Influence of local mechanical loadings paths on the oxidation assisted crack initiation of alloy 718

At high temperatures, alloy 718, like many other nickel-based superalloys, is sensitive to an oxidation-assisted intergranular crack (OAIC) growth mechanism. Former studies have pointed out that even if intergranular oxidation still occurred, intergranular crack initiation was inhibited due to specific mechanical loadings which were identified as Portevin-Le Châtelier (PLC) plastic instabilities. In the present work, key parameters triggering crack initiation or PLC instabilities in the [550–700°C] temperature range were determined for the studied grade by means of tensile tests on smooth specimens. Then, in order to assess the applicability of such a finding at a scale compatible with the material microstructure, a dedicated tensile V-shaped specimen was designed to generate different surface and sub-surface strain and strain rate histories. Thanks to a dual approach based on the observation of crack initiation location on this specific experimental specimen together with associated FE calculations, the critical mechanical loading paths inducing OAIC initiation have been specified. Thus, assuming that the metallurgical state is homogenous at the structure scale, a mapping of intergranular crack initiation is then obtainable

Influence of interstitials content on the diffusion of Niobium in alloy 718

Many studies have emphasized the beneficial effect of niobium on the physical metallurgy of Ni-Cr-Fe alloy 718. Among the different strengthening actions of niobium, such as solid solution hardening and carbide precipitation, the precipitation of niobium with nickel in a strengthening phase γ” (Ni3Nb) during the aging heat treatment has the largest influence on the mechanical properties of alloy 718. The improvement of the niobium distribution and diffusion in the Ni-matrix may allow a more homogenized repartition of γ” precipitates and seems then to be an effective way to upgrade the mechanical properties. As γ” precipitates decompose to the stable δ phase at very long aging times, the study of the effect of carbon, nitrogen and oxygen concentrations on precipitation and dissolution of the δ phase may give information on γ” precipitation and on niobium distribution. It is the purpose of the present work to examine the role that the alloy content of interstitial species plays with the niobium-rich δ phase evolution in alloy 718. Alloy 718 samples were heat treated under hydrogenated argon at 980°C for 0 to 96 hours in order to gradually curb the content of interstitial species by reaction with the reducing atmosphere. Chemical analyses realized by glow discharge mass spectrometry (GDMS) confirmed the reduction of the concentration of these species. Specimens were solution-treated for 1h at 1050°C in an inert atmosphere and furnace cooled. Some of the samples were then aged at 920°C for times ranging from 10 min to 1 hour. The precipitation was measured quantitatively in terms of volume fraction and the morphology of the precipitates was appreciated using scanning electron microscopy (SEM). The differences in the precipitation kinetics and in the microstructure evolution for each interstitial concentration are then discussed

Diffusion and segregation of niobium in fcc-nickel

Niobium is one of the major alloying elements, among the refractory elements, contributing to the strengthening of superalloys. Consequently, data about its behavior and its migration mechanism in fcc-Ni are essential knowledge to understand and control the strengthening in such alloys. We present in this work Nb interactions, solubility and diffusion in Ni performed by using the GGA approximation of the density functional theory. The substituted site is found to be the most favorable configuration in comparison to the tetrahedral and octahedral sites. The effect of temperature on solubility is discussed taking into account the thermal expansion of the lattice parameter and the vibrational contribution. Its diffusion mechanism is also discussed and compared to the literature. We finally discuss the segregation of Nb atoms on a 65-(012) symmetric tilt grain boundary

Etude comparative de différents superalliages base Ni pour ressorts de systèmes de maintien

