243 research outputs found
Magnetically affected texture and microstructure evolution during grain growth in zirconium
International audienceThe effect of a magnetic field on texture and microstructure development in cold rolled (80%) commercially pure zirconium (Zr701) was investigated. X-ray diffraction and EBSD measurements were utilized for the texture and microstructure characterization. The results revealed that a magnetic field promotes grain growth in the investigated material. During annealings at 550°C this is particularly apparent from the faster development of specific (0/180, 35, 30) texture components and the bigger mean grain size after magnetic annealing. The magnetic annealing at 700°C resulted in an asymmetry of the two major texture components. This is due to a magnetic driving force for grain growth arising from the anisotropic magnetic susceptibility of zirconium. During annealing at 700°C the abnormal grain growth occurred. This behavior is attributed to the higher mobility of grain boundaries between grains misoriented by 30° around [000. The magnetic field essentially enhanced the observed abnormal grain growth
Fast in-situ annealing stage coupled with EBSD: A suitable tool to observe quick recrystallization mechanisms
International audienceA heating stage has been developed to perform in-situ annealing in a SEM equipped with an EBSD system in order to study recrystallization mechanisms. High temperature treatments could then be performed inside the SEM, up to 1180 °C and with high heating and cooling rates (~ 100 °C s− 1). Samples were cooled down to room temperature to perform EBSD orientation mapping in between successive short-duration heat treatments. Microstructure evolution snapshots obtained this way allow gaining an insight into recrystallization mechanisms. The interest of such experiments is shown for two examples: static recrystallization of cold deformed pure tantalum and post‐dynamic evolution of hot-deformed Zircaloy4
Evolution of microstructure and twin density during thermomechanical processing in a γ-γ' nickel-based superalloy
International audienceMicrostructure evolution has been studied in the nickel-based superalloy PER®72 subjected to hot torsion, to annealing below the primary γ' solvus temperature and to annealing at a supersolvus temperature, with a special emphasis on grain size and twin content. Dynamic abnormal grain growth occurs before the onset of dynamic recrystallization. The resulting bimodal grain size distribution affects the grain-coarsening kinetics at the supersolvus temperature, so that the final microstructures depend on the former straining stages. As a consequence, the twin content does not follow a univocal relationship with the average grain size. The grain boundary velocity history before reaching the final grain size is a contributing factor, and this is notably related to the initial grain size distribution width. Dynamically recrystallized microstructures are by nature more homogeneous and thus give rise to lower rates in supersolvus grain coarsening, and accordingly lead to relatively lower twin densities
Comparison of grain growth mean-field models regarding predicted grain size distributions
Mean-field models have the ability to predict grain size distribution
evolution occurring through thermomechanical solicitations. This article
focuses on a comparison of mean-field models under grain growth conditions.
Different microstructure representations are considered and discussed,
especially regarding the consideration of topology in the neighborhood
construction. Experimental data obtained with a heat treatment campaign on a
316L austenitic stainless steel are used for material parameters identification
and as a reference for model comparisons. Mean-field models are also confronted
to both mono- and bimodal initial grain size distributions to investigate the
interest of introducing neighborhood topology in microstructure predictions
models. This article exposes that improvements in the predictions are obtained
in monomodal cases for topological models. In bimodal test, no comparison with
experimental data were performed as no data were available. But relative
comparisons between models indicate few differences in predictions. The
interest of neighborhood topology in grain growth mean-field models gives
overall small improvements compared to classical mean-field models when
comparing implementation complexity
About quantitative EBSD analysis of deformation and recovery substructures in pure Tantalum
