Wire drawing parameters such as area reduction and die angle have an important effect on damage generation. One of the most common defects appearing under certain critical configurations are the so-called ¿chevron cracks¿. These defects are difficult to identify since they are typically located at the wire core. To address this, great efforts have been made in the last two decades to develop numerical models capable of predicting damage evolution and failure. Continuum Damage Mechanics (CDM) models present the advantage of being coupled with the constitutive behaviour of the material. Among these models, Lemaitre¿s approach is one of the most widely used. In its original formulation, the model did not distinguish between tension and compression stresses in terms of damage accumulation. An additional parameter was then included to consider the crack closure effect under compressive stresses. However, this formulation is not valid for wire drawing, since it overestimates damage under these conditions. For this reason, a new damage model following Lemaitre¿s approach has been derived by redefining the crack closure effect under compressive stresses. In this way, under high hydrostatic compressive stresses, such as in the case of wire drawing, the model yields more realistic results in terms of damage accumulation. The model has been implemented in ABAQUS through user subroutines UMAT (for implicit cases) and VUMAT (for explicit cases). Single-pass wire drawing simulations have been performed to compare the original model with the new formulation.

In many practical situations, the distribution of residual stresses can have a paramount influence on the fatigue and fracture response of materials. In this paper, we describe a method for the local measurement of residual stresses. It consists of Digital Image Correlation (DIC) of Scanning Electron Microscope (SEM) images of a surface, before and after milling a slit using a Focused Ion Beam (FIB). The DIC algorithm used in this work is based on Fourier analysis, which can reach sub-pixel resolution. In order to calculate the internal stresses released during the milling process, the displacements detected with the DIC algorithm are fitted to Finite Element Method (FEM) simulations. Residual stresses measured by this method on a hard metal sample subjected to different laser surface treatments are successfully compared with X-ray diffraction measurements.

The results of existing constitutive models describing the behaviour of metal powder during compaction processes are very different. This reveals the high sensitivity of the mechanical behaviour of porous materials to the shape, arrangement and distribution of particles and pores. In order to clarify these discrepancies, the compaction behaviour under hydrostatic loads and high temperatures for a Nickel-based superalloy has been characterized. For this characterization, a numerical optimization procedure has been defined. In parallel, finite element models at a mesoscopic level have been built with the aim of estimating the parameters that define the mechanical behaviour of metallic powder under hydrostatic loads. Then a homogenization procedure has been used to compute the macroscopic behaviour of the powder. The results of both the experimental characterization and the mesoscopic models emphasize the limits of the analysed literature constitutive models to reproduce the compaction behaviour of the considered Astroloy powder.

Recently, there have been significant efforts to develop micro-mechanical models for a better understanding of in-service performance of WC-Co components. However, reliable information about the mechanical properties of individual features like grains or interfaces is still lacking. In this work, micro-beam testing has been used for analyzing the fracture strength of different WC-WC interfaces in a WC-6.5 wt%Co alloy. The method is based on machining cantilever beams by using Focused Ion Beam so that a single WC-WC interface is isolated from the rest of the microstructure. This machining is carried out in order to have the selected interface at a certain distance for the fixed end and perpendicular to the cantilever axis. CSL2 boundaries and randomly oriented boundaries have been identified by means of EBSD and subsequently tested by nanoindentation until fracture. Load-displacement curves confirm that CSL2 boundaries are stronger than the others and post mortem analyses indicates that the fracture mechanisms are different depending on the orientation between adjacent WC grains. This approach could be used to investigate the intrinsic strength of other interfaces present in hardmetals (i.e. WC/Co, FCC carbides/Co, FCC carbides/WC) and how it is related with processing parameters or in-service conditions.

Conventional HR-EBSD is attracting much interest due to its ability of measuring relative crystal misorientations and microstresses with great accuracy. However, this technique needs the use of simulated patterns in order to get absolute values of crystal orientation and stresses and thus expand its use to intergranular analyses. Simulation-based approaches have shown many limitations due to the poor correlation with the real patterns specially when Bragg simulations are considered. This paper presents an improved algorithm based on gradient-based correlation techniques that makes simulation-based HR-EBSD possible. Based on this new algorithm, a new pattern center calibration procedure is proposed and validated. Also, a new hybrid procedure that combines simulation-based HR-EBSD with conventional HR-EBSD is presented that enables an absolute determination of both orientations and stresses with improved accuracy. The hybrid HR-EBSD is used to analyze the martensitic transformation induced by plastic deformation in an as-quenched Ti-12wt.%Mo alloy.