Les systèmes de maintien situés sur les structures assemblages-combustibles des réacteurs nucléaires à eau sous pression (REP) sont constitués d'un empilement de lames qui agissent à la fois, comme élément accommodant les incompatibilités thermiques résultant des différences de coefficients de dilatation Acier, alliages de zirconium et principalement, comme système permettant de limiter les effets hydrodynamiques induits par le passage du fluide caloporteur à travers les assemblages. Actuellement, l'alliage 718 est le matériau constitutif de ces ressorts à lames. Il présente les performances en service nécessaires et suffisantes pour répondre aux sollicitations de ces systèmes ainsi qu'aux exigences des autorités de sûreté (dans les conditions actuelles de fonctionnement des REP). Or, dans le cadre de l'augmentation des performances générales des assemblages combustibles, l'emploi d'autres matériaux, dont les propriétés de relaxation sous flux neutronique sont supérieures à celles du 718, est envisagé par AREVA. Les matériaux étudiés sont principalement des superalliages base Ni, tels que les nuances 625+ et 725 qui à l'instar de l'alliage 718 durcissent par précipitation de phases secondaires, ainsi que des nuances d'alliage 718 riche en Molybdène. Cependant, bien que ces nouveaux matériaux présentent une relaxation sous flux neutronique améliorée, ils doivent répondre également à un cahier des charges strict, propre à leur utilisation en centrale : des propriétés mécaniques équivalentes, une bonne résistance à la corrosion sous contrainte (CSC) et une bonne résistance à la fragilisation par l'hydrogène (FPH) en milieu primaire de REP. Chacune de ces propriétés a été étudiée avec attention dans le double but de comparer ces matériaux entre eux et afin de cerner les paramètres clés contrôlant leur différence de comportement aussi bien en CSC qu'en FPH. ABSTRACT : Hold-down systems used in the fuel assembly of Nuclear Pressurized Water Reactor (PWR) are constituted by stiff springs. The role of the hold-down springs is to ensure the bond between the fuel assembly and the lower plate of the intern structure of the core, thus holding down the assembly on the bottom plate of the reactor, during all the exploitation and maintenance periods. Nowadays, alloy 718 is the constitutive material of these hold-down springs. Its properties in terms of mechanical behaviour, corrosion resistance… fill in the specifications required for such application in the present service conditions. However, in order to improve the common efficiency of fuel assemblies, the upgrading of their design as well as the use of new materials are advocated by the nuclear power plant company, AREVA. Though other Ni-base superalloys known for their good behaviour under neutronic radiation can be proposed as new materials, those superalloys must fill in all the application specifications in order to substitute alloy 718. So, sufficient mechanical properties, good resistance to Stress Corrosion Cracking (SCC) and good resistance to Hydrogen Embrittlement (HE) are also required to allow the replacement. All of these properties are carefully studied with the double aim to characterize and compare different superalloys, and to determine key parameters governing the SCC and HE behaviours of such alloys in primary water of PWR

Influence of interstitials content on the sensitivity of alloy 718 to oxidation assisted intergranular fracture

Alloy 718 is known to be sensitive to oxidation assisted intergranular cracking. It is also demonstrated that the occurrence of jerky flow (also called Portevin-Le Châtelier effect) stops the intergranular damaging mechanism. As dynamic strain ageing is known to be linked with the alloy content of interstitial species, the aim of the present work is to study the effect of carbon, nitrogen and oxygen concentrations on the mechanical behaviour of thin tensile specimens tested under oxidation conditions close to those encountered industrially for turbo machine disks. Thanks to heat treatments performed under reducing atmosphere, the content of interstitial species in tested alloy 718 samples is gradually curbed. Tensile specimens were then tested between 550 and 700°C for the strain rate range [10-5, 10-1] s-1. The key point of this work is that, for a given testing temperature, the tensile tests clearly demonstrated that the transition from an intergranular fragile fracture mode to a transgranular ductile one was always linked with the occurrence of Portevin-Le Châtelier phenomenon but for slower strain rates in comparison with what was observed on the as received aged material tested in the same conditions. This shift of the transition of fracture mode through the lower strain rates remained true until a threshold value of the heat treatment time under reducing atmosphere. Specimens heat treated over this value systematically exhibited a fully transgranular ductile fracture mode, whatever the plastic flow regime was. Implication of such a finding on the intergranular embrittlement of alloy 718 by high temperature oxidation is then discussed

