Materials Science Forum Vols. 702-703

Abstract: X-ray diffraction line profile analysis can be carried out on the hkl planes corresponding to the same texture component or the same crystallographic orientation fiber. It is shown that in textured polycrystalline materials or in thin films or multilayers X-ray line profiles measured on planes corresponding either to the main or the minor texture components can provide the Burgers vector population and dislocations densities in the different texture components separately. The experimental technique is outlined for textured specimens and the multiple convolutional whole profile method, i.e. the CMWP line profile analysis procedure, is presented for its capacity to determine the substructure pertaining to different texture components in textured samples.

Abstract: The accumulation of dislocations around hard particles such as martensite in Dual Phase steel has a prominent influence on the mechanical properties of multiphase steels. The origin of these so-called Geometrically Necessary Dislocations (GNDs) is either due to the transformation strain, or to strain gradients that arise during deformation. The generation of deformation-GNDs is explained by Ashby’s theory [1] regarding deformation of a plastic mass that contains dispersed undeformable particles. It is argued that the GNDs pile up locally against the ferrite-martensite interface. This work reports the calculated density of GNDs from high resolution Electron BackScatter Diffraction (EBSD) measurements. By measuring the lattice orientation within the grains, the lattice curvature can be quan-tified, which can be directly related to the presence of GNDs. The density of the GNDs can then be estimated either directly through kernel average misorientations, or through the calculation of the dislo-cation tensor. From this first approximation of the GND density a GNDD map has been obtained by two recently developed approaches. This map shows an enhanced dislocation density around the mart-ensite particles due to volume change during transformation. The kernel choice and step size depend-ency of the results are also investigated.

Abstract: An extension to a previously published, novel stereological method is reported which infers experimentally inaccessible components of the Nye GND tensor. Limitations imposed by electron-opacity of metals prevent direct measurement of four components of the Nye tensor, but it is possible to use additional experimentally-obtainable information in connection with underlying field equilibrium equations to estimate these additional components. This approach uses derivatives to the infinitesimal elastic distortion tensor to reduce error imposed by pattern center inaccuracy.

Abstract: During and after plastic deformation of metals, dislocations tend to evolve into generally well-defined structures that may include tangles, bands, cell walls, and various additional features. Observation of these structures by electron backscatter diffraction is only accomplished by analysis of changes in orientation from one position to the next. Excess (or geometrically necessary) dislocation densities can be inferred from 2D measurements or obtained directly from 3D measurements as indicated by Nye’s dislocation density tensor. Evolution of excess dislocation densities was measured for a split channel die specimen of aluminum alloy 7050 in the T7451 temper. Densities evolved by a factor or 1.6 for compressive deformations of 15%.

Abstract: Texture gradients are present in most samples, which are due to materials processing. Standard methods to evaluate texture gradients are based on the cut of samples, such as the X-ray investigation of surface textures against the texture inside a sheet. Bulk textures itself averaging over the whole sheet thickness are analysed by thermal neutrons. Both thermal neutrons and photons with high energies allow investigations non-destructively. The beam port Stress-Spec at the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) at Garching/Germany is equipped with a robot system based on a RX160 Stäubli robot, a Laser Tracker and a heavy basement. Samples up to 30kg can be investigated. Main restrictions are the available neutron flux, the detector efficiency and the detector size. Thus, the gauge volume is restricted to 1x1x1mm for ideal scattering conditions to measure in acceptable time scale. Photons with up to 200keV are known as high brilliant and high intense beam with similar penetration power than thermal neutrons. A typical set up of a high energy beamline for texture gradient investigations works without an Eulerian cradle so that restrictions in handling large sample are of less importance. The HZG materials science beamlines at Doris III and Petra III (Harwi-II@DorisIII and HEMS@PetraIII) are equipped with massif units for sample rotation and x-, y- and z- scanning for samples and additional equipments up to 200kg. Compared to thermal neutrons, which work with wavelengths between 1Å-2.5Å, the wavelength of high energy photons is small (0.05Å – 0.20Å). That leads on one hand to low scattering angles (1° - 10°) and on the other hand to an anisotropic ellipsoidal gauge volume. The local resolution of the synchrotron beam is much better than for thermal neutrons. In both methods corrections for constant gauge volume during pole figure scanning and for anisotropic absorption are of great importance.

