S. Zaefferer

A study of active deformation systems in titanium alloys: dependence on alloy composition and correlation with deformation texture

Abstract Active glide and twinning systems have been studied by transmission electron microscopy (TEM) in samples of three Ti-alloys, T40 (Ti+1000 ppm O), T60 (Ti+2000 ppm O) and TiAl6V4 deformed up to 5% by uniaxial or biaxial tension. The aim of the work was to understand more clearly which glide systems are activated during deformation of polycrystalline material and how they are related to the formation of the cold rolling textures. In order to estimate the stresses necessary for the activation of the observed glide systems, the resolved shear stresses for the different deformation systems were calculated from measured crystal orientations. The main results are: in TiAl6V4 〈a〉, basal slip has a lower critical resolved shear stress, τc, than prismatic slip. 〈c+a〉 pyramidal glide shows a very low τc, which is up to two times larger than that for prismatic slip. Nevertheless, 〈c+a〉 glide systems were only rarely activated and twinning systems were never activated. Therefore, deformation with c-components may be accommodated by β-phase deformation or grain boundary sliding. The observed c-type texture is due to the strong basal glide. In T40, τc for 〈c+a〉 glide is up to 13 times higher than that for prismatic glide. However, 〈c+a〉 glide and twinning were strongly activated, leading to the observed t-type texture. In T60, the high oxygen content completely suppressed twinning and strongly reduced 〈c+a〉 glide. The less developed t-type texture is due to the combination of 〈c+a〉 and basal glide.

Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg-Y alloys

Magnesium-yttrium alloys show significantly improved room temperature ductility when compared with pure Mg. We study this interesting phenomenon theoretically at the atomic scale employing quantum-mechanical (so-called ab initio) and atomistic modeling methods. Specifically, we have calculated generalized stacking fault energies for five slip systems in both elemental magnesium (Mg) and Mg-Y alloys using (i) density functional theory and (ii) a set of embedded-atom-method (EAM) potentials. These calculations predict that the addition of yttrium results in a reduction in the unstable stacking fault energy of basal slip systems. Specifically in the case of an I2 stacking fault, the predicted reduction of the stacking fault energy due to Y atoms was verified by experimental measurements. Wefind a similar reduction for the stable stacking fault energy of the {11¯

Comparison of damage development depending on the local microstructure in low alloyed Al-TRIP-steels, IF steel and a DP steel

Abstract A study of the microstructural effects on the void initiation and crack growth in aluminium alloyed TRIP (transformation induced plasticity) steels has been conducted. Understanding of this microstructural dependency of the crack development is necessary to increase the failure limit and the energy absorption rate of the material. Different microstructures in terms of phase volume fractions and spatial arrangement of these phases have been obtained by annealing. Additionally, different austenite stabilities, both geometrically as well as chemically based, have been obtained. First the macroscopic mechanical properties are linked to the microstructure. Furthermore, different characteristic prestraining levels are subsequently correlated to the microstructural damage evolution by observation of the samples with backscatter electron diffraction-based orientation microscopy in a high resolution scanning electron microscope. The experiments result in a prediction of the most critical microstructural parameters when high damage tolerance is needed.

Advanced Methods and Tools for Reconstruction and Analysis of Grain Boundaries from 3D-EBSD Data Sets

We report the recent development of a 3D orientation data post-processing software, which we refer to as QUBE. Amongst other functionalities, it offers the possibility to specify the spatial and orientational distribution of boundary normals. We describe a method to reconstruct a voxel-accurate and smooth 3D boundary triangle mesh by algorithmic means. A proof of concept is given by a benchmark on a generic dataset and we demonstrate a first result with the description of selected grain boundaries in an Fe-28%Ni sample.

A Study on the Microstructure Formation Mechanisms and Functional Properties of CdTe Thin Film Solar Cells Using Correlative Electron Microscopy and Atomistic Simulations

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