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 / 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

This research establishes a physics-based life determination model for the second generation single crystal superalloy PWA 1484 experiencing out-of-phase thermomechanical fatigue (TMF). The life model was developed as a result of a combination of critical mechanical tests, dominant damage characterization and utilization of well-established literature. The resulting life model improves life prediction over currently employed methods and provides for extrapolation into yet unutilized operating regimes. Particularly, the proposed deformation model accounts for the materials' coupled fatigue-environment-microstructure response to TMF loading. Because the proposed model is be based upon the underlying deformation physics, the model is robust enough to be easily modified for other single crystal superalloys having similar microstructure. Future use of this model for turbine life estimation calculations would be based upon the actual deformation experienced by the turbine blade, thereby enabling turbine maintenance scheduling based upon on a "retirement for a cause" life management scheme rather than the currently employed "safe-life" calculations. This advancement has the ability to greatly reduce maintenance costs to the turbine end-user since turbine blades would be removed from service for practical and justifiable reasons. Additionally this work will enable a rethinking of the warranty period, thereby decreasing warranty related replacements. Finally, this research provides a more thorough understanding of the deformation mechanisms present in loading situations that combine fatigue-environment-microstructure effects.

Thermomechanical fatigue life prediction is important in the design of Ni-base superalloy components in gas turbine engines and requires a stress-strain analysis for accurate results. Crystal viscoplasticity models are an ideal tool for this stress-strain analysis of Ni-base superalloys as they can capture not only the anomalous yielding behavior, but also the non-Schmid effect, the strain rate dependence, and the temperature dependence of typically large grained directionally-solidified and single crystal alloys. However, the model is difficult to calibrate even for isothermal conditions because of the interdependencies between parameters meant to capture different but similar phenomena at different length scales, many tied to a particular slip system. The need for the capacity to predict the material response over a large temperature range, which is critical for the simulation of hot section gas turbine components, causes the determination of parameters to be even more difficult since some parameters are highly temperature dependent. Rapid parameter determination techniques are therefore needed for temperature-dependent parameterizations so that the effort needed to calibrate the model is reduced to a reasonable level. Specific parameter determination protocols are established for a crystal viscoplasticity model implemented in ABAQUS through a user material subroutine. Parameters are grouped to reduce interdependencies and a hierarchical path through the groups and the parameters within each group is established. This dual level hierarchy creates a logical path for parameter determination which further reduces the interdependencies between parameters, allowing for rapid parameter determination. Next, experiments and protocols are established to rapidly provide data for calibration of the temperature-dependencies of the viscoplasticity. The amount of data needed to calibrate the crystal viscoplasticity model over a wide temperature range is excessively large due to the number of parameters that it contains which causes the amount of time spent in the experimentation phase of parameter determination to be excessively large. To avoid this lengthy experimentation phase each experiment is designed to contain as much relevant data as possible. This is accomplished through the inclusion of multiple strain rates in each experiment with strain ranges sufficiently large to clearly capture the inelastic response. The experimental and parameter determination protocols were exercised by calibrating the model to the directionally-solidified Ni-bas superalloy DS-CM247LC. The resulting calibration describes the material's behavior in multiple loading orientations and over a wide temperature range of 20 °C to 1050 °C. Several parametric studies illustrate the utility of the calibrated model.