High-resolution electron backscattered diffraction (HR-EBSD) is a powerful tool to describe microstructures at the sub-micrometric scale that achieves a higher degree of angular accuracy compared with conventional EBSD. However, such an EBSD technique is time-consuming and requires data-intensive computing to save and postprocess the results obtained after each scan. In the current work, a simple strategy to transform conventional results into high-resolution results is put forward in an averaging statistical layout. This makes it possible to measure the misorientations more precisely and, subsequently, the geometrically necessary dislocations by lowering the typical noise generated from Hough transformation-based conventional EBSD. Different steel microstructures are analyzed in light of this strategy. The calculated dislocation densities for those microstructures are used as input values for evaluating the initial dislocation density contribution to the yield strength in a newly developed mechanical model.

The oxidation of highly porous ceramic matrix composites (PCMCs) based on different Tyranno® fibers has been analyzed by means of thermogravimetry and electron microscopy. Both uncoated fibers and PCMC materials exhibit parabolic kinetics between 900°C and 1250°C, these being faster for Ti-doped than for Zr-doped Tyranno fibers. Oxide layers in Ti-doped fibers are porous and partially crystalline, whereas in Zr-doped materials a significant fraction of relatively coarse ß-SiC grains is still found embedded in the amorphous silica matrix. On the other hand, the CVD-SiC coatings exhibit higher oxidation rates from the outer surface than from the inner one, a phenomenon that has been associated not only with the more difficult access of oxygen to the inner face but also with the highly <111> textured structure of these coatings, for which very different oxidation rates have been published for the inward and outward directions. Cracking phenomena observed above 1100°C for long dwelling times do not lead to an acceleration of the oxidation process, which could be due to the simultaneous crystallization of the amorphous silica layers

This work presents a finite element analysis of the indentation size effect (ISE) experimentally observed in tests performed at submicron scale. A 3D model of a conical rigid surface indenting on a Nb single crystal at different depths has been developed. The bcc Nb material has been characterized within a finite-strain framework through a crystal plasticity model incorporating strain-gradient hardening. The hardness evolution for different material orientations and for different initial dislocation densities has been studied. The numerical results are compared with predictions of existing analytical models and with experimental results. (C) 2013 Elsevier B.V. All rights reserved.

The structure and crystallographic texture of zinc strips (Zn-Cu-Ti alloy) produced by the continuous horizontal twin roll strip casting method has been characterized. In longitudinal sections normal to the transverse direction, the strips display an approximately symmetrical chevron patterned structure of columnar grains inclined about 30 degrees from the rolling direction. In association with such structure, the macroscopic texture is mainly < 1 (1) over bar 00 > 'normal' (not cyclic) fibre texture tilted approximately +/-30 degrees around the transverse direction plus a similarly tilted weak < 0001 > fibre texture. A thin layer of small equiaxed grains with a strong (0001) basal texture is present at the free surfaces. The observed structure/texture combination agrees quite well with the expected macrostructure of solidification of the alloy in the twin roll casting process.

High resolution electron backscattered diffraction (HREBSD) is a novel technique for a relative determination of both orientation and stress state in crystals through digital image correlation techniques. Recent works have tried to use simulated EBSD patterns as reference patterns to achieve the absolute orientation and stress state of crystals. However, a precise calibration of the pattern centre location is needed to avoid the occurrence of phantom stresses. A careful analysis of the projective transformation involved in the formation of EBSD patterns has permitted to understand these phantom stresses. This geometrical analysis has been confirmed by numerical simulations. The results indicate that certain combinations of crystal strain states and sample locations (pattern centre locations) lead to virtually identical EBSD patterns. This ambiguity makes the problem of solving the absolute stress state of a crystal unfeasible in a single-detector configuration. (C) 2013 Elsevier By. All rights reserved.