As a next step the methodology can be used also to improve the crystal structure refinement of textured samples. From the experimental point of view the methods are particularly appropriate for TOF neutron diffraction or diffractometers equipped with position sensitive detectors, energy dispersive detectors or films.