Background and Objective: Nickel-base superalloys, strengthened by a high volume fraction of Ni3Al precipitates, have been the undisputed choice for turbine discs in gas turbines as they exhibit the best available combination of elevated temperature tensile strength and resistance to low cycle fatigue (LCF), which are essential for a disc alloy. Alloy 720LI is a wrought nickel-base superalloy developed for disc application and exhibit superior elevated temperature tensile strength and LCF properties. It is distinct from contemporary disc alloys because of its chemistry, (especially Ti, Al and interstitial (C and B) contents), processing and heat treatment. However, literature available in open domain to develop an understanding of these properties in alloy 720LI is rather limited. This study was taken up in this background with an objective of assessing the tensile and LCF properties exhibited by alloy 720LI within a temperature regime of interest and understand the structure-property correlations behind it. Tensile Behavior: The effect of temperature and strain rate on monotonic tensile properties were assessed at different temperature in the range of 25 – 750°C (0.67 Tm) at a strain rate of 10-4 s-1 and strain rate effects were explored in detail at 25, 400, 650 and 750°C at different strain rates between 10-5 s-1 and 10-1 s-1. Yield and ultimate tensile strength of the alloy remains unaffected by temperature till about 600°C (0.58Tm) and 500°C (0.51Tm), respectively, beyond which both decreased drastically. Negligible strain rate sensitivity exhibited by the alloy at 25 and 400°C indicated that flow stress is a strong function of strain hardening rather than strain rate hardening. However at 650 and 750°C, especially at low strain rates, strain rate sensitivity is relatively high. TEM studies revealed that heterogeneous planar slip involving shearing of precipitates by dislocation pairs was prevalent under strain rate insensitive conditions and more homogeneous slip was evident when flow stresses were strain rate sensitive. The planarity of slip is also considered responsible for the deviation in experimental data from the Ludwick–Hollomon power-law at low plastic strains in regimes insensitive to strain rate. Irrespective of strain rate sensitivity and degree of homogeneity of slip, fracture mode remained ductile at almost all the conditions studied. Dynamic Strain Ageing: Alloy 720LI exhibits jerky flow in monotonic tension at intermediate temperatures ranging from 250-475°C. After considering all known causes for serrated flow in materials, the instability in flow (Portevin-LeChatelier (PLC) effect) is considered attributable to dynamic strain ageing (DSA), arising from interactions between diffusing solute atoms and mobile dislocations during plastic flow. As the temperature range of DSA coincided with typical bore and web temperatures of turbine discs, its possible influence on tensile properties is considered in detail. No significant change in tensile strength, ductility, or work hardening is observed, due to DSA, with increase in temperature from smooth to serrated flow regime. However strain rate sensitivity, which is positive in smooth flow regime turned negative in the serrated flow regime. Analysis of serrated flow on the basis of critical plastic strain for onset of serrations revealed that in most of the temperature-strain rate regimes studied, alloy 720LI exhibits ‘inverse’ PLC effect which is a phenomenon that has not been fully understood in contrast to ‘normal’ PLC effect observed widely in dilute solid solutions. Other characteristics of serrated flow viz., stress decrement and strain increment between serrations are also analyzed to understand the mechanism of DSA. Though the activation energy determined using stress decrements suggest that carbon atoms could be responsible for locking of dislocations, based on its influence on mechanical properties and also on its temperature regime of existence, weak pinning of dislocations by substitutional solute atoms are considered responsible for DSA in alloy 720LI. LCF Behavior: LCF studies were carried out under fully reversed constant strain amplitude conditions at 25, 400 and 650°C with strain amplitudes ranging from 0.4-1.2%. Different cyclic stress responses observed depending on the imposed conditions are correlated to the substructures that evolved. Low level of dislocation activity and interactions observed in TEM is considered the reason behind stable cyclic stress response at low strain amplitudes at all temperatures. TEM studies also show that secondary γ’ precipitates that are degraded through repeated shearing are responsible for the continuous softening, observed after a short initial hardening phase, at higher strain amplitudes. Studies at 400°C show manifestation of DSA on LCF behavior at 400°C in the form increased cyclic hardening which tends to offset softening effects at higher strain amplitudes. Plastic strain dependence of fatigue lives exhibited bilinearity in Coffin-Manson plots at all temperatures. TEM substructures revealed that planar slip with deformation concentrated on slip bands is the major deformation mode under all the conditions examined. However, homogeneity of deformation increases with increase in strain and temperature. At 25°C, with increasing strain, increased homogeneity manifested in the form of increased number of slip bands. At 650°C, with increase in strain, increased dislocation activity in the inter-slip band regions lead to increased homogeneity. It is also seen that fine deformation twins that form at 650°C and low strain amplitudes play a role in aiding homogenization of deformation. Unlike other alloy systems where an environmental effect or a change in deformation mechanism leads to bilinearity in Coffin – Manson (CM) plots, our study shows that differences in distribution of slip is the reason behind bilinear CM plots. While the properties and behavior of alloy 720LI under monotonic and cyclic loading conditions over a range of temperatures could be rationalized on the basis of deformation substructures, the thesis opens up the door for further in-depth studies on deformation mechanisms in 720LI as well as other disc alloys of similar microstructure.

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