Precipitation kinetics analysis of oxide dispersion strengthened steels for their application as cladding material in Gen.IV power plants

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Effect of powder metallurgy synthesis parameters for pure aluminium on resultant mechanical properties

In this work, pure aluminium powders of different average particle size were compacted, sintered into discs and tested for mechanical strength at different strain rates. The effects of average particle size (15, 19, and 35 μm), sintering rate (5 and 20 °C/min) and sample indentation test speed (0.5, 0.7, and 1.0 mm/min) were examined. A compaction pressure of 332 MPa with a holding time of six minutes was used to produce the green compacted discs. The consolidated green specimens were sintered with a holding time of 4 h, a temperature of 600 °C in an argon atmosphere. The resulting sintered samples contained higher than 85% density. The mechanical properties and microstructure were characterized using indentation strength measurement tests and SEM analysis respectively. After sintering, the aluminium grain structure was observed to be of uniform size within the fractured samples. The indentation test measurements showed that for the same sintering rate, the 35 μm powder particle size provided the highest radial and tangential strength while the 15 μm powder provided the lowest strengths. Another important finding from this work was the increase in sintered sample strength which was achieved using the lower sinter heating rate, 5 °C/min. This resulted in a tangential stress value of 365 MPa which was significantly higher than achieved, 244 MPa, using the faster sintering heating rate, 20 °C/min

An original phylogenetic approach identified mitochondrial haplogroup T1a1 as inversely associated with breast cancer risk in BRCA2 mutation carriers

Abstract Introduction Individuals carrying pathogenic mutations in the BRCA1 and BRCA2 genes have a high lifetime risk of breast cancer. BRCA1 and BRCA2 are involved in DNA double-strand break repair, DNA alterations that can be caused by exposure to reactive oxygen species, a main source of which are mitochondria. Mitochondrial genome variations affect electron transport chain efficiency and reactive oxygen species production. Individuals with different mitochondrial haplogroups differ in their metabolism and sensitivity to oxidative stress. Variability in mitochondrial genetic background can alter reactive oxygen species production, leading to cancer risk. In the present study, we tested the hypothesis that mitochondrial haplogroups modify breast cancer risk in BRCA1/2 mutation carriers. Methods We genotyped 22,214 (11,421 affected, 10,793 unaffected) mutation carriers belonging to the Consortium of Investigators of Modifiers of BRCA1/2 for 129 mitochondrial polymorphisms using the iCOGS array. Haplogroup inference and association detection were performed using a phylogenetic approach. ALTree was applied to explore the reference mitochondrial evolutionary tree and detect subclades enriched in affected or unaffected individuals. Results We discovered that subclade T1a1 was depleted in affected BRCA2 mutation carriers compared with the rest of clade T (hazard ratio (HR) = 0.55; 95% confidence interval (CI), 0.34 to 0.88; P = 0.01). Compared with the most frequent haplogroup in the general population (that is, H and T clades), the T1a1 haplogroup has a HR of 0.62 (95% CI, 0.40 to 0.95; P = 0.03). We also identified three potential susceptibility loci, including G13708A/rs28359178, which has demonstrated an inverse association with familial breast cancer risk. Conclusions This study illustrates how original approaches such as the phylogeny-based method we used can empower classical molecular epidemiological studies aimed at identifying association or risk modification effects

Texture evolution analysis in oxide dispersion strengthened ferritic steel transformed by a tube pilgering process

International audienceFerritic Oxide Dispersion Strengthened (ODS) steels are investigated as potential materials for cladding tubes of advanced Sodium Fast Reactors. Pilgering process is used to cold-form the ODS seamless tubes with a large cross-section reduction in a double processing step. To estimate the influence of process parameters on the material, a microstructure analysis is carried out by Electron Backscattering Diffraction (EBSD) and X-ray Diffraction (XRD) methods. These data were used to obtain information about crystallographic texture, misorientation distributions, grain size, low and high angle boundary fractions and Schmid factor. Pilgered samples show a high texture formation with the well-known $\alpha$-fiber preferential orientation along the rolling direction. During the heat treatment process, grain morphology is restored from elongated grains to the almost equiaxed ones, while the texture presents unexpected increasing of fiber intensity. The remarkable temperature stability of this fiber is assumed to be linked to the texture memory of crystallographic structure during severe cold rolling process. The special attention was paid to the slip activation mechanism during plastic deformation applied by pilgering process. To predict this specific texture formation, modeling investigation of tube deformation process is performed using a polycrystalline self-consistent visco-plastic (VPSC) code. Pilgering forming is modeled by a sequence of deformation path considering the initial grain orientation sets obtained from EBSD maps results. It assumes a simple cyclic loading paths estimating the experimental loading associated to the pilgering process. The effect of initial and induced texture on the mechanical hardening is estimated and a set of hardening parameters is identified. Finally, the results of the traction tests along the rolling and transverse directions are compared to the numerical simulations

