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Because of its great importance in modern engineering and technology applications, steel continues to be highly relevant in the modern research field of nanocrystalline materials. Innovative processing methods and procedures are required for the production of such materials, which possess superior properties compared to their conventional counter parts. In this research, the original microstructure of a commercial C45 steel (Fe, 0.42–0.5 wt% C, 0.5–0.8 wt% Mn) was modified from ferritic–pearlitic to bainitic. Warm high pressure torsion for 5 rotations at 6 GPa and 350 °C was used to process the bainitic sample leading to an ultrafine/nano-scale grain size. A unique nano-crystalline microstructure consisting of equiaxed and elongated ferrite grains with a mean size smaller than 150 nm appeared in images taken by Transmission Electron Microscopy. Results of in-situ tensile testing in a scanning electron microscope showed very high tensile strength, on the order of 2100 MPa with a total elongation of 4.5% in comparison with 800 MPa and around 16% in the original state. Fracture occurred abruptly, without any sign of necking, and was typically caused by the stress concentration at a surface flaw. Also, stress concentrations near all surface defects were observed on the sample, visualized by the formation of shear bands. The fracture surface was covered with dimples, indicating ductile fracture. These properties are fully comparable with high strength, high alloyed steels.

One of the primary aims of the actinide community within nanoscience is to develop a good understanding similar to what is currently the case for stable elements. As a consequence, efficient, reliable and versatile synthesis techniques dedicated to the formation of new actinide-based nano-objects (e.g. nanocrystals) are necessary. Hence, a “library” dedicated to the preparation of various actinidebased nanoscale building blocks is currently being developed. Nanoscale building blocks with tunable sizes, shapes and compositions are of prime importance. So far, the non-aqueous synthesis method in highly coordinating organic media is the only approach which has demonstrated the capability to provide size and shape control of actinide-based nanocrystals (both for thorium and uranium, and recently extended to neptunium and plutonium). In this paper, we demonstrate that the non-aqueous approach is also well adapted to control the chemical composition of the nanocrystals obtained when mixing two different actinides. Indeed, the controlled hot co-injection of thorium acetylacetonate and uranyl acetate (together with additional capping agents) into benzyl ether can be used to synthesize thorium/uranium mixed oxide nanocrystals covering the full compositional spectrum. Additionally, we found that both size and shape are modified as a function of the thorium:uranium ratio. Finally, the magnetic properties of the different thorium/uranium mixed oxide nanocrystals were investigated. Contrary to several reports, we did not observe any ferromagnetic behavior. As a consequence, ferromagnetism cannot be described as a universal feature of nanocrystals of non-magnetic oxides as recently claimed in the literature.

Using automated crystal-orientation mapping (ACOM) in TEM, we have shown the formation of an ultrafine grained (UFG) structure with predominantly (>90%) high angle grain boundaries in C45 steel and Armco iron after high pressure torsion (HPT) at 350 °C. The UFG C45 steel exhibits a combination of high tensile strength and ductility and its mechanical properties were improved significantly compared to the analogously processed Armco iron. The higher strength of the processed C45 steel is attributed to the finer grain size, the highly fragmented cementite particles induced by the HPT deformation. The difference in ductility of the two materials is related to the distinct work hardening behaviors due to the different microstructural characteristics.

International audience; Recrystallization of a ball-milled ferritic ODS steel is studied towards its evolution from as-milled powder to consolidated state. This characterization has been made possible by using a combination of X-ray Diffraction (XRD) and an innovative method based on an Automated Crystallographic Orientation Mapping (ACOM) tool attached to a Transmission Electron Microscope (TEM). Focus Ion Beam preparation has been essential to obtain a thin section of the ODS steel powder particle and perform the ACOM-TEM study. Relevant temperatures regarding recovery and recrystallization during the heat treatment had first been identified with XRD profile analysis. Selected states were further characterized using ACOM-TEM that provides key information on microstructure, i.e. grain size and morphology, crystallite size, local texture and distortion. ACOM-TEM cartographies have revealed for the first time that the microstructure of as-milled ODS ferritic steel particles consists in very anisotropic grains containing undistorted domains and dislocation walls. This is in agreement with the nanosized crystallites measured by XRD results. The mutual benefits of XRD and ACOM-TEM methods to analyse and describe the microstructure are discussed as well as the reliability of dislocation density measurements provided by ACOM-TEM misorientation measurements. In addition, of the ACOM-TEM results, the microstructural evolution during the processing route is interpreted in terms of a competition between recovery, recrystallization, grain growth and precipitation.

Since 1991, in France, studies on the conditioning of iodine were carried out to assess the potential of several specific inorganic host matrices. The apatite family has been mainly studied because of its good chemical durability and its ability to confine iodine over geological time scales. A lead-bearing apatite, Pb10(VO4)4.8(PO4)1.2I2, and a calcium-bearing apatite, Ca10(PO4)6(OH)2-x(IO3)x, were selected on the basis of their incorporation rate (between 7 and 10 wt.%) and a satisfactory resistance to leaching (V0(50 °C, pure water) ∼ 10-2 g.m-2.d-1; Vr(50 °C, pure water) < 10-4 g.m-2.d-1). However, with such materials, the removal of open porosity requires non conventional sintering techniques like spark plasma sintering to decrease the surface exposed to water. This is why, in parallel, other matrices, like silver phosphate glasses, have also been investigated. To improve the chemical durability and thermal properties of these glasses, cross-linking reagents were added to their formulation.

