Maria Giulia Ratti

During the last fifteen years, CEA has acquired much experience in the control of the microstructure and the mechanical properties of ODS alloys for nuclear applications. Each major step of the production process has been studied to get the best compromise for the fabrication route of ODS materials. From this scientific background, two new Fe–13/18CrWTi ferritic ODS alloys have been designed to meet the needs of the fusion or GEN-IV programs. These new materials have been investigated at a semi- industrial scale with different industrial partners and consolidated as small plates. The aim of this paper is to present the recent CEA developments on ODS materials, and to show the first results obtained on the Fe–18Cr1WTi new ferritic ODS alloy. The fabrication route for these new materials is presented, along with the measured mechanical properties and the preliminary microstructure characterizations. These new materials look promising for nuclear applications and are considered by CEA as reference materials for the development of new ODS alloys.

The addition of titanium is well known to refine the precipitation of nano-phases (Y, Ti, O) in oxide dispersion strengthened (ODS) ferritic materials. In order to understand the influence of titanium on the nano-cluster nucleation, two powders of ferritic materials Fe–18Cr–1W were mechanically alloyed with a nano-powder of yttrium oxide Y2O3 in a planetary mill, one without titanium and one with 0.8 wt% of titanium. Several analyses were conducted by small angle neutron scattering (SANS) on powders after different milling conditions and heat treatments. The SANS technique appeared to be extremely useful to follow the precipitation of the nano-phases in ODS ferritic materials. The analysis of these tests indicates that the role played by titanium is clearly observed during the reprecipitation of the nano-oxides though the effect of titanium on the oxide dissolution during the milling is not obvious. Titanium is usually considered to refine the precipitation in ODS materials. This study shows that the nano-phases formed in powders with some titanium are much more resistant to the coarsening than the phases formed without titanium.

Ferritic 14%Cr and 18%Cr ODS steels produced at CEA in round bars or plates were tested mechanically. The present paper reports results obtained in tension, impact, fatigue, creep and toughness tests. These tests were carried out at various temperatures and in different directions. These materials show a pronounced anisotropy at all tested temperatures. No matter the loading, the transversal direction is always found to be far less resistant than the longitudinal one. This anisotropy is mainly observed in terms of damage mechanisms, with intergranular fracture preferentially occurring along the extrusion direction. This intergranular fracture mode leads to very low and anisotropic toughness values and to the absence of tertiairy creep stage, pointing out the unstable nature of fracture, even at high temperature. The unrealistically high values of the Norton exponent measured in creep suggests the existence of a threshold stress, which is consistent with the mainly kinematic nature of the stress as revealed by fatigue tests.

Creep experiments performed at 650 °C and 250 MPa on MA957 and CEA-developed 18%Cr ferritic Oxide Dispersion Strengthened (ODS) steels emphasize the particular creep behavior of ODS alloys. To understand the influence of oxide particles, we focused on the dislocation microstructure and their interaction mechanisms with precipitates. Microstructural characterization were performed using Transmission Electron Microscopy (TEM) on both un-deformed alloys and on 650 °C–250 MPa creep tested MA957 samples. No noticeable differences were observed, and dislocations seem to be anchored by precipitate particles. The dynamic behavior was studied by in situ TEM straining experiments at room temperature on MA957. Observation of dislocation motions indicates that interactions with particles, including pinning, control the flow stress at least at room temperature. At 650 °C, other mechanism is probably predominant.

It is usually accepted that the formation of nano-clusters inside Oxide Dispersion Strengthened (ODS) materials is due to the dissolution of the yttrium oxide during the mechanical alloying and to the precipitation of the nano-oxides during the consolidation process. A study was conducted to follow the dissolution and the re-precipitation of the phases after mechanical alloying and different heat treatments. A Fe–9Cr powder was milled during different milling times with 10 wt.% of yttria and/or titanium in a planetary mill. Using neutron and X-ray diffractions and the Rietveld method, the dissolution and re-precipitation of yttria was investigated. Even with a content of 10 wt.%, the yttria particles disappear during milling to form a structure which can be described as Fe crystallites coexisting with amorphous domains composed of Y and O atoms. After annealing Y and O atoms re-precipitate into the matrix to form yttria crystallites.

Abstract Nano-reinforced steels are considered for future nuclear reactors or for application at high temperature like the heat exchangers tubes or plates. Oxide Dispersion Strengthened (ODS) alloys are the most known of the nano-reinforced alloys. They exhibit high creep strength as well as high resistance to radiation damage. This article deals with the development of new nano reinforced alloys called Nitride Dispersed Strengthened (NDS). Those are also considered for nuclear applications and could exhibit higher ductility with a simplest fabrication way. Two main fabrication routes were studied: the co-milling of Fe–18Cr1W0.008N and TiH2 powders and the plasma nitration at low temperature of a Fe–18Cr1W0.8Ti powder. The materials were studied mainly by Small Angle Neutron Scattering. The feasibility of the reinforcement by nano-nitride particles is demonstrated. The final size of the nitrides can be similar (few nanometers) to the nano-oxides observed in ODS alloys. The mechanical properties of the new NDS show an amazing ductility at high temperature for a nano-reinforced alloy.

supervisors: L. Carminati, S. Resconi ; external referees: M.H. Genest, A. De Roeck ; coordinatore: F. Ragusa