Interface roughening in irradiated oxide dispersion strengthened steels

International audienc

Orientation relationships and interface structure of alpha '-Cr nanoclusters embedded in alpha-Fe matrix after alpha-alpha ' demixing in neutron irradiated Oxide Dispersion Strengthened material

A nanoscale characterization of neutron irradiated Oxide Dispersion Strengthened (ODS) alloys has been used to reveal the α-α′ demixing and the population of (Y, Ti, O) nanoparticles. Multivariate statistical analysis (MSA) processed energy filtered transmission electron microscopy (EFTEM) efficiently reveals the location of all nanophases. The α-α′ demixing did not only lead to the systematic formation of cube-on-cube semi-coherent Cr rich clusters but also to misfitting misoriented semi-coherent particles satisfying the orientation relationship [100] Cr//[111] Fe and [100] Cr//[112] Fe. The misorientation angle between the semi-coherent directions < 110 > Fe and < 110 > Cr was found to be 17°. © 2012 Elsevier B.V. All rights reserved

Characterization of microstrutural evolution of ODS alloys after aging treatment at 450DC and 500DC

International audienceOxide Dispersion Strengthened (ODS) steels envisaged as cladding materials in Generation IV fission nuclear reactors due to their good radiation resistance and improved high-temperature mechanical properties through the nano-oxides addition compared to conventional ferritic/martensitic steels. Cr-rich ferritic ODS alloys are in particular excellent candidates due to their high corrosion resistance, besides their excellent mechanical properties and creep resistance. However, these alloys, whose service temperature is in the range of 400-700C, can, face severe embrittlement problems particularly for lower temperatures. This embrittlement is attributed to the decomposition of the matrix into the alpha(Fe-rich) and alpha' (Cr-rich) phases. The effect of phase separation causes an increase in hardness and yield strength and reduction of the tensile elongation. Despite the significant body of research on microstructure and mechanical properties of ODS, there is still a significant lack of knowledge on this phenomenon in these alloys. In this work, Atome Probe Tomography (APT), Energy-Filtered Transmission Electron Microscopy (EFTEM) and thermoelectric power (TEP) were used to study the phenomenon of alpha' separation after aging at 450C and 500C for 5000 and 10000h hours in different Fe-18Cr and Fe-14Cr reinforced and non-reinforced steels, in order to study the effect of the Cr content, the duration and temperature on the kinetics of alpha alpha'-phase precipitation and differences between grades were indeed noticed. Moreover, we also study the behaviour of grain boundary after these heat treatments; we examine the relationships between the misorientation, the segregation and the grain boundary migration

Nano-cluster stability following neutron irradiation in MA957 oxide dispersion strengthened material

ODS steels are promising materials for Sodium cooled Fast Reactors since their fine distribution of nano-clusters confers excellent mechanical properties. However, the nano-feature stability needs to be assessed under neutron irradiation. Before irradiation, the characterizations show that nano-particles are finely distributed within the ferritic matrix and are identified to have a pyrochlore type structure. After irradiation of the MA957 alloy in the Phenix French reactor at 412 C up to 50 dpa and 430 C up to 75 dpa, transmission electron microscopy characterization reveals a very slight density fall but no distinguishable difference in nano-features size before and after irradiation. In addition, after both irradiations, the nano-oxides are still (Y, Ti, O) compounds with orientation relationship with the matrix. A multislice simulation of high resolution images suggests that nano-particles still have a fcc pyrochlore type structure after irradiation. A possible change of lattice parameter seems to be highlighted, possibly due to disordering by cascade effect. © 2013 Elsevier B.V. All rights reserved

Shape, orientation relationships and interface structure of beta-Nb nano-particles in neutron irradiated zirconium alloy

International audienceUnder neutron irradiation, radiation enhanced beta-Nb nano-precipitates develop within the α-grains of the Zr-Nb alloys. This radiation enhanced precipitation is of great interest since it may have some influence on the post-irradiation mechanical behavior of the material. In this paper the shape, the orientation relationship and the interface structure of such nano-particles are studied by means of both conventional and high-resolution transmission electron microscopy. The radiation damage was annealed out, thanks to a prior heat treatment and a creep test, in order to easily observe the beta-Nb nano-particles. The nano-particles exhibit a needle-like shape with a short thickness along the c-direction of 1.5 nm on average, a length and a width respectively of 6 nm and 3 nm, on average, in the basal plane. Using high-resolution TEM, a near Pitsch-Schrader orientation relationship is identified for a nano-particle. The interface atomic structure at various locations around the nano-particle has been accurately determined and an atomic model of the interface structure has been proposed

Amorphization of oxides in ODS steels/materials by electronic stopping power

International audienc

Atomic-scale Interface structure of a Cr-coated Zircaloy-4 material

International audienceHighly adherent, thin Cr coatings on Zr-based nuclear fuel claddings can be potentially used for the development of accident-tolerant fuels in light water reactors. To guarantee the successful implementation of Cr-coated Zr alloys as cladding tubes in nuclear power plants, the adhesive strength of the Cr coatings must be assessed. The interface between Cr and Zr was characterized via high-resolution transmission electron microscopy. We observed the formation of nanometer-thick Zr(Fe, Cr)2 poly-type, structured Laves phases at the interfacial region that display both C14 and C15 lattice symmetries. Although the crystallinity was preserved throughout the interfacial region, different atomic configurations were observed for all the interfaces studied. In most cases, coherent or semicoherent crystallographic relationships were observed, ensuring the adhesive strength of the coating

Comportement en conditions "extrêmes" d'un gainage de combustible nucléaire base zirconium revêtu de chrome

National audienc

Relaxation path of metastable nanoclusters in oxide dispersion strengthened materials

International audienceODS steels are a promising class of structural materials for sodium cooled fast reactor application. The ultra-high density of the strengthening nanoclusters dispersed within the ferritic matrix is responsible of the excellent creep properties of the alloy. Fine characterization of the nanoclusters has been conducted on a Fe-14Cr-0.3Ti-0.3Y2O3 ODS material using High Resolution and Energy Filtered Transmission Electron Microscopy. The nanoclusters exhibit a cubic symmetry possibly identified as f.c.c and display a non-equilibrium YTiCrO chemical composition thought to be stabilized by a vacancy supersaturation. These nanoclusters undergo relaxation towards the Y2Ti2O7-like state as they grow. A Cr shell is observed around the relaxed nano-oxides, this size-dependent shell may form after the release of Cr by the particles. The relaxation energy barrier appears to be higher for the smaller particles probably owing to a volume/surface ratio effect in reason to the full coherency of the nanocluster

Radiation-induced cavities in aluminium alloy imaged by in line electron holography

International audienceIn irradiated material, cavities result from the condensation of vacancies induced by collision cascades. The study of their formation is a relevant topic since a high density of cavities may alter significantly the material performance. In this work, a simplified version of in line holography was successfully applied for imaging cavities in ion-irradiated 6061 aluminium alloy. In transmission electron microscopy, the incoming electrons experience a phase shift owing to the potential variation induced by the cavities. The retrieval of this phase shift provides a convenient map to observe and highlight the cavities. Information on density of cavities can be easily obtained. In addition, interstitial clusters may also be detected