Tomáš Krajňák

Comparison of the microstructure and the mechanical properties of AX41 magnesium alloy processed by EX-ECAP via three different routes A, Bc and C

This work is devoted to the creep-resistant AX41 magnesium alloy (Mg-4 wt.% Al-1 wt.% Ca), processed by extrusion and consecutive Equal Channel Angular Pressing (EX- ECAP) up to total of eight passes, via route A, Bc and C at 220 °C. Beyond the deformation behavior the change of the microstructures was studied by electron back-scattering and X-ray diffraction methods. Significant grain refinement was found for all processing routes (grain sizes decreased below 1 pm after 8 passes). The X-ray line profile analysis performed by Convolutional Multiple Whole Profile (CMWP) fitting method has not revealed differences between the particular dislocation structures. Route A was found to be the most effective processing route from the point of view of grain size refinement and the room temperature strength. The influence of the texture and the dislocation structure on the plastic deformation processes is discussed in detail.

The Use of Acoustic Emission and Neutron Diffraction to Reveal the Active Deformation Mechanisms in Polycrystalline Magnesium and Comparison to Theoretical Modeling

A detailed analysis of the loading mode dependence of the deformation mechanisms in randomly textured cast magnesium is presented. An elasto-plastic self-consistent model (EPSC) is used to model the evolution of the dislocation structure and twinning. The results are quantitatively compared with experimental data obtained by in-situ neutron diffraction (ND) and acoustic emission (AE) methods. Both EPSC calculations and ND line profile analysis show an increased activity of prismatic slip with increasing strain and a loading mode dependence of the activity of the second-order pyramidal slip. The AE measurements and the modeling indicate different amount of nucleated twin variants and twinned volume in tension and compression, respectively.

Microstructure evolution in ultrafine-grained interstitial free steel processed by high pressure torsion

Commercial interstitial free steel was processed by high pressure torsion (HPT) at room temperature up to 5 revolutions. HPT resulted in strong grain refinement. The microstructure after HPT was inhomogeneous with refined grains mainly in regions near the specimen periphery, while coarse only slightly fragmented grains were observed in specimen centre. The microstructure inhomogeneity was continuously smeared out with increasing number of rotations by extending the fine grain region from specimen periphery towards its centre. However, even after 5 revolutions the microstructure remained inhomogeneous characterized by slightly coarser grains in central regions as compared to peripheral regions of the specimen. Positron annihilation spectroscopy (PAS) and X-ray line profile analysis (XLPA) were employed to characterize the structure inhomogeneity in individual specimens. Microstructure and dislocation density evolution were correlated with mechanical properties characterized by a detail microhardness measurement throughout the individual specimens.