Using Macrotexture Map to Visualize Texture Heterogeneity in Polycrystalline Aggregate

A new tool, macrotexture map, was developed to represent and visualize texture heterogeneity in polycrystalline aggregate. This is a critical tool for microstructure representation, useful in risk analysis, performance simulation, and hotspot identification. In contrast to orientation imaging microscope (OIM) map where each color represents a crystal orientation, each color in this macrotexture map represents a texture. Different color represent different texture and similar texture shall have similar color. Macrotexture map provide a unique function to quantitatively evaluate texture heterogeneity of large components, leading to a first-hand understanding of property heterogeneity and anisotropy. For an experienced user, these maps serve the same purpose in identifying high risk locations in the investigated component as medical imaging maps do for diagnosis purpose. This method will also serve as a starting point in mesoscale simulation with meshing sensitivity based on the texture heterogeneity. It will provide a bridge between texture characterization and behavior simulation of components with texture heterogeneity. Macrotexture map will offer a linkage between crystal plasticity simulation in small length scale and finite element/difference simulation in large length scale.

The ductile/brittle transition of polycrystalline aggregates in terms of their single crystal cubic properties

The Kröner solution is used to determine the shear modulus of any polycrystalline aggregate composed of cubic crystals. This solution from the self-consistent method takes the form of a cubic equation specified by the three elastic properties of the crystal. The ductile/brittle transition for homogeneous and isotropic materials in uniaxial tension is specified by terms of the shear modulus and the bulk modulus of the aggregate. These two basic results are then combined to specify the ductile/brittle transition of the polycrystalline aggregate in terms of its cubic crystal symmetry elastic properties. The various forms that the combined results take are developed and interpreted. Examples and detailed results are given for carbon-diamond, copper, tungsten, iron, silicon, lithium and platinum element type polycrystal materials. The tungsten and iron cases very closely bracket the ductile/brittle transition for all of the solids forming elements of the Periodic Table, one from the brittle side and the other from the ductile side.

Simulation of Complex Plastic Flows in Machining of Metal Polycrystals Using Remeshing

The simulation of machining of soft metals at the 100 microns-few mm length-scale requires capturing complex flow physics induced by the high ductility and polycrystalline aggregate nature of these metals. This work presents a remeshing and mesh-to-mesh transfer approach that can successfully simulate complex flows including highly sinuous flow with surface folding in polycrystalline aggregate cutting. The meshing scheme is both graded and adaptive, with the ability to automatically refine regions such as self-contacts. Notably, the presence of microstructure makes these simulations far more complex than their homogeneous counterparts, with several additional constraints on the remeshing algorithm. The approach is general, with no limitations on rake angle, grain-size, or friction coefficient, and does not use an artificial, predefined separation layer. The scheme accurately tracks individual grains and allows grain splitting in a manner consistent with imaging experiments. The plastic strain field, cutting-force evolution, and deformed grain shape from several annealed-copper cutting simulations are presented, representing a range of rake angles and friction coefficients as high as 0.5. The simulations accurately capture the thick chips, high cutting force, and highly undulating streaklines of flow that characterize sinuous flow, as well as the experimental observation that the sinuous flow is suppressed on using a high rake-angle for the cutting. Moreover, in grains that are split between the chip and residual surface, we can accurately capture the extreme grain stretching that is observed prior to splitting in imaging experiments. Remeshing also provides a way to accurately capture the residual surface plastic strains and strain gradients. The latter are particularly steep, with the strain falling from a value greater than 10 to 2 within a distance of 30 microns. The use of remeshing has numerous advantages over a predefined separation layer, including the fact that one can parametrically explore the effect of variables like the extent of yield stress inhomogeneity on the flow pattern with no limitations. Interestingly, the technique allows us to find the actual line of material separation in such cutting processes: As opposed to a horizontal line, this is typically an undulating curve with a deviation of about 0.06 of the undeformed chip thickness on either side of the horizontal. This fraction increases with the extent of sinuous flow. A simple, pseudograin model with spatial inhomogeneity in flow stress is used to represent the microstructure in the present work, but the present scheme can easily be used with more complex microstructural models as well.

Microstructural features effect on the evolution of cyclic damage for polycrystalline metals using a multiscale approach

In the context of polycrystalline metals, the damage process analysis is generally restricted to surface of a sample containing some hundreds of grains, due to the inherent difficulties microstructural analysis. Determination of the grain size influence, misorientation and neighboring grains effect remains difficult with experimental studies. In this work, the influence of microstructural characteristics of polycrystalline metals on the evolution of the damage process under cyclic loading is investigated. On the basis of the quaternion theory, the orientation of each (crystal) grain is represented by a single angle (theta). The relative misorientation of each grain is set by the ratio of its misorientation over the average value of polycrystalline aggregate. The relative volume of grain is used as a parameter representing the grain size. The damage evolution under cyclic loading is identified using the coupling of the crystal plasticity model with the Continuum Damage Mechanics (CDM) model. The damage evolution and its distribution on polycrystalline aggregate are calculated by means of numerical homogenization over each grain. The heterogeneity distribution and damage evolution for polycrystalline metals have been analyzed. The obtained results show that neighboring grains effects are larger and may change the tendency that larger grains with great misorientation are the most damaged.

The Microstructural Origin of Sinuous Flow in Metal Cutting

In situ, high-speed imaging experiments have revealed the existence of sinuous flow, a recently discovered mode of chip formation in machining. The origin and consequences of sinuous flow are still being investigated, but it is now known that sinuous flow involves extensive redundant plastic deformation, poor surface finish and paradoxically high cutting forces. Here, we use full-scale simulations to show that microstructure related inhomogeneity is a major cause of sinuous flow. The simulations are conducted in a Lagrangian FE framework, and use a simple pseudograin model to represent the metal workpiece as a polycrystalline aggregate. The model successfully captures all essential features of sinuous flow in metals like OFHC copper and CP aluminum, and points to the importance of including material microstructure in cutting simulations.

Application of the grain boundary formulation and image processing-based algorithm in micro-mechanical analysis of piezoelectric ceramic

The main subject of this paper is the micro-mechanical analysis of a piezoelectric ceramic. In micro-mechanical analyses, it is very important to have knowledge about the real and natural micro-structure of materials. Therefore, the barium titanate powder was prepared using the solid-state technique, and pellets and beams were manufactured by uniaxial and isostatic pressing. The boundary element method (BEM) is used in order to be combined with three different grain boundary formulations for investigation of micro-mechanics and crack nucleation and evaluation in piezoelectric ceramic. In order to develop a numerical programming algorithm, suitable models of polycrystalline aggregate have to be discretised for the BEM analysis. Hence, original comprehensive algorithms are designed on the basis of image processing methods. Several assumptions are made to model the grain boundary in micro-scale. In the first step, before having any cracks, the traction equilibrium and displacement compatibility are governing equations. When the onset micro-crack starts to initiate, one mixed-mode potential based cohesive law is applied to model grain boundaries and investigate the intergranular crack nucleation and evolution. Upon interface failure, a frictional law is utilised in order to study separation, sliding or sticking between micro-crack surfaces. Several numerical experiments on barium titanate polycrystalline aggregate are presented to show the effectiveness of image processing-based discretisation algorithms and grain boundary formulation in micro-mechanical analysis.