The authors introduce the multi-scale modelling approach of service properties and recent results that best reflect the complex behaviour of industrial materials. They describe the main modelling tools and highlight the essence of experimental checking at the appropriate scale. The authors describe in details the progress that has been accomplished in the prediction of the energy of atom-scale defects in steels, as essential input data to model the kinetics of microstructure evolutions under radiation. The authors present the multiscale prediction of mechanical properties along with the particular issue of the nuclear fuel.

Des examens par sondes atomiques classique et tomographique sur un acier HLE, a l'etat lamine a chauc et a l'etat lamine a froid et recuit, ont mis en evidence la segregation du carbone et du manganese a l'interface carbures NbC/matrice ferritique. Les caracteristiques de ces deux segregations relevent de mecanismes differents Celle du carbone, confinee a la peripherie immediate des particules de NbC, n'affecte que les plus grosses d'entre elles. Elle est vraisemblablement induite par le champ de contraintes elastiques associe a l'interface semi coherente, qui croit avec le volume de la particule Celle du manganese, en revanche, qui s'etend sur une plus grande distance a partir de l'interface et affecte toutes les particules, n'est pas encore comprise. La segregation interfaciale du carbone n'explique que partiellement le faible vieillissement apres ecrouissage de ces aciers au niobium, car une quantite appreciable de carbone en solution solide a egalement ete mise en evidence. Ces resultats illustrent l'aptitude de la sonde atomique a l'etude des tres petites particules et de leur interface dans les aciers tres faiblement allies.

Both conventional and 3D atom-probes were applied to the investigation of grain-boundary (GB) segregation phenomena in two-phase nickel base superalloys Astroloy. 3D images as provided by the tomographic atom-probe reveal the presence of a strong segregation of both boron and molybdenum at grain-boundaries. Slight carbon enrichment is also detected. Considerable chromium segregation is exhibited at gamma'-gamma' grain-boundaries. All these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Experiments show that segregation occurs during cooling and more probably between 1000 degrees C and 800 degrees C. Boron and molybdenum GB enrichments are interpreted as due to an equilibrium type-segregation while chromium segregation is thought to be induced by gamma' precipitation at GB's and stabilised by the presence of boron. No segregation of zirconium is detected.

A new three-dimensional (3D) atom probe was used to study grain-boundary segregation and ordering phenomena in a two-phase (gamma-gamma') Ni-based superalloy, namely Astroloy. The tomographic images show that the grain-boundary serration is due to gamma' precipitates which develop along boundaries during cooling. These 3D reconstructions also exhibit segregations of B, C, Mo and Cr at grain boundaries. They also reveal that the degree of order in gamma' particles which are located along boundaries is preserved up to the grain boundary.

Duplex stainless steels are subject to embrittlement after long term aging at temperatures below 500°C. This embrittlement is usually attributed to the α-α' phase separation occuring in the ferrite, though the role of G-phase particles, when present, may not be negligible. In order to try to restore their mechanical properties to that of the as-quenched conditions, specimens were reannealed above the Fe-Cr miscibility gap. The mechanical effect of various reannealing treatments is reported, and correlated with transmission electron microscopy and atom probe field ion microscopy microstructural investigations.

The microstructural evolution of Mo bearing and Mo free cast duplex stainless steels, induced by long term aging in the range 350–4507deg;C has been studied using atom probe and field ion microscopy and transmission electron microscopy. The salient features of this evolution at 350 and 400°C are spinodal decomposition and G phase precipitation in the ferrite. The nucleation of the G phase is enhanced by spinodal decomposition. Other intermetallic phases observed at higher temperatures may also be formed at 400°C. In the ferrite/ austenite interface, an intermetallic film, possibly an icosahedral phase, is formed after aging for 30 000 h at 400 and 450°C in Mo bearing steels, whereas interfacial M23C6 carbides precipitate in the Mo free materials. The observed evolution of the spatial and compositional parameters of the spinodal decomposition in Mo bearing steels is described and its relationship with the evolution of mechanical properties is outlined.MST/1185

Alloy 690 is used to replace Alloy 600 for the fabrication of tubes for steam generators of french pressurized water nuclear reactors. In order to reduce the dispersion in tensile properties observed for the first Alloy 690 industrial tubes, and which had already been noticed for Alloy 600, a joint research programme has been carried out by Electricité de France (Département Etude des Matériaux) and Valinox Montbard.

