Frédéric Christien

Influence of Stress on Intergranular Brittleness of a Martensitic Stainless Steel

Phosphorus intergranular segregation can strongly decrease grain boundary cohesion in steels. Martensitic stainless steel 17-4 PH was tempered 4 hours at 600°C and aged for long times at 320°C (up to 15700 h). Tempering treatment at 600°C leads to phosphorus intergranula segregation, although there is no sign of intergranular brittleness because of the low hardness of the steel. Ageing treatment at 320°C makes the steel much harder ( α’ p ecipitation) and it is then possible to obtain intergranular ruptures. It was observed that the s hift of the ductile-to-brittle transition temperature due to ageing at 320°C was more important if the ageing treatment was carried out under a tensile stress of 500 MPa, even though the hardness l evel i unchanged by the stress. This shift was attributed to an increase of phosphorus concentr ation in the grain boundaries that lie perpendicularly to the tensile stress axis, which corres ponds to the longest side of the Charpy specimen. The interpretation of this effect is based on the following assumption: the segregation free energy of phosphorus is increased for grain boundaries normal to the tensile axis. Tempering treatment at 600°C leads to phosphorus segregation in all the grain boundaries. But, during ageing at 320°C, the application of a tensile stress make the phosphorus diffuse from grain boundaries that lie parallel to the tensile axis toward grain boundari es that lie normal to the tensile axis. This assumption was confirmed by intergranular segregation m easurements made on specimens that were heat treated under a flexion load. Introduction Structure materials often undergo stresses during service. This i s the case of the martensitic stainless steel 17-4 PH used in nuclear power plants which is likel y to remain in service for very long times at temperatures about 300°C (10 4 – 10 hours). It has been shown that intergranular segregation of phosphorus decreases the grain boundary cohesion of 17-4 PH steel, which leads to a dramatic increase of its ductile-to-brittle transition tempera ture (DBTT) [1–3]. The stress applied to the steel during service can reach several hundreds MPa and, up to now, its effect on phosphorus segregation and mechanical properties of steels has not been extensi v ly studied. This paper presents some preliminary results concerning the influence of st ress on intergranular phosphorus segregation and consequently on the DBTT of the 17-4 PH steel. Material Microstructure. 17-4 PH is a martensitic stainless steel. Its chemical c omposition is given in Table 1. The microstructure of the steel is illustrated in Fig. 1: the forme r austenitic grains ( γ grains) are divided in lath packets ( α grains). The steel is usually annealed at 1050°C, which leads to a γ grain size of 17 μm. Table 1: Chemical composition of 17-4 PH [% wt]. C Si Mn Ni Cr Mo Cu Co Nb N S P 0,031 0,31 0,81 4,82 15,61 0,03 3,12 0,03 0,21 0,03 0,02 0,016 Defect and Diffusion Forum Online: 2003-02-14 ISSN: 1662-9507, Vols. 216-217, pp 275-284 doi:10.4028/www.scientific.net/DDF.216-217.275

Quantification of Grain Boundary Segregation Monolayers by X-ray Spectroscopy in a Scanning Electron Microscope

P. Nowakowski*, F. Christien*, M. Allart*, Y. Borjon-Piron*, R. Le Gall*, J.C. Menard**, H. Mantz*** * Laboratoire Genie des Materiaux et Procedes Associes (LGMPA), Universite de Nantes, Polytech’Nantes, Rue Christian Pauc, BP 50609, 44306 Nantes Cedex 3, France ** Carl Zeiss NTS sas, 27 rue des peupliers, 92752 Nanterre Cedex, France *** Carl Zeiss NTS GmbH, Research & Development, Carl-Zeiss-Strase 56, D-73447 Oberkochen, Germany

Intergranular Fatigue Cracking Enhanced by Impurity Segregation

It was previously reported that fatigue life of some alloys can be dramatically reduced if the grain boundaries contain a high level of impurity segregation before fatigue tests. In this paper the susceptibility of single phase brass samples (90Cu10Zn) to this form of damage is studied. After cold drawing of as cast brass bars, fatigue samples were heat treated at 800°C during 30min to promote recrystallization and impurity segregation at grain boundary. The samples were then tested under high frequency bending fatigue test at 200°C. After cracking, fracture surfaces were studied using both scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The SEM micrographs showed that the fractures were mostly intergranular. Chemical composition of intergranular cracks surface were analyzed using EPMA at low accelerating voltage. A high concentration of sulfur was found on most of grain boundary facets. The internal stress in alloys after fatigue was qualitatively estimated using electron backscattering diffraction in scanning electron microscopy. A high level of local misorientation was found near most grain boundaries. The mechanism of intergranular cracks formation during fatigue is discussed taking into account both the segregation of sulfur at grain boundaries and accumulation of plastic strain at grain boundaries

The Use of Electron Probe MicroAnalysis to Determine the Thickness of Thin Films in Materials Science

