Papers by Author: Radomír Kužel

Abstract: X-ray diffraction (XRD) studies of ECAP (equal-channel angular pressing) materials were performed after annealing and by in-situ measurements in XRD high-temperature chamber for samples prepared by different number of passes and number of revolutions, respectively. Main attention was given to Cu and Cu-Zr samples. Significant dependence on number of passes was found for ECAP samples. In-situ measurements were focused not only on temperature dependence but also on time evolution of the diffraction line profiles. Evaluation in terms of dislocation densities, correlation and crystallite size and its distribution was performed by our own software MSTRUCT developed for total powder diffraction pattern fitting. Abnormal growth of some grains with annealing is well-known for copper and leads to the creation of bimodal microstructure. Therefore a special care must be given to the evaluation and a model of two Cu components (larger and smaller crystallites) was fitted to the data if an indication of some crystallite growth appears either in the XRD line profile shape or in two-dimensional diffraction patterns.

Abstract: The modification of the Transformed Model Fitting /TMF/ [1] method developed for evaluation of the single-line profiles from high-resolution neutron powder diffractometers is proposed [2]. More sophisticated real-space model [3] is used for the distortion and crystallite size broadening of the diffraction lines. The new PC program based on this model has been tested on neutron diffraction profiles collected in situ upon tensile deformation tests of the plain ferritic steel. The deformation test was performed in the incremental mode in which each individual deformation step was followed by unloading. The neutron diffraction spectra were collected both upon loading and unloading and behavior of the diffraction profiles in elastic as well as in plastic region of the deformation curve was examined in detail. Comparison of microstress values in loaded/unloaded state and in elastic and plastic region offers interesting possibility to separate the contribution of the type II and type III microstresses.

Abstract: In the present work we studied microstructure of ultra fine grained (UFG) pure Mg and UFG Mg-based alloys. The initial coarse grained samples were deformed by high pressure torsion (HPT) using pressure of 6 GPa. Such deformation leads to formation of UFG structure in the samples. The severe plastic deformation results in creation of high number of lattice defects. Therefore, we used positron annihilation spectroscopy (PAS) for defect characterizations. PAS represents a well developed non-destructive technique with high sensitivity to open volume defects like vacancies, vacancy clusters, dislocations etc. In the present work we combined PAS with TEM and XRD to obtain complete information about microstructure of the UFG samples studied. We have found that microstructure of HPT-deformed Mg contains two kinds of regions: (a) ”deformed” regions with UFG structure (grain size 100-200 nm) and high number of randomly distributed dislocations, and (b) ”recrystallized” regions with low dislocation density and grain size of few microns. It indicates some kind of dynamic recovery of microstructure already during HPT processing. On the other hand, homogenous UFG structure with grain size around 100 nm and high density of homogeneously distributed dislocations was formed in HPT-deformed Mg-9.33 wt.%Gd alloy. After characterization of the as-deformed microstructure the samples were subsequently isochronally annealed and the development of microstructure with increasing temperature and recovery of defects were investigated.

Abstract: Bulk samples of pure Mg and Mg-Gd alloys were prepared by high-pressure torsion (HPT). The HPT made samples exhibit ultra fine grained (UFG) structure with grain size around 100 nm. Results of microstructure investigations of the UFG samples obtained by positron lifetime (PL) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) are presented. In particular, lattice defects introduced by HPT were characterized. The data obtained at atomistic level are compared with macroscopic properties given by microhardness measurements.

Abstract: In the present work, positron annihilation spectroscopy (PAS) is employed for microstructure investigations of various ultra fine grained (UFG) metals (Cu, Ni, Fe) prepared by severe plastic deformation (SPD), namely high-pressure torsion (HPT) and equal channel angular pressing (ECAP). Generally, UFG metals prepared using both the techniques exhibit two kinds of defects introduced by SPD: dislocations and small microvoids. The size of the microvoids is determined from the PAS data. Significantly larger microvoids are found in HPT deformed Fe and Ni compared to HPT deformed Cu. The microstructure of UFG Cu prepared by HPT and ECAP is compared and the spatial distribution of defects in UFG Cu samples is characterized. In addition, the microstructure of a pure UFG Cu prepared by HPT and HPT deformed Cu+Al2O3 nanocomposite (GlidCop) is compared.

Abstract: Textured thin film of PbTiO₃ on glass and both textured and stressed films of TiB₂ on iron substrate were investigated by two-dimensional reciprocal space mapping. The texture and residual stress parameters were found by fitting of the measured intensity in the reciprocal space map by the simulated data. Two different types of texture were found in TiB₂ for different values of residual stress. The relevance of resulting parameters was checked using different models. Simulation of the data involved a proper empirical texture correction, the Pearson VII profile function, the irradiated volume correction giving a possibility of the film thickness determination, the background, and other correction factors (Lorentz, polarisation).

Abstract: The thickness measurement based on absorption of X-rays in thin films has been tested on a polycrystalline titanium nitride films deposited on a tungsten carbide substrate. Intensities of three reflections from each material were measured for incidence angles of the primary beam ranging from 0.5° to 35°. After experimental correction for texture effects, data from the TiN film and the WC substrate were fitted by known functions using least squares routines. The substrate reflection intensities were found to be more suitable for determining the thickness of the overlaying thin film. The average thickness of TiN film (2.00± 0.17 µm) determined from the substrate reflections was in fair agreement with the average value obtained from optical microscopy (2.2± 0.8 µm). The thickness values determined from the TiN thin film reflections are very unreliable due to a high sensitivity of measurements to disturbing instrumental and sample effects at small angles.

Abstract: Two methods were used for the analysis of submicro-crystalline copper prepared by high-pressure (6 GPa) severe plastic deformation. Conventional powder XRD study was carried in terms of the modified Williamson-Hall (WH) plots and shown large line-broadening anisotropy. A new high-resolution small-angle transmission diffuse scattering method was applied for determination of crystallite size. Two methods were used for the evaluation of the diffuse scattering - calculation of the autocorrelation function of the crystalline shape by the transformation of the measured signal and by the direct fitting. Bimodal block size distribution has to be assumed for annealed samples. This was also confirmed by classical back-reflection film method as well as by the long tails of conventional diffraction profiles.