Linear chemically sensitive electron tomography using DualEELS and dictionary-based compressed sensing

We have investigated the use of DualEELS in elementally sensitive tilt series tomography in the scanning transmission electron microscope. A procedure is implemented using deconvolution to remove the effects of multiple scattering, followed by normalisation by the zero loss peak intensity. This is performed to produce a signal that is linearly dependent on the projected density of the element in each pixel. This method is compared with one that does not include deconvolution (although normalisation by the zero loss peak intensity is still performed). Additionaly, we compare the 3D reconstruction using a new compressed sensing algorithm, DLET, with the well-established SIRT algorithm. VC precipitates, which are extracted from a steel on a carbon replica, are used in this study. It is found that the use of this linear signal results in a very even density throughout the precipitates. However, when deconvolution is omitted, a slight density reduction is observed in the cores of the precipitates (a so-called cupping artefact). Additionally, it is clearly demonstrated that the 3D morphology is much better reproduced using the DLET algorithm, with very little elongation in the missing wedge direction. It is therefore concluded that reliable elementally sensitive tilt tomography using EELS requires the appropriate use of DualEELS together with a suitable reconstruction algorithm, such as the compressed sensing based reconstruction algorithm used here, to make the best use of the limited data volume and signal to noise inherent in core-loss EELS

Evolution of microstructure and mechanical properties of medium Mn steels during double annealing

A double annealing process was applied to cold rolled medium Mn steel. The evolution of both microstructure and mechanical properties during the second annealing were analysed. Austenite reverted transformation (ART) was observed during intercritical annealing. It was shown that a complex ultra-fine microstructure composed of three phases (retained austenite/martensite/ferrite) was formed and two types of morphologies were detected (lath-like and polygonal). Furthermore, a high volume fraction of retained austenite (22%), which was stabilized at room temperature, was the origin of a TRIP effect. A good balance between strength and ductility can be achieved by optimizing the heat treatment. The various results are discussed and some mechanisms are proposed to explain the observations

Evolution of microstructure and mechanical properties of medium Mn steels during double annealing

A double annealing process was applied to cold rolled medium Mn steel. The evolution of both microstructure and mechanical properties during the second annealing were analysed. Austenite reverted transformation (ART) was observed during intercritical annealing. It was shown that a complex ultra-fine microstructure composed of three phases (retained austenite/martensite/ferrite) was formed and two types of morphologies were detected (lath-like and polygonal). Furthermore, a high volume fraction of retained austenite (22%), which was stabilized at room temperature, was the origin of a TRIP effect. A good balance between strength and ductility can be achieved by optimizing the heat treatment. The various results are discussed and some mechanisms are proposed to explain the observations

Selective oxidation of Fe-Al (8wt.%) binary alloy during continuous annealing

International audienceAluminum is one of the alloying elements whose proportion in advanced high-strength steels (AHSS) can be high. This study focuses on the reaction mechanisms occurring during continuous annealing of AHSS, with emphasis on the behavior of aluminum. To provide a better understanding of these mechanisms, we used a binary Fe-Al (8 wt.%) model alloy.The samples were annealed in a laboratory furnace with a temperature profile relevant to the galvanizing line practice. Samples were heated to 800°C and held at this temperature for 60 s before being cooled to room temperature. The rapid cooling of the samples, that occurs when the annealing furnace is stopped, acts as a quench for reactions. The gas atmosphere consisted of a mixture of N2 – 5 vol.% H2 with a dew point of 0 °C.With the chosen annealing conditions, native iron oxides are reduced, selective internal and external oxidation of aluminum occurs, and metallic iron particles are formed on the alloy surface. The annealed samples were characterized using several complementary analysis techniques. The surface of annealed samples was observed in a Field Emission Gun Scanning Electron Microscope to obtain high resolution images. Thin cross-sections were extracted from the oxidized samples using a Focused Ion Beam Microscope and characterized in a Transmission Electron Microscope. X-ray Photoelectron Spectroscopy was used to analyse the extreme surface of the samples.An external oxide layer was found between the iron particles. Internal oxidation is visible on cross-sections. The composition, morphology and depth of both internal and external oxidations was studied. Elementary mechanisms of the oxidation reaction are proposed and discussed

Further Insight into Interfacial Interactions in Iron/Liquid Zn-Al System

International audienceIn the galvannealing process, steel strips are immersed in molten zinc containing 0.100 to 0.135 wt pct Al at 450°C. The coating obtained is composed of a thin intermetallic compounds' layer called the inhibition layer (200 nm) covered with a thick zinc layer (10 lm). The nature of this inhibition layer has been investigated here for a galvanizing bath with a low Al content. The inhibition layer formed on industrial low-alloyed steels was characterized by transmission electron microscopy and atom probe tomography. The inhibition layer is composed of a thin Fe 2 Al 5 Zn x layer (20 nm), covered with a thicker d layer (200 nm). The Fe 2 Al 5 Zn x layer is discontinuous at the lowest bath Al content. Small precipitates (20 nm in diameter) with a stoichiometry corresponding to Fe 3 Al-containing Zn were also found for the first time in the d phase. The microstructure of the inhibition layer can be described with diffusion paths drawn in the Al-Fe-Zn ternary section at 450°C. This means that all interfaces of the inhibition layer are at thermodynamic equilibrium. The Fe 2 Al 5 Zn x layer is formed on the steel surface before the d layer. The nucleation and growth of the Fe 3 Al-Zn particles probably occur in the liquid metal at the same time as d

Refonder le rapport aux personnes. "Merci de ne plus nous appeler usagers"

Rapport à la ministre des affaires sociales et de la santé Marisol Touraine et la ministre déléguée aux personnes handicapées et à la lutte contre l'exclusion Marie-Arlette Carlott