Papers by Author: Günter Gottstein

Abstract: In order to improve deep drawability of Al-Mg-Si alloy sheets for automotive body panel by texture control, the effects of Mn addition and warm rolling on recrystallization texture were investigated for rolled and T4 treated sheets and the r-value was related to overall texture through thickness. Recrystallization texture after T4 treatment varied significantly depending on alloy composition and rolling condition. Planar anisotropy of r-value could be predicted relatively exactly from recrystallization texture by the Taylor full constraints model for all specimens. Mn addition led to an increase of average r-value in both cold and warm rolled materials, while warm rolling resulted in a decrease of average r-value in spite of relatively high predicted r-values in the surface layer.

Abstract: The ferrite to austenite phase transformation in microalloyed steel was studied, with a special focus on the orientation relationship between prior ferrite and subsequent austenite. Also the role of growth selection and preferred nucleation was investigated in this context. Their effects were examined at partial phase transformation.

Abstract: The plastic behaviour of hexagonal metals such as magnesium is very anisotropic and substantially affected by crystallographic texture. In this work, samples of pure magnesium with different starting textures were hot rolled at 400°C in one rolling pass to different final thicknesses of 5%, 10%, 30%, 50%, 70%, 90%. The microstructure and macrotexture evolution were examined by optical microscopy (OM) and X-ray diffraction, respectively. Further, microtexture analysis was done by EBSD. Texture modelling with LAPP using an FC Taylor model, based on the expected active slip and twinning modes was performed. The results were compared with the measured textures.

Abstract: Diffusion controlled creep in nanostructured materials is considered for the case when grain growth occurs concurrently. The Nabarro-Herring and Coble mechanisms that would predict creep rate reduction are re-considered to include the effect of grain-growth induced vacancy generation. It is shown that under such conditions creep is accelerated during an initial stage of grain growth as compared to the case of constant grain size. This creep enhancement stage is followed by a period of reduced creep rate. The predicted strain rate behaviour resembles primary and secondary creep.

Abstract: We present a new analysis of the relative rate of growth or shrinkage of grains in a two-dimensional network, based on the classical Von Neumann-Mullins (VN-M) analysis. We find that an analysis of the stability of the grain shape during shrinkage or growth shows that any change in the regular 2D grain leads to changes in the shape. We also re-examine a recent analysis that claims to have invalidated the VN-M relationship, but find that it is still valid, and that the cited analysis, in fact, confused a second order correction with a first order problem, partly because their derivation was in error. The erroneous magnitude of the discrepancy led them to use unphysical issues to explain the discrepancy. The way in which the curvature is distributed along the perimeter of a grain only gives rise only to second order corrections to the rate of change of area as a function of grain topology (number of sides).

Abstract: The motion and geometry of connected grain boundary systems with triple junctions in aluminium -10 ppm magnesium was investigated in-situ with a special designed SEM heating stage. The results show that triple junctions can have a marked influence on grain boundary motion. The grain area change with annealing time was from a hot stage in the SEM. An analysis of the experimental data reveals that there is no unique relationship between growth rate and the number n of grain sides (Von Neumann-Mullins relation). This is attributed to the effect of triple junction drag on grain growth.

Abstract: In magnetically anisotropic materials a driving force for grain boundary migration can be induced by an external magnetic ¯eld. It is experimentally shown that annealing of locally deformed Zn single crystals in a suitably directed high magnetic ¯eld results in a growth of new individual grains. Velocities of some solitary moving grain boundaries were measured and their absolute mobilities were estimated at a single temperature. Results are discussed in terms of preferential grain orientation and boundary character.

Abstract: The motion of a curved grain boundary with a “surface triple junction” (“free surface – boundary - free surface”) in aluminum bicrystals is studied. The effect of the “surface triple junction” on grain boundary motion is discussed in the terms of the equilibrium of boundary and junction velocity. Boundary motion in samples with different boundary curvature revealed a strict proportionality of boundary velocity and driving force. This result corroborates the fact that in the entire investigated temperature range the “surface” triple junction does not affect the boundary motion.

Abstract: The motion of <111> tilt and mixed tilt-twist grain boundaries with misorientation angles in the range between 34° and 42° in pure Al bicrystals was measured over the temperature range between 310 and 610°C. The experiments revealed that the change of the set of boundary planes in the curved moving tilt boundary does not affect its motion. The shape of the curved moving part of the mixed tilt-twist boundary was measured and compared with analytically calculated boundary shape. The results have shown that an increase of the twist component along the curved mixed boundary in studied geometrical configuration does not affect its steady-state motion. Similar to the behaviour of <111> pure tilt boundaries, the mobility of <111> mixed tilt-twist grain boundaries in the vicinity of special misorientation å7 depends on the misorientation angle in a non-monotonic fashion.