R. Coppola

A microstructural comparison of two nuclear-grade martensitic steels using small-angle neutron scattering

Abstract Results are presented of a small-angle neutron scattering (SANS) study on two 10–13% Cr martensitic stainless steels of interest for nuclear applications, viz. DIN 1.4914 (MANET specification, for fusion reactors) and AISI 410. The investigation has focussed principally on microstructural effects associated with the differences in chromium content between the two alloys. The size distribution functions determined from nuclear and magnetic SANS components for the two steels given identical heat treatments are in accord with an interpretation based on the presence of ∼ 1 nm size CCr aggregates in the microstructure. Much larger (∼ 10 nm) scattering inhomogeneities with different magnetic contrast are also present and tentatively identified as carbides.

Microstructural investigation of 12% Cr martensitic steel for NET by means of small angle neutron scattering

Abstract A microstructural investigation of the modified DIN 1.4914 martensitic steel MANET, developed as a possible structural material for prototype fusion reactors, has been carried out by means of small-angle neutron scattering (SANS) technique. The results of this study concern the presence both of carbides in the tempered material and He bubbles in a sample implanted with α-particles. These results show that the adopted technique can provide useful information on the average size of these microstructural defects.

Study of γ′-precipitation kinetics in alloy 800 at 575° C by small angle neutron scattering☆

Abstract Small-angle neutron scattering has been used to study the characteristics of γ′-precipitation during ageing at 575°C in several alloy 800 heats, differing in the initial chemical composition; the effect of a pre-ageing treatment at 700°C and of creep tests have been studied by the same technique. For each investigated sample the volume fraction average size and size distribution function of the γ′-precipitates were determined and quantitatively compared with the prediction of the Lifshitz-Slyozov and Wagner model.

Study of residual stress in a ferritic steel electron beam test weldment using neutron diffraction

This paper describes the results of a programme of experimental neutron diffraction residual stress studies. The aim of the programme was to characterize the residual stress distribution which may arise in a component due to electron beam welding. The programme comprises of three studies, one on a complete weld test-block and one on each of two cut sections. The studies confirm that major non-uniform stresses may be present in such weldments.

Neutron diffraction study of internal stresses in brazed divertor structures for ITER

Abstract The strain and the stress are evaluated in a divertor mock-up of carbon fiber composite (CFC) brazed to a molybdenum cylindrical pipe. The main components of stress in the graphite are obtained in a point close to and in a point far from the brazed region. The problems characteristic of CFC, that is alignment of the matrix induced by the fibers and the occurrence of an amorphous phase, are overcome. Neutron diffraction turns out to be a powerful tool to investigate bulky materials also in case of complex components. This result is valid not only for fusion problems but also for many fields of material engineering.

Structure evolution during heat treatments of 12% Cr martensitic steel for net

Abstract The structure evolution of martensitic 12% Cr steel has been followed in the range from room temperature up to 600°C by means of X-ray diffraction with analyses of the grain textures and of the diffraction line profiles, together with observations in optical and scanning electron microscopy and with hardness measurements. The behaviour of hardness appears referable to two stages in the transformation of martensite to ferrite, involving subsequent disappearance of martensite laths of different thickness and characterized by specific texture variations. Carbide precipitation in the ferritic volumes appears determining for the hardness change above 400°C, with concomitant effects on the hardness drop above 500°C depending on internal stress, dislocation density decreases. Some effects referable to elements with large diffusivities are also indicated.

Small angle neutron scattering investigation of the ageing process in Al-Mg-Si alloy

SANS measurements are presented to investigate the ageing process of a polycrystalline Al-Mg-Si alloy a 210°C. The growth of needle-shaped precipitates as a function of ageing time is observed, in accordance with TEM observations. The precipitated fraction estimated from the size distribution function agrees with resistivity measurements previously performed on the same alloy. Bidimensional scattering profiles are also given in the case of a single crystal; the data analysis is in progress.

High-resolution neutron diffractometer for internal stress measurements

Abstract This contribution presents a recently developed project to build a new dedicated diffractometer for residual-stress determination at Laboratoire Leon-Brillouin (CEA-CNRS). Details of the project, in particular the use of a positionsensitive multidetector, are described. The expected performance of the new instrument, which is presently under construction, is discussed with reference to materials studies.

Solute Cr atom distribution and fracture behaviour of MANET steel

Abstract The effects of heat treatments at 700°C on the toughness behaviour have been investigated in MANET steel by means of Charpy impact tests and SEM observations with microanalysis. The ductile to brittle transition temperature, the upper shelf energy and the slope in the ductile to brittle transition depend strongly on the time of heating as a consequence of the different fracture mechanisms evolving with the thermal treatment. The transition, from fracture mainly conditioned by lath boundaries to fracture depending mainly on processes at the PAG boundaries, appears correlated with the specific characteristics of the distribution of the Cr atoms in solution.

Investigation on He-bubble growth in α-implanted 1.4914 steel

Abstract The growth of He bubbles in steel 1.4914 (NET reference material) was investigated by means of transmission electron microscopy (TEM); TEM observations were carried out on samples implanted with high-energy α-particles as well as on non-implanted samples. These results are presented and discussed in view of small-angle neutron scattering (SANS) experiments to be performed on implanted samples of the same material.