Abstract: The two most important advances in two-dimensional x-ray diffraction (XRD²) are area detectors for collecting 2D diffraction patterns and algorithms in analyzing 2D diffraction patterns. The VÅNTEC-500 area detector represents the innovation in detector technology. The combination of its large active area, high sensitivity, high count rate, high resolution and low noise, makes it the technology of choice for many applications, including texture analysis. A 2D diffraction pattern contains information in a large solid angle which can be described by the diffraction intensity distribution in both 2θ and g directions. The texture information appears in a 2D diffraction pattern as intensity variation in g direction. The intensity variation represents the orientation distribution of the crystallites in a polycrystalline material. The diffraction vector orientation regarding to the sample orientation can be obtained by vector transformation from the laboratory space to the sample space. The fundamental equations for texture analysis are derived from the unit vector expression in the sample space.

Abstract: Texture formation during an austempering treatment of a TRIP-assisted steel was studied by in-situ texture measurements with a high energy source (synchrotron). Samples from a cold rolled sheet were subjected to a complete heat treatment cycle for TRIP steels including reheating to the intercritical (α+γ) temperature region, isothermal soaking and bainitic holding (austempering) at 400°C for 600s. At specific points of the thermal cycle {200}γ, {220}γ {222}γ, {331}γ and {200}α, {211}α and {220}α Debye rings were recorded and the corresponding incomplete pole figures were calculated. The latter were used to derive the orientation distribution functions (ODFs) of BCC and FCC phases at specific steps of the annealing process after assuming the orthotropic sample symmetry. The acquired data for the texture evolution during the α–γ–α phase transformation showed that during the reheating for intercritical annealing the gamma phase with {011} orientation is among the first to nucleate from the recrystallized α phase during heating and the Goss and Cube orientations are among the principal gamma phase components which transform to BCC phase after cooling.

Abstract: A 3D microstructure, measured by high-energy x-ray diffraction microscopy, is used as an input to a parallelized viscoplastic Fast Fourier Transform code (VPFFT) to simulate a tensile test. Distributions of strain, damage accumulation, neighbor interactions, and Schmid factor mismatch throughout the microstructure are calculated. These results will form the basis of a direct comparison to microstructure maps that track plastic deformation in the real sample.

Abstract: Abstract. Anisotropy of physical and mechanical properties of textured polycrystalline materials strongly depends on microstructural characteristics, such as subgrain sizes, lattice deformations, etc. Generalized Pole Figures (GPF) are an attempt to estimate the anisotropy of these properties; so, the energy stored during plastic deformation is a key parameter in primary recrystallization. In this work, the technique to measure GPF (measurements and software) was implemented for X-Ray diffraction and applied to study of property anisotropy of a Fe50%Ni alloy. GPF’s of texture, crystallite size, stored energy and diffraction peak shift, (among others) have been characterized. The Full Width at Half Maximum (FWHM) of obtained instrumental functions shows that defocusing is significant for polar angle higher than 50°. The mixing parameter of the pseudo-Voigt function using in fitting, presents important dispersions.

Abstract: We study the dislocation and twin substructures in a high manganese twinning-induced-plasticity steel (TWIP) by means of electron channeling contrast imaging. At low strain (true strain below 0.1) the dislocation substructure shows strong orientation dependence. It consists of dislocation cells and planar dislocation arrangements. This dislocation substructure is replaced by a complex dislocation/twin substructure at high strain (true strain of 0.3-0.4). The twin substructure also shows strong orientation dependence. We identify three types of dislocation/twin substructures. Two of these substructures, those which are highly favorable or unfavorable oriented for twinning, exhibit a Schmid behavior. The other twin substructure does not fulfill Schmid’s law.