Papers by Author: David Rafaja

Abstract: A new method for a fast analysis of heavily deformed, multicomponent ferritic/pearlitic steels microstructure based on XRD measurements had been developed. Its practical application has been examined and proven during wire rod production of a high-strength eutectoid non-alloyed steel grade containing 0.81 weight percent carbon. For individual technological conditions, the lattice strains and their anisotropy were analysed quantitatively by means of fast X-ray diffraction measurements and correlated with the results of comprehensive mechanical testing. Obtained relationships between the microstructure characteristics and mechanical properties were described using physically based models and used to establish a material specific database for prediction of the mechanical properties from X-ray diffraction data. Depending on the deformation state different parameters have to be applied for the material’s macroscopic properties prediction. Additionally, the fast microstructure analysis can provide more detailed information in the case of deviations from the as-required material’s properties due to technological aberrations.

Abstract: The influence of Spark Plasma Sintering / Field Assisted Sintering Technology applying pulsed direct current up to the root-mean-square current densities of 129 A/cm² on the interfacial reactions in Al - Fe - Al stacks was investigated at temperatures between 500°C and 600°C. Independently of the current density and current direction, thin Al₁₃Fe₄ and wide Al₅Fe₂ phases were detected in the diffusion couples. The Al₅Fe₂ phase consisted of columnar grains having a {001}-fiber texture. Al₁₃Fe₄ was found in the form of discontinuous spots at the Al/Al₅Fe₂ interface. The interface between Al₅Fe₂ and Fe was highly fringed. The layer growth kinetics of Al₅Fe₂ was parabolic. The growth rate was strongly enhanced in the SPS/FAST experiments as compared to the conventional diffusion experiments, independently, on the current direction. It is suggested that the enhanced growth rates are a result of temperature gradients existing in a typical Spark Plasma Sintering device. Possible effects of thermomigration and electromigration are discussed.

Abstract: The combination of a TRIP steel with the MgO stabilized ZrO2 ceramics (MgO•ZrO2) is regarded as a promising way to increase the energy absorption in engineering materials. An additional contribution to the energy absorption in the counterparts, i.e. in the TRIP steel and in MgO•ZrO2, is expected to arise at the interfaces between the individual materials. However, the mutual crystallographic orientation of the TRIP steel and MgO•ZrO2 at their interface plays a crucial role both for the adhesion of the counterparts and for the energy absorption process. In this work, the interfaces between the TRIP steel and MgO•ZrO2 were studied on simplified systems, which were prepared in form of the TRIP steel thin films that were deposited using the magnetron sputtering on various substrates, e.g. Si wafer, MgO•ZrO2 and the Al2O3/ZrO2 composites. The substrates were both single-crystalline (Si wafer) and polycrystalline (MgO•ZrO2, Al2O3/ZrO2). The basic characteristics of the thin films (morphology, thickness, chemical composition) were obtained from the scanning electron micrographs and from the energy dispersive analysis of the characteristic X-rays (EDX). X-ray diffraction (XRD) and transmission electron microscopy with high-resolution (HRTEM) that was complemented by the Fast Fourier Transform (FFT) of the HRTEM micrographs were employed as the crucial experimental methods for the microstructure analysis of these thin films. XRD was used for the phase analysis and for the global texture analysis. The global texture analysis was performed via the pole figure measurements. FFT/HRTEM was used for the characterisation of the local orientation relationships between the TRIP steel and the respective substrate and for the visualisation of the interfaces between individual crystallites.

Abstract: Formation of microstructure defects at the phase boundaries in TRIP steels was investigated with the aid of microstructure analysis on a TRIP steel crystal, which was grown by the Bridgman technique. The microstructure studies comprised scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and transmission electron microscopy with high resolution (HRTEM). Initial XRD measurements revealed that the crystals under study consist of austenite and ferrite with extremely strong preferred orientations. Subsequent XRD pole figure measurements and EBSD scans have shown that the orientation relationship between austenite and ferrite can be described by the Nishiyama-Wassermann model. For a detailed description of the microstructure of the Bridgman crystal, the orientation distribution of crystallites within the individual phases was investigated using the XRD reciprocal space mapping and the rocking curve measurements. These experiments have shown that the density of microstructure defects is much lower in ferrite than in austenite. The direct information about the defect structures at the phase boundaries between austenite and ferrite was obtained from the TEM micrographs, which revealed complicated micro-twin structures at the boundaries between the neighbouring phases. HRTEM discovered very narrow stripes of ferrite embedded in austenite that were regarded as a source of the microstructure defects in austenite.

Abstract: Capability of the X-ray scattering for study of low-dimensional structures is illustrated on few examples. They are focused to the phase analysis, residual stress measurement, calculation of the stress-free lattice parameters, investigation of the anisotropic lattice deformation and preferred orientation in UN thin films. Further, the study of concentration profiles in functionally graded hard-metals and investigation of the multilayer degradation caused by soft annealing are discussed.