“Angular resolution expected from iCHORD orientation maps through a revisited ion channeling model”

International audienceCrystalline orientation maps are obtained in a Focused Ion Beam (FIB) microscope using the ion CHanneling ORientation Determination (iCHORD) method, which relies on the channeling phenomenon observed in ion-induced secondary electron images. The current paper focuses on the angular resolution that can be expected from such orientation maps, obtained using a revisited ion channeling model. A specific procedure was developed to evaluate the angular resolution, based on the distribution of orientation errors when evaluating controlled sample disorientation. The main advantage is that no external reference is required. An angular resolution of 1° is obtained on a nickel based sample using standard acquisition conditions. This value fulfills most of the needs in terms of microstructural characterization usually carried out by Electron Back Scattered Diffraction

Effect of cold rolling on phase separation in 2202 lean duplex stainless steel

International audienceThe effect of plastic deformation (cold rolling) on the thermal aging of a lean 2202 duplex stainless steel at intermediate temperatures (280 °C-450 °C) was studied using a combination of atom probe tomography and thermoelectric power measurements. In the undeformed condition, spinodal decomposition of ferrite was shown to be the prime aging process with an activation energy of 206 kJ/mol. Plastic deformation enhanced the aging and the austenite partly transformed into martensite. When no precipitation or phase transformation was observed in austenite, both ferrite and martensite decomposed. The spinodal decomposition kinetics of ferrite increased with increasing deformation rate. The FeCr-rich martensite also decomposed at 450 °C, via a nucleation and growth process, owing to a decrease of the Cr content in the matrix. Chromium nitrides, G-Phase, and Cu-rich precipitates were observed in the decomposed martensite

Effect of cold rolling on phase separation in 2202 lean duplex stainless steel

International audienceThe effect of plastic deformation (cold rolling) on the thermal aging of a lean 2202 duplex stainless steel at intermediate temperatures (280 °C-450 °C) was studied using a combination of atom probe tomography and thermoelectric power measurements. In the undeformed condition, spinodal decomposition of ferrite was shown to be the prime aging process with an activation energy of 206 kJ/mol. Plastic deformation enhanced the aging and the austenite partly transformed into martensite. When no precipitation or phase transformation was observed in austenite, both ferrite and martensite decomposed. The spinodal decomposition kinetics of ferrite increased with increasing deformation rate. The FeCr-rich martensite also decomposed at 450 °C, via a nucleation and growth process, owing to a decrease of the Cr content in the matrix. Chromium nitrides, G-Phase, and Cu-rich precipitates were observed in the decomposed martensite

About the automatic measurement of the dislocation density obtained by R-ECCI

International audienceA proof of concept of a new method for automatic characterization of the dislocation density from scanning electron microscopy images is presented. A series of backscattered electron images are acquired while the sample is rotated. For each pixel of the region of interest, the variation of the grey-level intensity as a function of the rotation angle, called the intensity profile, is calculated. This profile can be used to determine the nature of each pixel (dislocation, matrix or noise), such that an automatic dislocation density can be determined within the region of interest. The method is well adapted for dislocation densities ranging from 1012 to 1014 m−2. The simulation of a volume containing dislocations enabled the determination of the maximum and minimum densities attainable as well as the theoretical and experimental measurement errors related to the projection of this volume on a two-dimensional image. The theoretical measurement error due to the projection of dislocation on a surface, is 3% for low dislocation densities (1012m−2) and 20% for higher dislocation densities (1014 m−2). Experimentally, measurement errors are limited by image analysis conditions, which leads to total measurement errors of 15% for 1012m−2 and 34% error for 1014 m−2. These uncertainties were obtained considering a given analyzed depth value, that could not be experimentally verified. This uncertainty on the depth value leads to large errors bars in the final measurement, which can reach an order of magnitude

Fast high-temperature consolidation a novel way to understand the microstructural heterogeneities in nano-reinforced ferritic steels

International audienceOxide-Dispersion Strengthened (ODS) ferritic steels were produced by powder metallurgy using a field-assisted sintering technique called spark plasma sintering (SPS). A multiscale characterization using electron microscopy combined with an in situ synchrotron X-Ray Diffraction allowed to understand the evolution of the heterogeneous microstructure. The influence of oxides precipitation was quantified by nanoscale observations to explain the microstructure heterogeneities in ferritic ODS alloys

Effect of interstitial carbon distribution and nickel substitution on the tetragonality of martensite: A first-principles study

By using first principles calculations, the effects of carbon distribution and of 25 at.% of nickel substitution on the tetragonality of ferrous martensite have been investigated. Different carbon concentrations have been considered (0≤x c ≤ 12.5 at.%). All calculations were based on the Projector Augmented Wave (PAW) method of the Density Functional Theory (DFT). The Special Quasirandom Structure (SQS) method has been used to model the disordered structures of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems. Our calculations demonstrate that the lattice parameters a and c vary linearly with carbon concentration, up to the high value of x c = 12.5 at.% (for X = 1). The nickel substitution enhances the expansion of parameter c of the Fe 16 C 2X system up to 37%. This trend is correlated with the shear moduli calculated for bcc-iron and the disordered Fe25 at.%Ni structure. The mixing energies of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems indicate that the unmixing of martensite by spinodal decomposition is favoured by the nickel substitution. It is shown that carbon-carbon interactions are mostly repulsive until the distance of two bcc-iron lattice parameters.Etude de la décomposition spinodale dans les aciers martensitiques Fe-X-