International audienceThe aim of this work is to present a quantitative analysis of features involved in recovery during annealing of deformed Tantalum. In pure metals where crystalline defects usually have high mobility, dislocation annihilation and rearrangement occur to a great extent prior to recrystallization. Therefore a complete understanding of recrystallization cannot be accomplished without an advanced knowledge of the recovery phenomenon. Depending on whether dislocations induce a measurable curvature in the crystal lattice or not, they are called Geometrically Necessary Dislocations (GNDs) or Statistically Stored Dislocations (SSDs) respectively. In the present work only GNDs are considered. For this purpose electron backscatter diffraction (EBSD) is an advantageous technique to obtain statistically representative results when compared to Transmission Electron Microscopy (TEM). However, a quantitative analysis of GNDs from EBSD data is not straightforward. Since local misorientations are induced by the curvature of the crystal lattice caused by GNDs, GNDs analysis can be done using local misorientations. However the values obtained from this analysis are step size dependent and influenced by the measurement noise. Reasoning on the basis that when the step size tends to zero, local misorientation should also tend to zero, measurement noise can be estimated [1]. The measurement noise appears to notably be very much dependent on the amplitude of local misorientations, which must be considered in the perspective of GND density calculatio
Level-Set modeling of grain growth in 316L stainless steel under different assumptions regarding grain boundary properties
Two finite element level-set (FE-LS) formulations are compared for the
modeling of grain growth of 316L stainless steel in terms of grain size, mean
values and histograms. Two kinds of microstructures are considered, some are
generated statistically from EBSD maps and the others are generated by
immersion of EBSD data in the FE formulation. Grain boundary (GB) mobility is
heterogeneously defined as a function of the GB disorientation. On the other
hand, GB energy is considered as heterogeneous or anisotropic, respectively
defined as a function of the disorientation and both the GB misorientation and
the GB inclination. In terms of mean grain size value and grain size
distribution (GSD), both formulations provide similar responses. However, the
anisotropic formulation better respects the experimental disorientation
distribution function (DDF) and predicts more realistic grain morphologies. It
was also found that the heterogeneous GB mobility described with a sigmoidal
function only affects the DDF and the morphology of grains. Thus, a slower
evolution of twin boundaries (TBs) is perceived
Strain induced abnormal grain growth in nickel base superalloys
International audienceUnder certain circumstances abnormal grain growth occurs in Nickel base superalloys during thermomechanical forming. Second phase particles are involved in the phenomenon, since they obviously do not hinder the motion of some boundaries, but the key parameter is here the stored energy difference between adjacent grains. It induces an additional driving force for grain boundary migration that may be large enough to overcome the Zener pinning pressure. In addition, the abnormal grains have a high density of twins, which is likely due to the increased growth rate
Déconvolution des spectres de perte d'énergie des électrons
Cet article traite de la déconvolution des spectres de perte d'énergie des électrons (EELS) qui sont utilisés pour la caractérisation des matériaux. On présente en premier lieu le modèle physique des spectres EELS, puis les méthodes de déconvolution utilisées. Des extensions des méthodes d'Egerton [2] sont notamment proposées. Une comparaison des différentes méthodes est effectuée sur des spectres EELS issus d'échantillons de nickel d'épaisseurs différentes
EBSD coupled to SEM in situ annealing for assessing recrystallization and grain growth mechanisms in pure tantalum
International audienceAn in situ annealing stage has been developed in-house and integrated in the chamber of a Scanning Electron Microscope equipped with an Electron BackScattered Diffraction system. Based on the Joule effect, this device can reach the temperature of 1200°C at heating rates up to 100°C/s, avoiding microstructural evolutions during heating. A high-purity tantalum deformed sample has been annealed at variable temperature in the range 750°C-1030°C, and classical mechanisms of microstructural evolutions such as recrystallization and grain coarsening phenomena have been observed. Quantitative measurements of grain growth rates provide an estimate of the mean grain boundary mobility, which is consistent with the value estimated from physical parameters reported for that material. In situ annealing therefore appears to be suited for complementing bulk measurements at relatively high temperatures, in the context of recrystallization and grain growth in such a single-phase material
Effect of grain orientation on the development of dislocation substructures during cold-deformation of pure Tantalum. Link with static recrystallization and recovery.
International audienceRecrystallization and recovery of pure polycrystalline tantalum are highly influenced by the intragranular dislocation structure developed during plastic deformation. A tantalum oligocrystal has been deformed by compression at room temperature. The resulting intragranular dislocation structures have been characterized using a FEG scanning electron microscope coupled with an EBSD system. Based on these experimental observations and crystal plasticity simulations, the development of dislocation substructures is related to the crystallographic stability of grain orientations
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