Electron Probe MicroAnalysis (EPMA) was born around 1950 when Raymond Castaing, a French graduate student working under the supervision of Andre Guinier, built his first microanalyser (Castaing & Guinier, 1950; Castaing, 1951; Grillon & Philibert, 2002). The principle of EPMA is to bombard the sample surface with a focused electron beam and to collect the X-rays emitted from the sample. The X-rays are dispersed using Bragg diffraction on a mobile monochromator, which enables to get the whole X-ray spectrum from zero to more than 10 keV. The technique of dispersion is called WDS (Wavelength Dispersive X-ray Spectroscopy). The spectrum is made of continuous background (Bremstrahlung emission) and characteristic peaks, which allow elemental qualitative and quantitative analysis of the material. EPMA-WDS can be carried out in dedicated instruments called “microprobes” (usually fitted with 4 WDS spectometers) or in high current Scanning Electron Microscopes (SEM) equipped with one single WDS spectrometer. More recently, Energy Dispersive X-ray Spectroscopy has been developed and has nowadays become a cheap and widespread technique. Most SEM in materials science laboratories are equipped with an EDS spectrometer. EPMA-EDS performances are far below those of EPMA-WDS with regards to sensitivity and spectral resolution, although it can achieve very good qualitative and quantitative results in many cases. EPMA quantification of the sample composition is usually possible by using a standard material of known composition. Unfortunately there is in general no direct proportionality between the concentration of an element and the X-ray emission intensity (peak height) coming from this element. Since the early years of EPMA, many models have been proposed in the literature to correlate the concentration of an element and the associated X-ray emission intensity: ZAF (Philibert & Tixier, 1968), MSG (Packwood & Brown, 1981), PAP and XPP (Pouchou & Pichoir, 1987; Pouchou et al., 1990)... The XPP model (implemented in several microanalysis software packages) has now reached a good level of maturity and reliability, even in difficult situations involving light elements and strong bulk absorption. It is based on an accurate calculation of the (z) curve, which describes the depth-dependence of the X-ray emission intensity of a particular line in the sample. In EPMA analysis of

Gallium Distribution in Gallium-Coated Aluminum for Brazing Application

This work deals with the study of a new aluminum brazing process, called Galluminium. This technique, consisting in using gallium as a solder, is advantageous since it enables to braze at ambient air without flux. Indeed if the gallium coating is done mechanically, it descales the alumina layer and acts as a barrier against further reoxidation. Nevertheless, liquid gallium causes a severe aluminum embrittlement since it penetrates into the aluminum grain boundaries. We will show in this paper that this phenomenon has only a little impact on the mechanical resistance of the brazed joints since the amount of deposited gallium is enough low to avoid a severe embrittlement and the heat treatment (from 200°C to 600°C during several minutes) during brazing process dissolves gallium in the bulk.

Recent Developments in the Study of Grain Boundary Segregation by Wavelength Dispersive X-Ray Spectroscopy (WDS)

It was recently shown [1] that EMPA-WDS (Electron Probe MicroAnalysis by Wavelength Dispersive X-ray Spectroscopy) can be used to detect and to accurately quantify monolayer surface and grain boundary segregation. This paper presents the last developments of this application. It focuses on the measurement of sulphur grain boundary segregation in nickel on fractured surfaces. A special attention was paid to the quantification of the sulphur coverage, taking into account the non-normal incidence of the electron beam on a fracture surface. Sulphur grain boundary segregation kinetics was measured at 750°C in nickel to document the quantitative possibilities of the technique.

The Influence of Phase Transformations on Creep Resistance in Fe-Ni-Cr Alloys for Reformer Tube Applications

Heat resistant steels of the HP-series have widespread uses in the petrochemical industry in pyrolysis and reformer furnaces. The alloys are carbon-rich Fe-Ni-Cr alloys, with additions like Mn, Si, Nb, Ti, W... The typical microstructure of as-cast HP alloys is an austenite matrix with intergranular eutectic-like primary chromium carbides of the M7 C3 type and niobium carbides of the MC type. Upon ageing, phase transformations occur. Intragranular secondary M23 C6 carbides precipitate, which is thought to restrict dislocation motion, and intergranular M7 C3 transforms into M23 C6 . Under certain thermal conditions, a partial transformation of the primary niobium carbides into a nickel-niobium silicide called G phase can occur. These phases may play a critical role during creep, but neither their role on mechanical properties nor the mechanisms of phase transformations are clearly identified. The aim of this study is to understand the role of each phase or phase transformation in the creep resistance of HP alloys. Consequently, a critical review of phase formation and transformations in such alloys is presented using a set of experimental and modelling techniques (electron microscopy, Castaing microprobe, creep tests at high temperature and neural networks modelling of mechanical properties...).Copyright

Effect of self-interstitial diffusion anisotropy in electron-irradiated zirconium: A cluster dynamics modeling

Irradiation of metals leads to the formation of point-defects (vacancies and selfinterstitials) that usually agglomerate in the form of dislocation loops. Due to the elastic interaction between SIA (self-interstitial atoms) and dislocations, the loops absorb in most cases more SIA than vacancies. That is why the loops observed by transmission electron microscopy are almost always interstitial in nature. Nevertheless, vacancy loops have been observed in zirconium following electron or neutron irradiation (see for example [1]). Some authors proposed that this unexpected behavior could be accounted for by SIA diffusion anisotropy [2]. Following the approach proposed by Woo [2], the cluster dynamics model presented in [3] that describes point defect agglomeration was extended to the case where SIA diffusion is anisotropic. The model was then applied to the loop microstructure evolution of a zirconium thin foil irradiated with electrons in a high-voltage microscope. The main result is that, due to anisotropic SIA diffusion, the crystallographic orientation of the foil has considerable influence on the nature (vacancy or interstitial) of the loops that form during irradiation.