M.H. Mathon

A SANS investigation of the irradiation-enhanced α–α′ phases separation in 7–12 Cr martensitic steels

Abstract Five reduced activation (RA) and four conventional martensitic steels, with chromium contents ranging from 7 to 12 wt%, were investigated by small angle neutron scattering (SANS) under magnetic field after neutron irradiation (0.7–2.9 dpa between 250 and 400 °C). It was shown that when the Cr content of the b.c.c. ferritic matrix is larger than a critical threshold value (∼7.2 at.% at 325 °C), the ferrite separates under neutron irradiation into two isomorphous phases, Fe-rich (α) and Cr-rich (α′). The kinetics of phase separation are much faster than under thermal aging. The quantity of precipitated α′ phase increases with the Cr content, the irradiation dose, and as the irradiation temperature is reduced. The influence of Ta and W added to the RA steels seems negligible. Cold-work pre-treatment increases slightly the coarsening of irradiation-induced precipitates in the 9Cr–1Mo (EM10) steel. In the case of the low Cr content F82H steel irradiated 2.9 dpa at 325 °C, where α′ phase does not form, a small irradiation-induced SANS intensity is detected, which is probably due to point defect clusters. The α′ precipitates contribute significantly to the irradiation-induced hardening of 9–12 wt% Cr content steels.

Microstructural analysis of 9% Cr martensitic steels containing 0.5 at.% helium

Abstract Microstructural examinations by transmission electron microscopy and small angle neutron scattering were performed on 100 μm thick specimens of 9Cr–1Mo (EM10) and modified 9Cr–1Mo (T91) martensitic steels homogeneously implanted with 23 MeV α particles to a concentration of 5000 appm. Two implantation temperatures were selected, 250 and 550 °C, which correspond respectively to the lower and higher bounds of the operation temperature range foreseen for the window of accelerator driven systems devoted to waste transmutation. 250 °C is also the maximum operating temperature of the European spallation source window. The TEM samples were punched out from implanted tensile specimens following testing, which revealed, as detailed in a companion paper [P. Jung et al., these Proceedings], drastic hardening and complete ductility loss for the specimens implanted at 250 °C. Helium bubbles were detected in both materials implanted at 250 and 550 °C and bubble size distributions as well as number densities were determined. Furthermore, it was found that the bubbles are at thermodynamic equilibrium. Based on the microstructural results, it is shown that the high degree of hardening of specimens implanted at 250 °C is due to the high density of tiny helium bubbles they contain. It is furthermore suggested that the brittle, intergranular fracture mode displayed by these specimens results from the combined effects of pronounced intragranular hardening and weakening of prior austenite grain boundaries due to helium.

Influence of the cold rolled reduction on the stored energy and the recrystallization texture in a Fe–53%Ni alloy

Abstract The influence of the cold rolled reduction on stored energy was studied in Fe–53%Ni using neutron diffraction. Measurements of diffraction peak broadening were undertaken to study the influence of stored energy on mechanisms that govern the nucleation and growth of the cube texture in a heavily cold rolled Fe–Ni alloy.

Residual strain distribution in Zircaloy-4 measured by neutron diffraction and estimated by homogenization techniques

Abstract Neutron scattering provides a volume measurement of the elastic strain distribution in polycrystals. Phase average elastic strains have been measured on a plastically deformed specimen of Zircaloy-4, and an excellent agreement was obtained with the prediction of the affine self-consistent scheme for nonlinear elasto-viscoplasticity.

IN-SITU NEUTRON DIFFRACTION STUDY OF THE CUBE CRYSTALLOGRAPHIC TEXTURE DEVELOPMENT IN Fe53%-Ni ALLOY DURING RECRYSTALLIZATION

