Hasso Weiland

Grain-scale micromechanics of polycrystal surfaces during plastic straining

This is a study on grain-scale micromechanics of polycrystal surfaces during plastic straining. We use Al–Mg–Si sheets (alloy AA6022) as model material. The work aims at understanding the relationship between microstrain heterogeneity and surface roughness in plastically strained polycrystals in terms of the surface and through-thickness microstructure. Experiments were conducted on polycrystals with identical composition but different processing and microstructures. We performed tensile and bending tests on sheet samples cut in transverse and rolling directions. We investigated the plastic surface microstrains (photogrametry), surface topography (confocal microscopy), particle distribution (metallography, SEM), microtexture (EBSD), and grain size distribution (EBSD) in the same sample regions. We also conducted in-situ straining experiments where the microtexture, surface topography, and stress–strain behavior were simultaneously determined. The results reveal a relationship between the heterogeneity of plastic surface microstrains, roughness, and microstructure. In particular a correlation could be established between microstrains and banded microtexture components (Cube, Goss, {111}[uvw]).

Three dimensional characterization and modeling of particle reinforced metal matrix composites: part I Quantitative description of microstructural morphology

In this first of a two part sequence of papers, 3-D microstructures of Si particle reinforced aluminum matrix composites are computationally constructed by assembling digitally acquired micrographs obtained by serial sectioning. The material samples considered vary in volume fraction and in particle size. Furthermore, equivalent microstructures with actual particles replaced by ellipses (in 2-D) or ellipsoids (in 3-D) are computationally simulated for efficiency. The equivalent microstructures are tessellated by a particle surface based algorithm into a mesh of Voronoi cells. Various 3-D characterization functions are developed to identify particle size, shape, orientation and spatial distribution in the actual materials and to compare with 2-D micrographs. Through this analysis, differences between 2- and 3-D characterization are established. Results indicate that it may not be sufficient to use 2-D section information for characterizing detailed microstructural features like particle shapes, orientations and near-neighbor distances. The second part of this sequence of papers will describe the important relationship of these features to damage evolution in these same materials. This sequence of papers is perhaps one of the first on 3-D physical characterization of the phase and damage structure for this class of materials.

A geometric approach to modeling microstructurally small fatigue crack formation: I. Probabilistic simulation of constituent particle cracking in AA 7075-T651

Microstructurally small fatigue crack (MSFC) formation includes stages of incubation, nucleation and microstructurally small propagation. In AA 7075-T651, the fracture of Al7Cu2Fe constituent particles is the major incubation source. In experiments, it has been observed that only a small percentage of these Fe-bearing particles crack in a highly stressed volume. The work presented here addresses the identification of the particles prone to cracking and the prediction of particle cracking frequency, given a distribution of particles and crystallographic texture in such a volume. Three-dimensional elasto-viscoplastic finite element analyses are performed to develop a response surface for the tensile stress in the particle as a function of the strain level surrounding the particle, parent grain orientation and particle aspect ratio. A technique for estimating particle strength from fracture toughness, particle size and intrinsic flaw size is developed. Particle cracking is then determined by comparing particle stress and strength. The frequency of particle cracking is then predicted from sampling measured distributions of grain orientation, particle aspect ratio and size. Good agreement is found between the predicted frequency of particle cracking and two preliminary validation experiments. An estimate of particle cracking frequency is important for simulating the next stages of MSFC formation: inserting all particles into a microstructural model for these stages is computationally intractable and physically unnecessary.

Serial sectioning method in the construction of 3-d microstructures for particle-reinforced MMCs

A serial-sectioning technique is described for obtaining detailed three-dimensional microstructural images from two-dimensional sections of Si-reinforced Al-Si-Mg alloys. Optical micrographs of a series of microstructural sections were generated by the gradual removal of material layers. These micrographs were then enhanced and digitized in a quantitative image analysis system. Three-dimensional microstructure models were then computationally constructed by assembling the digitally acquired two-dimensional micrographs. Serial sectioning was found to be an inexpensive and effective means of accurately depicting heterogeneous microstructures at high resolution. A few relevant observations on material properties also were made during sectioning. Particle cracking was found to occur in larger particles that are located in clustered areas and have their longest dimension aligned with the tensile axis.