It is demonstrated that a population of non-interacting and non-coalescing defects develops according to a continuity equation ∂ρ∂t+∂∂x(ρ · υ) = 0 where ρ is the partial density of defects of length x at time t and υ is their growth rate. This equation predicts two types of basically different behaviours: in type I, defect nucleation reaches a constant (steady state) value, while in type II it increases exponentially with time. The analysis of the experimental defect length distribution curves with the help of the model's equations allows the growth law υ(x) to be identified and the influence of metallurgical and testing conditions upon nucleation and growth to be determined. The model is applied to the creep of precipitation-strengthened Al-base alloys and to the slow tensile testing of an Fe-base austenitic alloy at elevated temperature; it is demonstrated that the former behaves according to type I and the latter to type II.

The intergranular segregation of Sb and Ni in αFe was studied by scanning Auger electron spectroscopy in ultra-high purity FeSb and ternary Fe1% NiSb and Fe2% NiSb alloys. The segregation isotherms of Sb at 550°C are considerably steepened by Ni additions which also reduce the bulk Sb content at which the boundaries are saturated with Sb. For a given bulk Ni content, the Ni coverage increases linearly with the Sb coverage, in qualitative agreement with Darken and Simkovich's equations derived from the Gibbs adsorption isotherms. Better quantitative agreement is obtained with the regular solution model of co-segregation. The results appear consistent with a strong NiSb attractive interaction (βNiSbgb∼- 10 kJ·mole−1) and reveal the existence of a weaker but appreciable SbSb attraction (βsbgb∼- 5,4 kJ·mole−1).

The segregation of S and other residual elements (S, Ca) to the grain boundaries and to the free surface of commercial purity nickel (99.3%) have been studied quantitatively by means of Auger electron spectroscopy. This technique allows the evaluation of the concentrations required for saturation of both types of interfaces at thermodynamic equilibrium, the segregation free energy of sulfur, and the influence of third elements on sulfur segregation. The kinetics of S segregation to these two types of interfaces are initially proportional to the square root of time in agreement with McLean's model. The segregation free energy of S to the grain boundaries, −98 kJ mole−1, is approximately half that to the free surface, −180 kJ mole−1. The saturation coverage of S at these two types of interfaces are approximatively equal, i.e. 45 at%. Considering the free surface, this value is equal to that obtained by adsorption from a S containing atmosphere. The presence of other elements such as C and Ca drastically alters the segregation kinetics of S. The repulsive S-C and attractive S-Ca chemical interactions respectively retard and accelerate the S build up on the surface. Also the S-Ca synergism is able to induce transient maxima of S coverage larger than the equilibrium saturation coverage of the binary Ni-S surface.

The thermodynamics of co-segregation and precipitation of P and alloying elements (transition metals M and carbon) involved in temper embrittlement of steels are studied quantitatively on the basis of the regular solution model for co-segregation. The equations of this model are fitted to the available Auger data for grain boundary segregation in high purity iron-base alloys and commercial steels, allowing the determination of the intrinsic segregation energies ΔGi o and of the binary βP gb, βc gb and ternary βPC gb, ßMP gb interaction coefficients in the grain boundaries. This analysis shows that Ni, Cr, and Mo do not segregateper se in iron whereas Mn does weakly, and that the segregation of these elements is essentially driven by that of P through the strongβMP gb attractive interaction energyat the boundaries. This energy, which increases in the order Ni, Mn, Cr, Mo, is remarkably close to the bulk values βMP B in the corresponding phosphides as calculated on the basis of solubility data. The scavenging of P by M elements with largebulk M-P interactions is shown to play a determining role in low Mo and high (12 pct) Cr steels. The beneficial role of carbon is complex since it drives Mo to the grain boundaries due to the large Mo-C attraction, but it also strongly opposes P segregation due to the large repulsive P-C interaction.