Kinetic study of the low temperature internal oxidation of nickel based model alloys exposed to PWR primary water

Two Ni-Fe-Cr ternary alloys have been oxidized in simulated pressurized water reactor primary water at 360°C for 1000 h. The chemical composition of those alloys were chosen in order to be representative of the one of chromium depleted areas under the oxide scale of industrial alloys (e.g. alloy 600) exposed in the same conditions. The resulting oxidized structures (corrosion scale and underlying metal) were characterized using complementary analytical methods (FEG-SEM, TEM, SIMS, optical microscopy). On the one hand, the characterized external oxide layer is very close to the one observed on industrial nickel-base alloys, hence validating the use of such model alloys. On the other hand, both free oxygen and oxides have been detected at grain boundaries several micrometers under the metal/oxide interface. Implications of such a finding on the involved transport mechanisms for oxygen and the intergranular stress corrosion cracking resistance of nickel-base alloys are then discussed

Quantitative assessment of intergranular damage due to PWR primary water exposure in structural Ni-based alloys

Two nickel-based alloys, alloy 718 and alloy 600, known to have different resistances to IGSCC, were exposed to a simulated PWR primary water environment at 360 °C for 1000 h. The intergranular oxidation damage was analyzed in detail using an original approach involving two characterization methods (Incremental Mechanical Polishing/Microcopy procedure and SIMS imaging) which yielded a tomographic analysis of the damage. Intergranular oxygen/oxide penetrations occurred either as connected or isolated penetrations deep under the external oxide/substrate interface as far as 10 μm for alloy 600 and only 4 μm for alloy 718. Therefore, assessing this damage precisely is essential to interpret IGSCC susceptibility

Effects of the Cr-depletion on the stress state of the sublayer of ni-base alloys oxidized in high temperature water

The exposure in high-temperature water (300–360°C) of Alloy 600 is known to induce not only generalized corrosion, but also degradation at the underlying base metal such as intergranular penetrations of oxygen and\or oxides and Cr-depletion. Possible consequences of Cr-depletion are the creation of local stresses in the underlying metal due to local variations of the lattice parameter, and the formation of physicals properties gradient, like the elastic modulus or thermal expansion coefficient. In order to assess the effects caused by the Cr-depletion on an exposed Alloy 600, finite element (FE) calculations and physical properties characterisations on synthetic alloys with a chemical composition representative of different Cr-depleted layers, were performed. The levels of the calculated stresses were then discussed regarding to the other features of oxidation involved in the high temperature water stress-corrosion cracking mechanism

New-generation biocompatible Ti-based metallic glass ribbons for flexible implants

We introduce five new biocompatible Ti-based metallic glass (MG) compositions with different metalloid and soft metal content for a synergistic improvement in corrosion properties. Without any potentially harmful elements such as Cu, Ni or Be, these novel alloys can eliminate the risk of inflammatory reaction when utilized for permanent medical implants. Excluding Cu, Ni or Be, which are essential for Ti-based bulk MG production, on the other hand, confines the glass-forming ability of novel alloys to a moderate level. In this study, toxic-element free MG alloys with significant metalloid (Si–Ge–B, 15–18 at.%) and minor soft element (Sn, 2–5 at.%) additions are produced in ribbon form using conventional single-roller melt spinning technique. Their glass-forming abilities and their structural and thermal properties are comparatively investigated using X-ray diffraction (XRD), synchrotron XRD and differential scanning calorimetry. Their corrosion resistance is ascertained in a biological solution to analyze their biocorrosion properties and compare them with other Ti-based bulk MGs along with energy dispersive X-ray. Ti60Zr20Si8Ge7B3Sn2 and Ti50Zr30Si8Ge7B3Sn2 MG ribbons present a higher pitting potential and passivation domain compared with other Ti-based MG alloys tested in similar conditions. Human mesenchymal stem cell metabolic activity and cytocompatibility tests confirm their outstanding cytocompatibility, outperforming Ti-Al6-V4