When do oxide precipitates form during consolidation of oxide dispersion strengthened steels?

International audienceThe processing of oxide dispersion strengthened (ODS) steels involves ball milling, where the oxide forming species are driven in solid solution. Precipitation of the nanometre-scale oxides occurs during subsequent annealing and consolidation. This paper reports in-situ Small-Angle X-ray Scattering measurements of the formation of these precipitates during heating of cold-compressed as-milled powders. Clusters are already initially present, and precipitation starts at 300 degrees C. The maximum precipitate density is achieved at 600 degrees C, followed by very slow coarsening at higher temperature. These results open the way to understand the coupled evolution of precipitation and crystalline defects during heating and consolidation of ODS steels. (C) 2016 Elsevier B.V. All rights reserved

About Elastoplastic Nonlocal Formulations with Damage Gradients

International audienceFEM results of softening materials are well known to show pathological mesh dependency. The main goal of this work is to revisit and propose efficient nonlocal damage gradient enhanced formulations able to avoid mesh dependency in the context of elastoplastic damage models with destination to industrial applications. This formulation is presented and studied for simple tension tests, with various spatial discretizations. Numerical aspects and implementation in ABAQUS-standard environment are discussed. The structure of the nonlocal element needed for those formulations is presented. For a given set of meshes, the ability of the proposed formulation to control the size of the necking zone is studied. In the same time the independence of the global dissipation to the mesh size is checked. Theoretical and practical limits of the proposed approach are highlighted

Competition between intragranular and intergranular deformation mechanisms in ODS ferritic steels during hot deformation at high strain rate

Oxide Dispersed Strengthened (ODS) ferritic stainless steels present well-known fine grains microstructures where dislocation movement is hindered by a dense precipitation of nano-oxides particles. Previous research, on the thermomechanical behavior at high temperature and strain rates, was focused on torsion tests (Karch in J Nucl Mater 459:53–61, 2014). Considering texture evolution and grain shape as indicators of the intragranular dislocation glide activity, it was shown that, for high temperature and strain rate, intragranular deformation was in competition with intergranular accommodation. The latter phenomenon was related to early damaging at grain boundaries. The occurrence of a transition phenomenon from an intragranular to an intergranular deformation mechanism, with increasing temperature, was recently confirmed by neutron diffraction spectroscopy (Stoica in Nature Commun 5:5178, 2014). In the present paper, hot extrusion (HE) tests are performed, avoiding damage due to the high stress triaxiality, and allowing further investigation of intragranular and intergranular plasticity at large strains. Three ferritic steels exhibiting various precipitation size anddensitywere hot extruded.Microstructure evolution at different stages of deformation is investigated using the Electron Back-Scattered Diffraction (EBSD) technique. After extrusion at 1373 K (1100°C), the microstructure of ODS steels consists of a mixture of small round shape grains and larger elongated grains containing low-angle grain boundaries. Texture measurements show the appearance of the a-fiber (\110[//extrusion direction) and an increase in its intensity during the extrusion process in the larger grains. The fragmentation of the large elongated grains by Continuous Dynamic Recrystallization (CDRX) partially occurs in ODS materials depending on precipitation reinforcement. For smaller grains, plastic deformation has no effect on crystallographic orientation and grain shape, indicating a grain boundary accommodation phenomenon as the major deformation mechanism. Precipitation density not only impacts the intragranular dislocation glide activity, but also reduces CDRX kinetics in coarse grains