Laboratoire de physico-chimie de l’e´tat solide, U.M.R. 8648, Baˆt. 410, 91405 Orsay, France(Received February 24, 2000)(Accepted in revised form April 3, 2000)Keywords: Texture; Recrystallization and recovery; Iron-nickel alloyIntroductionMany investigations have been carried out on the recrystallization crystallographic texture of rolledmetallic materials. However, the formation mechanism of the recrystallized grains with a preferredorientation has not been clarified yet. In particular, in the case of fcc metals and alloys from mediumto high stacking fault energy such as FeNi alloys, the cube texture {100},001. development duringrecrystallization is not clearly understood in spite of intensive studies. Several mechanisms have beenproposed to explain this phenomenon and two main alternative theories are generally proposed: orientednucleation (1) and oriented growth (2). Nevertheless, these theories are not satisfactory to explain fullythe cube texture development during recrystallization and in particular, they can not give any infor-mation on the time scale of the phenomenon. In this purpose, “in situ” texture measurements duringrecrystallization have been performed.The Fe53%-Ni alloy is widely used as soft magnetic sheet materials. After high reduction, cold rolledtexture is characterized by three main components considered as a typical copper-type texture (3): the“brass” {110} ,112. (B), the “aluminium” {123} ,634. (S) and the “copper” {112} ,111. (C)components. After annealing, a typical strong cube texture {100},001. development is observed.The aim of this paper is to study the crystallographic texture evolution of the Fe-53%Ni alloy duringannealing and the influence of the annealing temperature on the recrystallization kinetics. Then, the useof neutron diffraction is particularly requisite on the one hand to achieve good statistic measurement,even for small texture volume fraction and, on the other hand to use complex sample surroundings, asa furnace, necessary for the “in-situ” recrystallization study.Experimental ProcedureAfter hot rolling, the Fe-53%Ni alloy was cold rolled with a 95% reduction, until a final thickness sheetof about 0.2 mm. So in order to perform “in-situ” texture measurements by neutron diffraction, a pileup of 50 pieces (1cm 3 1cm) is realized in a vanadium (almost “transparent” for neutron diffraction)cubic container (1cm

Small Angle Neutron Scattering Study of Irradiated Martensitic Steels: Relation Between Microstructural Evolution and Hardening

Martensitic/ferritic steels (containing 7–13 % Cr) are candidate materials for internal structures in pressurized water, fast breeder, and fusion reactors. Approval for use requires verification of structural stability under neutron irradiation in relation to the evolution of mechanical properties. In this context, several conventional and Reduced Activation (RA) martensitic materials were neutron irradiated at 325°C up to 6 dpa. They were investigated by Small Angle Neutron Scattering (SANS) under a magnetic field after various doses. It was shown that when the Cr content of the b.c.c. ferritic matrix was larger than a critical threshold value (∼ 7.2 at.% at 325°C), the ferrite separated under neutron irradiation into two isomorphous phases, Fe-rich (α) and Cr-rich (α′). The kinetics of phase separation is much faster than under thermal aging. The quantity of precipitated α′ phase increases with the Cr content and the irradiation dose. In the case of steel with the lowest Cr content (F82H) irradiated at 5.6 dpa at 325°C, the α′ phase does not form, and the SANS signal suggests a small contribution due to vacancy clusters. It was believed that these could contribute to the “black dots” observed by TEM. Furthermore, we studied the microstructural features responsible for the secondary hardening phenomenon detected in the as-quenched F82H martensitic steel during irradiation or annealing. In addition, the microstructural evolution of the Oxide Dispersion Strengthened (ODS) steel MA957, which presents an excellent hardening/ductility compromise after irradiation, has been also characterized. The stability of the oxides has been elucidated, and an important α′ volume fraction has been detected. The contribution of α′ to the irradiation-induced hardening was assessed. This, although not negligible, is not the critical factor in normalized and tempered or cold-worked steels. However, it may be the main contribution to hardening in MA957.

Modeling of structural hardening in oxide dispersion-strengthened (ODS) ferritic alloys

AbstractBased on a rather simple macroscopic and statistical model, experimentally observed variations of yield stress at room temperature in various ODS alloys were theoretically reproduced. For the first time, yield stress values of ODS steels were calculated by taking into account: (1) two interaction mechanisms between dislocations and nanoprecipitates (shearing or bypassing, simultaneously, depending on the particle size); and (2) the whole, possibly multimodal, nanoparticle distributions experimentally determined by SANS. The relative importances of the various strengthening mechanisms can be easily deduced from these calculations.

SANS study of the microstructural evolution of martensitic steels under thermal ageing and neutron irradiation

SANS technique has been used to study the microstructural evolution of martensitic steels containing 7-12% Cr, under thermal ageing and neutron irradiation. Some of the investigated alloys are industrial low activation martensitic (LAM) materials, where alloying elements have been substituted by elements with low radiological impact. For materials quenched from the austenitic phase, a direct correlation was found between the hardening and the number density of M2C precipitates induced by thermal ageing at 500 °C. Irradiation-accelerated spinodal decomposition of the B.C.C. ferrite was observed in alloys containing large (= 11%) chromium content.

Elastic Strain Study in Quartzites Using Neutron Diffraction

Abstract The residual elastic strains determination by neutron diffraction has been performed on quartzite samples from a former texture study. The measured complete residual strain tensor has been considered as representative of the “natural” finite plastic deformation. Such tensors fitted to plastic tensors have been introduced in texture modelings. The results demonstrate the potential of complete and meaningful strain tensor determinations in rocks.