Niobium is a strong carbide forming element which is often used in microalloyed steels to control the grain size during thermomechanical treatments and to provide strengthening through precipitation processes. A detailed microscopic investigation is one of the keys for understanding the first stages of the precipitation sequence, thus Transmission Electron Microscopy (TEM) is required. The main difficulty of TEM studies is due to the nanometre scale dimensions of the particles, which makes their detection, structural and chemical characterization delicate. Model Fe- (Nb0.06%,C0.05%) and Fe-(Nb0.05%,C0.03%,N0.03%) ferritic alloys subjected to isothermal annealing treatments have been investigated. High Resolution TEM (HRTEM) and conventional TEM (CTEM) were used to characterise the morphology, nature and location of precipitates. Volume fraction measurements and a statistical approach to the determination of precipitate size histograms have been investigated using Energy Filtered TEM (EFTEM) and High Angle Annular Dark Field (HAADF) imaging. Chemical compositions were quantified by Electron Energy Loss Spectroscopy (EELS). The evolution of precipitate composition with time and temperature is compared with previous simulations obtained from new thermodynamic models based on equilibrium boundary conditions.

The behaviour from 1500 K up to melting of nanocrystalline (nc) thorium dioxide, the refractory binary oxide with the highest melting point (3651 K), was explored here for the first time using fast laser heating, multi-wavelength pyrometry and Raman spectroscopy for the analysis of samples quenched to room temperature. Nc-ThO2 was melted at temperatures hundreds of K below the melting temperature assessed for bulk thorium dioxide. A particular behaviour has been observed in the formed liquid and its co-existence with a partially restructured solid, possibly due to the metastable nature of the liquid itself. Raman spectroscopy was used to characterize the thermal-induced structural evolution of nc-ThO2. Assessment of a semi-empirical relation between the Raman active T2g mode peak characteristics (peak width and frequency) and crystallites size provided a powerful, fast and non-destructive tool to determine local crystallites growth within the nc-ThO2 samples before and after melting. This semi-quantitative analysis, partly based on a phonon-confinement model, constitutes an advantageous, more flexible, complementary approach to electron microscopy and powder x-ray diffraction (PXRD) for the crystallite size determination. The adopted experimental approach (laser heating coupled with Raman spectroscopy) is therefore proven to be a promising methodology for the high temperature investigation of nanostructured refractory oxides.

cited By 3; International audience; The in situ sintering of a powder of Pb3(VO4) 1.6(PO4)0.4 composition was performed in an Environmental Scanning Electron Microscope. The electric current induced by the electron beam was found to reduce the effective temperature of sintering as well as to accelerate the kinetics of shrinkage of a cluster composed of sub-micrometric grains of material. The presence of the residual current flow in the cluster during observation for in situ experiments helps to reduce the apparent sintering temperatures from 50 to 150 °C compared to conventional heating conditions without current. © 2011 Elsevier B.V. All rights reserved.

International audience; High Resolution Transmission Electron Microscope and Electron Energy Loss Spectroscopy and have been used to characterize the structure and chemical composition of niobium carbonitridcs in the ferrite of a Fe-Nb-C-N model alloy at different precipitation stages. Experiments seem to indicate the coexistence of two types of precipitates: pure niobium nitrides and mixed sub-stoichiometric niobium carbonitrides. In order to predict the chemical composition of these precipitates, a thermodynamical formalism has been developed to evaluate (i) the nucleation and growth rates (classical nucleation theory) and (ii) the chemical composition of nuclei and existing precipitates. A model based on the numerical resolution of former equations, is used to compute precipitates size distribution evolution at a given temperature. The predicted compositions are in very good agreement with experimental results.

Contributions of different strengthening mechanisms to yield strength of carbon steels C 10 (0.1 % C) and C 45 (0.45 % C) with ultrafine-grained microstructures have been analyzed in this work based on the precision investigation of the microstructure by electron microscopy, 3D atom probe tomography, and mechanical properties. Estimated values of yield stress showed satisfactory agreement with the experimental results. It is shown that significant contribution to strengthening is made by carbides. In particular, as the carbon content increases in carbon steels, the volume fraction of cementite increases and, accordingly, there is a bigger deviation of the yield strength value from the classical Hall-Petch relationship. The relative contribution of grain boundary strengthening into the yield stress is reduced due to an increased proportion of precipitate strengthening.

Miscible (Al/Nb) and immiscible (Cu/Nb) nanometallic multilayer systems were characterized by means of transmission electron microscopy techniques, primarily by automated crystallographic orientation mapping, which allows for the resolution of crystal structures and orientations at the nanoscale. By using this technique, distinctive Nb orientations in relation to the crystallographic state of the Al and Cu layer structures can be observed. Specifically, the Al and Cu layers were found to consist of amorphous, semi-amorphous, and crystalline regions, which affect the overall multilayer microstructure.