Papers by Author: Günter Gottstein

Abstract: Grain boundary triple junctions are the structural elements of a polycrystal. Recently it was recognized that they can strongly impact the microstructural evolution, and therefore there engender new opportunities to control and to design the grain microstructure of fine-grained and nanocrystalline materials due to their effect on recovery, recrystallization and grain growth. The measurement of triple junction energy and mobility is thus of great importance. The line energy of a triple junction constructs an additional driving force of grain growth. Taking the triple line energy into account, a modified form of the Zener force and the Gibbs-Thomson relation can be derived to reveal the influence of the triple line energy on second phase particles and the change of the equilibrium concentration of vacancies in the vicinity of voids at a grain boundary. The impact of triple junctions on the sintering of nanopowders is discussed. The role of “grain boundary - free surface” triple lines in the adhesive contact formation between spherical nanoparticles is considered. It is shown that there is a critical value of the triple line energy above which the nanoparticles do not stick together. Based on this result, a new nanoparticle agglomeration mechanism is proposed, which accounts for the formation of large agglomerates of crystallographically aligned nanoparticles during the nanopowder processing.

Abstract: In the current study the nucleation of Cube grains during recrystallization in a commercial Al-Fe-Si alloy was investigated by in-situ and ex-situ annealing experiments at different temperatures. Both methods confirm that the Cube grains can be nucleated during RX both at large deformed Cube bands and other locations. During annealing only about one third of the Cube nuclei from these other locations developed into Cube oriented recrystallized grains owing to an unfavorable growth environment. Such nuclei needed also a longer incubation time due to their small size compared to nuclei formed at large Cube bands which can form nuclei very fast owing to the well-developed substructure and transition zones promoting nucleation. The growth rate of all Cube nuclei was found to be strongly influenced by their environment. This effect became more pronounced with decreasing annealing temperature due to an increasing amount of recovery.

Abstract: Two binary alloys, Mg-1Ce and Mg-1Gd (wt.%), were subjected to severe deformation via. single-pass rolling, followed by annealing treatments at different temperatures. Optical microscopy, X-ray diffraction and electron backscatter diffraction techniques were applied to characterize the respective texture and microstructure evolution. Correlations between the material composition and the deformation, recrystallization and grain growth events were established. Mg-1Ce displayed typical split basal textures post rolling with little modification during the transition from deformation to recrystallization, eventually producing a predominantly basal texture. On the other hand, Mg-1Gd produced significant texture modification, starting from a split basal deformation texture, which was eventually replaced by a RD-TD double split texture. The texture modification in the Mg-1Gd alloy was attributed to favorable grain growth during the recrystallization and grain growth events.

Abstract: The effect of the triple line energy on grain growth was studied by means of computer simulations with a network model. The results showed that the driving force stemming from the triple lines can influence significantly the evolution of grain growth.

Abstract: Texture evolution during static primary recrystallization of an austenitic Fe-28Mn-0.28C TWIP steel was analyzed. The cold-rolled material, which showed a Brass-type texture at medium (30% and 50%), and additionally a γ-fiber at high (80%) deformation degrees, was subjected to isothermal annealing at 700°C. The influence of rolling degree/starting texture on the development of particular texture components was studied. After recrystallization a weakened, retained rolling texture was observed for the examined reduction levels. In addition to the deformation components, Brass and Goss, further α-fiber components were formed mainly by annealing twinning leading to the development of this fiber.

Abstract: The migration of planar grain boundaries induced by a magnetic field was measured in specially grown zinc bicrystals (99.995%). Particularly, symmetrical and asymmetrical <> tilt grain boundaries with rotation angles in the range between 60° and 90° were investigated. Boundary migration was measured in-situ in the temperature range between 330°C and 415°C and the absolute values of grain boundary mobility were obtained. The results revealed that grain boundary mobility essentially depends on the misorientation angle and the inclination of the boundary plane. An application of a magnetic field during the annealing of cold rolled (90%) Zn-1.1%Al sheet specimens substantially affected the texture and microstructure evolution. This effect is attributed to the additional magnetic driving force for grain growth arising due to the magnetic anisotropy of zinc.

Abstract: The recrystallization behavior of 50% cold rolled Fe-22%Mn-0.376%C alloy during annealing at 560°C, 630°C and 700°C was investigated. Microhardness tests were applied for characterization of the recrystallization kinetics, X-ray diffraction and EBSD measurements were utilized to characterize the crystallographic texture and the grain microstructure. The obtained experimental data were evaluated in terms of the JMAK model. The obtained values of the Avrami exponent varied in the range between 0.70 and 1.37. The inhomogeneous grain microstructure after recrystallization is interpreted in terms of non-randomly distributed nuclei. Shear bands, lamellar lines intersecting with mechanical twins and grain boundaries with localized high misorientation gradients were identified to be preferential nucleation sites. No pronounced texture was observed after annealing at 630°C.

Abstract: The results of investigations of magnetically driven grain boundary migration in high purity (99.995%) zinc bicrystals are presented. In-situ measurements were conducted by means of a specially designed and fabricated polarization microscopy probe. The migration of planar tilt grain boundaries with various misorientation angles in the range between 60° and 90° was studied. The absolute grain boundary mobility and its temperature dependence was measured in the regime between 330°C and 415°C and the corresponding migration activation parameters were determined. The results revealed that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The migration activation enthalpy was found to vary between 1.18 eV and 2.15 eV. The obtained activation parameters comply with the compensation law, i.e. the migration activation enthalpy changes linearly with the logarithm of the pre-exponential factor.

Abstract: In the present study, a modified cellular automaton CORe (Cellular Operator for Recrystallization) was used to predict the recrystallization (RX) texture and microstructure of 70% cold rolled commercial AA8079L alloy at 300°C. The nucleation of the Cube orientation is of considerable scientific interest since the Cube texture component influences significantly the anisotropy of material properties. Experimental data collected during this investigation were used for subsequent modeling. By studying the annealed material by EBSD, an average nucleation rate at Cube bands was established and used in the model. The simulated microstructure reveals, in general, good agreement with experiment. The texture prediction shows the right tendency, but the modeled intensity of the Cube texture is about 2 times smaller than in experiment.

Abstract: The results of recent experimental, theoretical and computer simulation studies of the thermodynamics and kinetics of grain boundaries and grain boundary junctions are presented and discussed.