Papers by Author: Günter Gottstein

Abstract: A 3D Vertex Model has been successfully implemented to investigate the evolution of a special grain assembly during grain growth. The model considers the mean curvature as driving force for the motion of the vertices and allows the consideration of all parameters affecting the motion of the system, i.e., grain boundary energy and line tension of the triple lines, as well as grain boundary (GB), triple line (TL) and quadruple point (QP) mobility as well. The used special configuration makes it possible to study the influence of all structural elements of a grain boundary network on the evolution of the system by allowing the steady-state motion of the boundaries of a shrinking grain. In the present work the different mobilities have been systematically varied and the evolution of the grain size with time has been studied as a function of TL and QP mobility. The results of the simulations are finally linked to the different kinetic regimes reached by the system.

Abstract: The stress induced migration of planar grain boundaries in aluminium bicrystals was measured. Symmetrical <100> tilt grain boundaries with misorientation angles in the range between 5.7° and 17.8° were examined. Boundary migration under a shear stress was observed to be ideally coupled to the lateral translation of grains. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with the respective boundary geometry. The temperature dependence of grain boundary mobility was measured, and the corresponding activation parameters were determined. The activation enthalpy of boundary migration was found to be independent of misorientation angle in the investigated misorientation range and amounts to H=1.44 eV.

Abstract: A novel high temperature heating method in combination with automated EBSD-data acquisition is presented. A commercially available infrared laser is utilized to heat samples up to a temperature of about 1000°C in high vacuum in a SEM while acquiring EBSD-data of the microstructure. First results on the γ-α-γ phase transformation between 840°C and 865°C in a microalloyed ferritic low carbon steel is presented.

Abstract: The kinetic and structural behavior of symmetrical <100> tilt grain boundaries with rotation angles 8.4°, 12.0°, 14.3° and 16.0° were investigated in-situ in a hot stage SEM in the temperature range between 380°C and 640°C. The results revealed that depending on the rotation angle the boundary either remained straight, became faceted or curved under the driving force provided by the boundary surface tension during high temperature annealing. The transition “facetedcurved boundary” was also found to depend on temperature. The observed behavior is attributed to the anisotropy of grain boundary energy with respect to boundary inclination.

Abstract: Three recent investigations in the field of thermodynamics of grain boundaries and grain boundary junctions are presented. 1. The grain boundary excess free volume (BFV) along with the surface tension belongs to the major thermodynamic properties of grain boundaries. A special technique, recently developed, makes it possible to measure the BFV for practically any grain boundary and provides a way of estimating the grain boundary excess free volume for grain boundaries of different classes with rather high accuracy. The experimental values of the BFV measured for different grain boundaries are compared and discussed. 2. A new approach will be presented that makes it possible to correctly measure the grain boundary triple line tension. For this the topography at an equilibrated triple junction was measured by atomic force microscopy. Preliminary results of grain boundary triple line energy measurements are presented. 3. The problem is discussed whether it is possible to achieve an equilibrium grain size during grain growth in single phase alloys. Various approaches to the problem are considered. It is shown that the most realistic possibility to stabilize the grain size in a polycrystal is by impurities with negative grain boundary adsorption.

Abstract: Pure Cu, CuZr and an Al-alloy were processed by Equal Channel Angular Pressing (ECAP) at room temperature applying route Bc. Microstructure evolution during ECAP and subsequent annealing was investigated. The deformed and annealed states were characterized by EBSD, TEM and microhardness tests. The microstructure variation was recorded and compared to the behavior of conventional cold rolled material. The study revealed a very low thermal stability of ECAP deformed pure Cu samples compared to cold rolled material with same total strain. However, the thermal stability was significantly improved by alloying with Zr. In contrast, ECAP deformed Al-alloy showed higher thermal stability than cold rolled material.

Abstract: The recrystallization behavior of 71% cold rolled aluminum alloy 3103 was investigated by measuring the crystallographic texture and the grain microstructure during heat treatment at 288°C, 310°C and 330°C in a magnetic field of 17 T. The results revealed that the application of a magnetic field substantially enhances recrystallization kinetics.

Abstract: Modeling and simulation of recrystallization, grain growth, and related phenomena are important tools for the fundamental understanding of microstructural evolution and prediction of engineering properties. In particular for ultra fine grained and nanocrystalline materials proper account of microstructural evolution is essential for the optimal processing of these materials. It is shown that for modeling of softening phenomena it is important to discriminate between discontinuous primary recrystallization and discontinuous grain growth owing to their quite different underlying physics. Recent developments in recrystallization modeling and simulation of grain growth are addressed, in particular nucleation of recrystallization and junction effects in grain growth. Major progress is also expected from atomistic modeling and quantum-mechanical computations for making available specific material properties.

Abstract: Eutectic AlNi, AlFe, alloys exhibit plastic strains to failure (usually in the range of 1%- 5%), that those of structural alloys. We have developed a technique to measure strains at the scale of the microstructure and have used this method to assess the variation in failure properties with microstructure. This method is capable of using the grayscale information in the image of a gridded sample to obtain sub-pixel marker displacement, and can therefore accurately determine small strain values. Microstructures that exhibit large variation in local strain distribution tend to have higher variability in tensile properties, particularly tensile ductility, compared to microstructures that accumulate strain more uniformly. Orientation and morphology of lamellar plates in lamellar colonies play, also, a role in influencing the distribution of strain. Local grain orientation, phase distribution and segregation are factors influencing the strain distribution, and therefore the properties of these materials.

Abstract: Texture evolution and microstructure development of hot extruded magnesium alloy AZ31 deformed by PSC and uniaxial deformation at select temperatures and a constant strain rate of 10-4 s-1 were investigated and compared using X-ray techniques, electron back scattered diffraction (EBSD) and optical microscopy. At a deformation temperature of 200 °C both deformation routes resulted in a similar crystallographic texture and showed a heterogeneous microstructure consisting of highly deformed zones appearing as huge and/or elongated grains containing twins and shear bands embedded in a very fine-grained microstructure. High temperature deformation (400 °C) gave rise to completely different deformation textures for the two processes. Uniaxial deformation tended to randomize the initial extrusion texture, whereas in PSC a prismtexture {10-10}<11-20> prevailed. The flow stress was found to be strongly dependent on loading conditions and deformation modes.