There has been continued interest over the last few years in the relationships between the yield stress and the microstructural parameters of lath martensites and bainites. As far as the grain size is concerned, various researchers have concluded that a Hall-Petch relationship exists between the yield stress and the average diameter of the packets of the parallel elongated martensite-bainite grains which are termed laths. Other work has shown a reciprocal relationship between the yield stress and the lath width.

Abstract The grain boundary composition of nickel base alloy 600 has been studied by means of Auger electron spectroscopy. After being cathodically charged with hydrogen in a water saturated salt bath at 200 C, all the specimens could be intergranularly fractured in the Auger chamber. Phosphorus was the only element found to segregate at the grain boundaries of the two materials studied in all conditions of heat treatment considered; sulfur appeared essentially as a contaminant, which built up on the surfaces after fracture. The segregation of P was shown to be of the equilibrium (McLean) type, whereas Si did not segregate appreciably to the grain boundaries. The results, discussed in connection with published corrosion and stress corrosion data, explain the influence of P and the virtual absence of influence of Si on the sensitivity of alloy 600 to intergranular corrosion in HNO3 + Cr6+ solutions. They also indicate that the segregation of P is not the cause of intergranular stress corrosion cracking of this alloy in pure water and caustic environment, and that hydrogen embrittlement is very unlikely to be the mechanism of this phenomenon.

X-ray microanalysis associated with scanning transmission electron microscopy has been used to study the partition of Mn between reverted austenite and tempered martensite laths in 6 and 9% Mn cryogenic steels tempered in the intercritical range. This partition is much smaller than that dictated by the equilibrium Fe-Mn phase diagram, which shows that austenite and ϵ-martensite can be stabilised in dispersed form even though their Mn content is much smaller than required by Schumann's diagram for martensitic transformation of bulk austenites. It is suggested that the very small size of these austenite islands has a stabilizing influence with respect to the γ → α′ transformation.

Molybdenum considerably reduced reversible temper embrittlement (RTE) of 12%Cr martensitic stainless steels by scavenging P in the matrix, which decreased P segregation to the grain boundaries. Contrary to the case of many low-alloy steels, Mo additions as large as 1 wt-% were still beneficial, due to the lower C content which allowed more Mo to remain in solid solution in the materials studied. The thermodynamics of the co segregation and precipitation processes of P, Mo, and Cr were analysed quantitatively and shown to be in good agreement with previously proposed models. In particular, the Mo-P and Cr-P chemical interactions were shown to be primarily responsible for the segregation of Mo and Cr, which means that their intrinsic segregation energies in Fe are very small. The segregation of P being too small to drive a segregation of Mo in the materials studied, the embrittling potency of P was unaffected by Mo additions and the impact transition temperature was a unique linear function of the intergranular P content, independent of the nominal Mo content. The embrittling potency of P was similar to that in lowalloy steels (6K/at.-%P), but since the transition temperature of a virtually P segregation-free condition is much higher in 12%Cr than in low-alloy martensitic steels, RTE is potentially more dangerous in the former materials.

A brief review of the general metallurgical properties of temper brittleness in low alloy steels emphasizes that this intergranular embrittlement is sensitive to essentially two categories of independent variables: the chemical composition of the grain boundaries (segregation of the solutes on an atomic scale), as well as the ‘mechanical-microstructural’ parameters of the alloy (microstructure and strength of the matrix, morphology of the carbides and grain boundaries, etc.). This paper, essentially devoted to the former, reviews and discusses the available segregation data in the light of recently proposed models. The segregation potencies and embrittling powers of the various impurities are compared, and their dependence on the alloy’s metallic components through chemical interaction between both types of solutes is particularly emphasized. The link between segregation and solubility in multicomponent systems and its use as a predictive means for impurity segregations is outlined. The discussion aims at showing that temper embrittlement is in no way a unique phenomenon, specific to low alloy steels, as was formerly considered, but rather a strikingly illustrative case of multicomponent segregation induced embrittlement, where some metallic alloying elements, although not embrittling per se , can drastically enhance the segregation of residual impurities, while some others can be used as scavengers to alleviate it.