Papers by Author: Günter Gottstein

Abstract: A new approach to dynamic recrystallization (DRX) is introduced. It is based on the assumption that the critical conditions for DRX and the arrest of DRX grain boundaries are related to the development of mobile subboundaries. The theoretical predictions are compared to experimental results during incipient and steady-state DRX. The grain size sensitivity of the DRX grains establishes the desired link between deformation and DRX microstructure.

Abstract: Recent results of experimental research into stress induced grain boundary migration in aluminum bicrystals are briefly reviewed. Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains for any <100> tilt boundary in the entire misorientation range (0-90°). Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of migration activation parameters. Grain boundaries can act during their motion under the applied stress as sources of lattice dislocations that leads to the generation and growth of new grains in the boundary region. The rate of stress induced boundary migration decreases with increasing solute content in aluminum. Both the migration activation enthalpy and the pre-exponential mobility factor were found to increase with rising impurity concentration.

Abstract: In a previous work [ we introduced the geometry of a granular system that allowed the study of the effect of a finite mobility of the quadruple and triple junctions on grain boundary migration. One of the most important conclusions of this work was that the triple junctions drag more effectively the motion of the grain boundaries than the quadruple junctions. Nevertheless, this conclusion was drawn without consideration of the grain size. For this reason, this conclusion might be contradictory with our understanding of the grain boundary junctions because while the effect of the triple lines is inverse linear with the grain size that of the quadruple junctions is proportional to the inverse square of the grain size and thus, quadruple junctions are expected to drag more effectively, at least, for very small grain sizes. In the present investigation, we studied comprehensively the effect of grain size on the evolution of the granular system under the assumption of a finite mobility of the boundary junctions. For this purpose, several network model simulations were carried out for different grain sizes ranging from nanoto micrometers using a fully periodic grain arrangement. The results seem to corroborate that the triple junctions drag more effectively the motion of the grain boundaries, however, for very low junction mobility and grain sizes the effect appears to be indistinguishable. It was also observed that for very low quadruple junction mobility the geometry of the granular system undergoes a severe transformation which results in the unfulfillment of the equation derived in [.

Abstract: The impact of a magnetic field on texture and microstructure evolution in Zn-1.1%Al was investigated. Specifically oriented cold rolled (85%) sheet specimens were annealed in a magnetic field of 17 Tesla at 340°C for 15, 30 and 90 minutes. X-ray diffraction and EBSD-measurements were utilized to characterize crystallographic texture and grain microstructure. The results confirmed that grain growth in magnetically anisotropic zinc can be substantially affected by a magnetic field. This manifested itself by significant changes in the development of the grain growth texture during magnetic annealing compared to annealing at zero field. The magnetically induced texture changes are caused by the generation of an additional magnetic driving force which arises from a difference in magnetic free energy density between differently oriented grains. The grain microstructure evolution was also essentially affected by a magnetic field such that grains with energetically favoured orientations grow faster and their fraction becomes larger, than that of grains with disfavoured orientations. The results will be also discussed with respect to the magnetically altered grain boundary character distribution.

Abstract: A cellular automaton and a vertex model were used, respectively, for the simulation of recrystallization and grain growth in a Fe-0.374%C-21.64%Mn alloy. The results of the recrystallization simulations revealed that the preferential nucleation during the annealing of the rolled sheet occurs at shear bands, which is corroborated by experimental observations. Subsequently, grain growth simulations were carried out with a 2D vertex model. The model used experimental data as input for its validation in this specific steel. The simulations showed a good agreement with the experimental results.

Abstract: The current study introduces a thermodynamically correct approach which allows a direct precise measurement of the grain boundary triple line tension. The experimental technique utilizes the measurement of the surface topography of a crystal in the vicinity of a triple junction and grain boundary groove on thin wires by atomic force microscopy. The grain boundary triple line tension of a random triple line was measured to be in the order of 10^-9 J/m.

Abstract: A novel set-up developed to continuously observe and measure stress driven grain boundary migration is presented. A commercially available tensile/compression SEM unit was utilized for in-situ observations of mechanically loaded samples at elevated temperatures up to 850°C by recording orientation contrast images of bicrystal surfaces. Two sample holders for application of a shear stress to the boundary in bicrystals of different geometry were designed and fabricated. The results of first measurements are presented.

Abstract: Stress induced grain boundary migration was experimentally investigated in aluminum bicrystals. Migration of planar symmetrical <100> and <111> tilt boundaries under a shear stress was observed to be accompanied by a lateral translation of the adjacent grains. This coupling proved to be the typical migration mode for all investigated boundaries, no matter whether low-or high angle, low Σ CSL coincidence or non-coincidence boundary. The migration-shear coupling was also observed for asymmetrical tilt boundaries. Measurements of the temperature dependence of coupled boundary migration revealed that there is a specific misorientation dependence of the migration activation parameters. Contrary to expectations, a high angle Σ7 tilt boundary moved under an applied stress, but produced practically no shear during its migration.

Abstract: The magnetically driven motion of planar symmetrical and asymmetrical <> tilt grain boundaries in high purity (99,995%) zinc bicrystals was measured in-situ by means of a po­la­rization microscopy probe in the temperature range between 330°C and 415°C and the corres­pon­ding migration activation parameters were obtained. The results revealed that grain boundary mobi­lity essentially depends on the misorientation angle and the inclination of the boundary plane. The magnetic annealing of the cold rolled (90%) Zn-1.1%Al sheet specimens resulted in an asymmetry of the two major texture components. This effect is attributed to a magnetic driving force for grain growth. The grain microstructure evolution was also essentially affected by a magnetic field.

Abstract: The recrystallization behavior of a 50% cold rolled Fe-23.2Mn-0.57C alloy was investigated during annealing at temperatures between 560°C and 670°C. The recrystallization kinetics were characterized by microhardness tests. X-ray diffractions and EBSD measurements were used to characterize the grain microstructure and texture evolution during annealing. The obtained experimental data were evaluated in terms of the JMAK model. The annealing texture in the investigated temperature range was very weak.