Three dimensional characterization and modeling of particle reinforced metal matrix composites part II: damage characterization

This second paper of a two part sequence, attempts to quantitatively characterize 3-D microstructural damage in particle reinforced metal matrix composites using a combination of computational and experimental tools. It is perhaps the first studies providing quantitative 3-D characterization of phase and damage morphology for comparison with 2-D micrographs. Materials with different volume fractions and particle sizes, at different levels of deformation are considered. The serial sectioning method is used to obtain micrographs of a series of parallel sections of the sample material. 3-D computer images of the particles and the associated damage in the microstructure are constructed by digitally assembling the section micrographs. Equivalent microstructures with actual particles and cracks replaced by ellipses or ellipsoids are simulated for enhanced computational efficiency. The equivalent microstructures are meshed into Voronoi cells by a surface based algorithm. Various characterization functions of geometric parameters are generated to identify the damage size, shape, orientation and spatial distribution both in 2- and in 3-D. A sensitivity analysis is conducted to explore the influence of the morphological parameters on damage. Particle size, orientation and local volume fraction are found to play the most significant roles in the cracking process. Experimental observations of damage are compared with predictions by a probabilistic damage model viz. the Weibull model. Representative material elements, which correspond to the characteristic sizes for local continuum representation, are investigated through the use of variograms and marked correlation functions.

In-situ surface characterization of a binary aluminum alloy during tensile deformation

A metal surface that is not contacting a tool typically roughens with increasing plastic strain. This form of roughening, which is often referred to as orange peel, results from non-uniform plastic flow of surface grains that are not constrained by a tool surface and hence are free to deform out of the surface plane. Although a substantial body of literature exists on orange peel, there appears to have been little work on the characterization of individual grain surface topography at the nanometric scale resulting from tensile deformation. The information gained from tracking changes in grain topography on the sub-micron scale stands to provide insight into both surface and near-surface grain deformation which could aid in the development of finite element models of non-uniform plastic flow of metals. From a materials design standpoint, a knowledge of the topographical changes that occur on various grains could provide insight as to how metal alloys can be manufactured so that free grain roughening is minimized. The purpose of this article is to present a technique that combines atomic force microscopy and orientation imaging microscopy to measure grain surface topography during in-situ tensile testing.

Recrystallization and Texture Development in Hot Rolled 1050 Aluminum

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.

Micron-Resolution 3-D Measurement of Local Orientations Near a Grain-Boundary in Plane-Strained Aluminum Using X-Ray Microbeams

Abstract X-ray microbeams have been used to perform nondestructive measurements of the local orientations and microstructure in single-crystal and grain-boundary regions in a columnar Al (0.2 wt.% Mg) tri-crystal deformed 20% in plane-strain. The measurements were made using a recently developed differential-aperture X-ray microscopy (DAXM) technique providing high angular resolution determinations of local orientations with micron 3-D spatial resolution using focused microbeams. The X-ray microbeam technique is described, three-dimensional spatially resolved pole-figures and lattice rotation distributions in single-crystal and grain-boundary regions are presented, and the potential of micron resolution 3-D X-ray structural microscopy for plastic deformation investigations is discussed.

Alloying effects on dislocation substructure evolution of aluminum alloys

The constitutive response of aluminum alloys is controlled by the evolution of dislocation substructure including mobile and forest dislocation density, cell size distribution and morphology, and misorientation angle between neighboring cells. The present study focuses upon the small strain regime and compares the measured microstructural evolution of 3003, 5005, and 6022 aluminum alloys during deformation. Room temperature tensile deformation experiments were performed on industrially manufactured specimens of each alloy and the evolving microstructure was compared with the mechanical response. The dislocation structure evolution was characterized using transmission electron microscopy and orientation imaging of deformed specimens. It was observed that structural evolution is a function of lattice orientation and the character of neighboring grains. In general, the dislocation cell size and misorientation angle between dislocation cells evolves systematically with deformation at relatively small strain levels. # 2003 Elsevier Ltd. All rights reserved.

Simulations of deformation and recrystallization of single crystals of aluminium containing hard particles

The deformation of a single crystal of aluminium in the Goss orientation {011}100 containing a coarse particle of silicon was modelled by using a finite-element (FE) code based on the crystal plasticity approach. The simulations clearly captured the heterogeneous deformation of the aluminium matrix, resulting in a region of high deformation in the vicinity of the hard particle, surrounded by a region where the amount of deformation was significantly lower. The evolution of the corresponding deformation substructure during annealing was simulated using a Monte Carlo technique. The simulations clearly demonstrated the discontinuous evolution of the subgrains in the deformation zone to form recrystallization nuclei around the hard particle, and the subsequent growth of these nuclei to consume the matrix region around the particle. For plane strain compression up to ezz = -0.4 that was used in this study, the deformation texture components near the particle consisted of rotations up to 20° from the initial Goss orientation about the transverse direction. Recrystallization simulations captured the formation and growth of nuclei from the deformation heterogeneities existing near the hard particle and predicted a significant strengthening of the orientations present in the particle deformation zone. The simulation results are shown to capture many of the experimentally observed features of deformation and recrystallization textures in aluminium single crystals containing coarse particles of silicon.