Papers by Author: Hasso Weiland

Abstract: Crystallographic texture analysis is a microstructure characterization technique essential for relating microstructure attributes to the anisotropy of properties in crystalline matter. As such, it is widely used in research as well as in industrial materials and process design. Additionally, texture analysis is also used in industrial applications to monitor the consistency of a process for e.g. the anisotropy in mechanical properties from product to product. This overview outlines the reduction of quantitative texture data for the use in industrial settings.

Abstract: Control of grain size during recrystallization of aluminum alloys is critical when tailoring material properties for structural applications. Most commonly the grain size is controlled by adding alloying elements which form second phases during homogenization heat treatments small enough to impose a Zener drag on the grain boundary mobility. These phases are known as dispersoids and are in the 10 to 200 nm in diameter range. In Al-Zn alloys, zirconium has been successfully used in controlling the degree of recrystallization after solution heat treatments. It is commonly understood that the Al3Zr dispersoids of about 20 nm in diameter present in the microstructure are the key features affecting grain boundary mobility. With the success of controlling recrystallization in Al- Zn alloys, zirconium has been added to other alloy systems, such as Al-Cu-Mn, and a similar retarding effect in recrystallization kinetics has been observed as seen in the Al-Zn systems. However, in Al-Cu-Mn alloys, zirconium bearing dispersoids are not observable in the microstructure. Consequently, additional microstructural effects such as solute drag need to be considered to explain the experimental observations. In this paper, the role of zirconium additions in aluminum alloys will be summarized.

Abstract: The softening process consists of recovery and recrystallization. Despite the significant effect of recovery on the softening, recovery has not enjoyed the attention as much as recrystallizaion has mostly due to difficulties in the microstructural characterization of recovery. The present study introduced GOS qualitatively and quantitatively to gauge the microstructural evolution occurring during annealing. Then the GOS analysis was applied to discern alloying effects on recrystallization in hot deformed Al-Cu-Mg alloys. Recovery seems accountable for the retardation of recrystallization in the Mn containing alloys. By adding Zr to Al-Cu-Mg-Mn, recrystallization could be further inhibited.

Abstract: In this work we present deformation experiments of polymer-coated polycrystalline aluminium sheets. We observe that the straining is accompanied by the development of microstructural defects at the sample surface as well as in the interface between the metal and the different polymers. These defects are due to a variety of dynamical mechanisms which are essentially induced by bulk plasticity of the metal substrate. They micromechanically interact with the polymer coating and transfer some of the metallic roughness to the coating and to the surface.

Abstract: A new technique for investigating 3D grain growth in polycrystalline materials using white x-ray microdiffraction with micron point-to-point spatial resolution is presented. This technique utilizes focused polychromatic x-rays at the Advanced Photon Source, differential aperture depth-profiling, CCD measurements, and automated analysis of spatially-resolved Laue patterns to measure local lattice structure and orientation. 3D thermal grain growth studies of hotrolled aluminum have been initiated to demonstrate the capabilities of this method. Complete 3D grain orientation maps were obtained from a hot-rolled aluminum polycrystal. The sample was then annealed to induce grain growth, cooled to room temperature, and re-mapped to measure the thermal migration of all grain boundaries within the same volume region. Initial observations reveal significant grain growth above 360°C, involving movement of both low- and high-angle boundaries. Systematic measurements have been obtained of the as-rolled grain structure and of the microstructural evolution after annealing at successively higher temperatures. Small second-phase precipitates have been identified. Such measurements will provide the detailed 3D experimental link needed for testing theories and computer models of 3D grain growth in bulk materials.

Abstract: Aluminum alloys exhibit recrystallization kinetics that vary strongly with composition. The conventional understanding is that certain alloying elements, e.g. chromium, retard grain boundary motion due to the formation of fine dispersions of second phase particles, giving rise to particle drag of boundaries. There is countervailing evidence, however, that suggests that solute drag provides a stronger influence on grain boundary mobility. This paper presents new evidence for a pronounced effect of solute based on experiments in which individual boundaries migrate under the driving pressure of stored energy from prior plastic strain. As supported by the literature, boundaries exhibit a maximum mobility for a 38-39 degree <111> misorientation in initial annealing experiments. Specifically, this mobility maximum is asymmetric with a sharp cutoff below 38-39 degrees but a more gradual decrease at misorientations beyond 40 degrees. The occurrence of other, smaller mobility peaks is discussed within the context of the sharpening of evolving maxima with discussed within the context of the sharpening of evolving maxima with increased recrystallization. The presence of a minimum at 38-39 degrees is found at both higher temperatures and higher solute concentrations. This transition from a local mobility maximum to a minimum is discussed within the context of recent theories solute drag activity.

Abstract: The evolution of texture as a function of recrystallization has been characterized for hotrolled AA1050. Samples prepared from hot rolled sheet were annealed isothermally for sufficient time to allow complete recrystallization. The microstructural variation and texture evolution in the samples was observed by automatic indexing of Electron Back Scatter Diffraction (EBSD) patterns in a Scanning Electron Microscope (SEM). The spatial orientation variation within the deformed microstructure of nucleation, growth and orientations of recrystallized grains was examined. The orientation spread within grains was found to be a useful quantity for partitioning recrystallized and unrecrystallized regions. Also the effect of deformation texture on the evolution and growth of various recrystallization texture components was analyzed. The analysis is aimed at obtaining a correlation between the deformation microstructure, texture development and recrystallization kinetics in the hot-rolled condition. Preliminary results suggest only a weak correlation between the rate of recrystallization and the deformation texture component.

Abstract: Engineering properties of aluminum alloys can be optimized by a combination of thermomechanical processes. For example, a series of deformation and recrystallization processes is used to improve the deep drawing characteristics of aluminum sheet for beverage can fabrication or the forming characteristics in automotive applications. In other cases, suppression of recrystallization is desired: fracture toughness of high strength aluminum alloys is better for an unrecrystallized material than for a recrystallized material of comparable yield strength. Industrial control of recrystallization mainly focuses on control of texture for formability, control of grain size and degree of recrystallization for surface appearance and damage related properties. Such control is achieved by manipulating nucleation and growth of new grains. Nucleation density and crystallographic orientation of new grains is determined by inhomogeneities of the deformed structure. Grain boundary mobility is controlled by second phases, which at the correct size will exert a pinning force on the boundary. Additionally, solutes impose a drag on moving grain boundaries. The following will review the various applications of recrystallization in manufacturing of wrought aluminum products, summarize the current understanding of recrystallization processes in aluminum alloys, and point out needs for further understanding and necessary requirements for simulation of recrystallization processes.