Local Texture Variations Research Articles

Modeling mechanical behavior of an additively manufactured metal structure with local texture variations: a study on model form error

The repetitive rapid solidification that occurs in metal additive manufacturing (AM) processes creates microstructures distinctly different from wrought materials. Local variability in AM microstructures (either by design or unintentional) raises questions as to how AM structures should be modeled at the part-scale to minimize modeling error. The key goal of this work is to demonstrate a posteriori error estimation applied to an AM part. It is assumed that the actual microstructure is unknown and an approximate, spatially uniform material model is used. Error bounds are calculated for many reference models based on AM microstructures with elongated grain morphology and localized or global fiber textures during a post-processing step. The current findings promote confidence that a posteriori model form error estimation could be used effectively in mechanical performance simulations of AM parts to quickly obtain quantitative error metrics between an approximate model result and many microstructure-based reference models. The a posteriori error estimation introduces significant time savings compared to computing the full reference model solutions. Tight bounds on model form error are obtained when texture variations in the reference models occur on large length scales. For materials with property variation at small length scales, multi-scale error estimation techniques are needed to properly account for the many interfaces present between areas with different properties.

Local texture-microstructure correlation due to deformation localization in ECAP-processed nickel

A correlation between the local microstructure and texture is investigated in ultrafine grained nickel of 99.6wt% purity deformed via Equal Channel Angular Pressing (ECAP). Annealing at 700K induces discontinuous recrystallization and a layered microstructure with alternating recrystallized coarse- and remaining ultrafine grained areas is formed. A large scale analysis of as-deformed Ni applying orientation-imaging microscopy revealed local texture variations on a similar scale. The shear band formation via plastic flow localization induces systematic variations of local texture and microstructure which affects subsequent recrystallization. The correlation of fiber components of the shear texture and the spatial distribution of specific orientations before and after annealing allows to identify regions of decreased thermal stability. The dominant texture feature after annealing is found to be a prominent rotated cube component, which is a consequence of nucleation and growth of new grains on the cost of the defected microstructure during recrystallization. The formation of the observed anisotropy as a consequence of the heterogeneous distribution of crystal orientations is discussed within the context of dynamic recrystallization, plastic instabilities, stored energy and recrystallization nucleation mechanisms.

Delaunay triangulation-based pit density estimation for the classification of polyps in high-magnification chromo-colonoscopy

In this work we propose a method to extract shape-based features from endoscopic images for an automated classification of colonic polyps.This method is based on the density of pits as used in the pit pattern classification scheme which is commonly used for the classification of colonic polyps. For the detection of pits we employ a noise-robust variant of the LBP operator. To be able to be robust against local texture variations we extend this operator by an adaptive thresholding. Based on the detected pit candidates we compute a Delaunay triangulation and use the edge lengths of the resulting triangles to construct histograms. These are then used in conjunction with the k-NN classifier to classify images.We show that, compared to a previously developed method, we are not only able to almost always get higher classification results in our application scenario, but that the proposed method is also able to significantly outperform the previously developed method in terms of the computational demand.

Orientation effects in nanoindentation of single crystal copper

Numerical simulations and experimental results of nanoindentation on single crystal copper in three crystallographic orientations [(1 0 0), (0 1 1) and (1 1 1)] using a spherical indenter (3.4 μm radius) were reported. The simulations were conducted using a commercial finite element code (ABAQUS) with a user-defined subroutine (VUMAT) that incorporates large deformation crystal plasticity constitutive model. This model can take full account of the crystallographic slip as well as the orientation effects during nanoindentation. Distributions of the out-of-plane displacements and shear stresses as well as shear strains were obtained for indentation depths of up to 310 nm. The experimental studies were conducted using an MTS Nano Indenter (XP) system from which the load–displacement relationships were obtained while the surface topography as well as the surface profile along a line scan of indents were obtained using a Digital Instruments (Dimension 3100) atomic force microscope (AFM). The top views of the indent pile-up patterns under the spherical indenter show two-fold, three-fold, and four-fold symmetries for the (0 1 1), (1 1 1), and (1 0 0) orientations, respectively. Attempt was made to relate the anisotropic nature of the surface topographies around the indents in different crystallographic orientations of the single crystal copper specimens with the active slip systems and local texture variations. A reasonably good agreement had been obtained on several aspects of nanoindentation between the experimental and numerical results reported in this investigation as well as similar results reported in the literature. Thus, material properties of single crystal copper can be determined based on an appropriate numerical modeling of the nanoindentation on three crystallographic orientations.

Uniformity of Grain Coarsening in Submicron Grained Al-Sc Alloy Containing Local Variations in Texture

The effect of fine particles on the uniformity of grain coarsening in a submicron grained Al-Sc alloy containing significant local variations in texture has been investigated using high resolution EBSD. The alloy was processed by severe plastic deformation and low temperature ageing to generate a fine-grained (0.8 µm diameter) microstructure containing either a dispersion of nanosized Al3Sc particles or a particle-free matrix. The initial processing generated a uniform grain size distribution, but the distribution of grain orientations was inhomogeneous with the microstructure containing colonies of grains consisting predominantly of either HAGBs or LAGBs with the latter possessing orientation gradients of up to 10 o/µm. Despite the marked differences in boundary character between these regions, the alloy undergoes slow and uniform grain coarsening during annealing at temperatures up to 500 oC with no marked change in the grain size distribution, boundary distribution and texture. A model of grain coarsening that takes into account the influence of fine particles on the kinetics of grain growth within an orientation gradient is outlined. The model predicts that a large volume fraction of fine particles (large f/r-value) tends to homogenize the overall rate of grain coarsening despite the presence of orientation gradients in the microstructure.

Synthesis of progressively-variant textures on arbitrary surfaces

We present an approach for decorating surfaces with progressively-variant textures . Unlike a homogeneous texture, a progressively-variant texture can model local texture variations, including the scale, orientation, color, and shape variations of texture elements. We describe techniques for modeling progressively-variant textures in 2D as well as for synthesizing them over surfaces. For 2D texture modeling, our feature-based warping technique allows the user to control the shape variations of texture elements, making it possible to capture complex texture variations such as those seen in animal coat patterns. In addition, our feature-based blending technique can create a smooth transition between two given homogeneous textures, with progressive changes of both shapes and colors of texture elements. For synthesizing textures over surfaces, the biggest challenge is that the synthesized texture elements tend to break apart as they progressively vary. To address this issue, we propose an algorithm based on texton masks, which mark most prominent texture elements in the 2D texture sample. By leveraging the power of texton masks, our algorithm can maintain the integrity of the synthesized texture elements on the target surface.

Microstructure mapping of interconnects by orientation imaging microscopy

Orientation imaging microscopy (OIM) provides a detailed description of the variation in crystallographic structure over the surface of single crystal or polycrystalline bulk materials. This technique has been used in the investigation of interconnect lines and contact pad material in a number of aluminum metallizations on silicon oxide/silicon substrates. The specimens were examined in a scanning electron microscope fitted with apparatus for collecting electron back-scatter diffraction patterns (EBSPs). OIM scans were performed over various regions of each specimen. The technique has provided information regarding the local texture variations between different regions of a thin film structure for both patterned and unpatterned metallizations. It was concluded that significant modification of the microstructure may occur subsequent to the initial metal deposition and that this modification can differ between the unpatterned regions and interconnect lines of varying widths.