No description available.

The polycrystalline nickel-base superalloys RR1000 and Udimet 720Li (U720Li) were developed for turbine disc applications. These alloys contain a higher volume fraction of the ordered γ' phase (close to 50%) when compared to previous generation alloys (~ 25%) in order to ensure that they retain high strength at operating temperatures exceeding 700°C. The increased percentage of precipitates in the material leads to higher levels of constraint between matrix and the precipitates, and this will have consequences for the deformation mechanisms of the aggregate. It is therefore important to understand how the increased volume fraction of precipitates affects the deformation behaviour of the material. This is not only crucial for the design of the optimum microstructure, but also for lifing models, which predict the lifetime of a component. It is the aim of the present work to improve the understanding of the deformation behaviour of these alloys by focussing on the influence of the γ' precipitate size. These alloys usually comprise a complex trimodal γ' size distribution, which complicates studies on the dependence of the deformation behaviour on the precipitate size. Hence, simplified model microstructures were generated here with a unimodal γ' size distribution. The model microstructures were subjected to in-situ loading experiments with neutron diffraction at temperatures of 20°C, 500°C and 750°C. Neutron diffraction measurements during loading revealed the elastic lattice strain response of both the γ and the γ' phases, which can indicate changes in their respective deformation behaviour. These measurements showed a load transfer from γ to γ' for some test conditions, which indicated that γ was able to deform with noticeably less deformation in the γ' phase. With a larger γ' precipitate size and/or higher test temperature, the tendency for load transfer increased. A post-mortem analysis of the deformed microstructures using advanced electron microscopy techniques (EBSD, ECCI, TEM) showed that shearing of the γ' precipitates dominated the deformed microstructures at 20°C and 500°C and was also observed after deformation at 750°C. Deformation was less localised in the microstructures with large γ' precipitates, which might be correlated with the increased trend for load transfer. The onset of multiple slip or the activation of Orowan looping as an additional deformation mechanism are suggested as possible explanations for these observations.

No description available.

Directionally solidified (DS) nickel-base superalloys are used in high temperature gas turbine engines because of their high yield strength at extreme temperatures and strong low cycle fatigue (LCF) and creep resistance. Costly inspecting, servicing, and replacing of damaged components has precipitated much interest in developing models to better predict service life. Turbine blade life modeling is complicated by the presence of notches, dwells, high temperatures and temperature gradients, and highly anisotropic material behavior. This work seeks to develop approaches for predicting the life of hot sections of gas turbines blade material CM247LC DS subjected to LCF, dwells, and stress concentrations while taking into consideration orientation and notch effects. Experiments were conducted on an axial servo-hydraulic MTS® testing machine. High temperature LCF tests were performed on smooth and notched round-bar specimens in both longitudinal and transverse orientations with and without dwells. Experimental results were used to develop and validate an analytical life prediction model. An analytical model based on a multiaxial Neuber approach predicts the local stress-strain response at a notch and other geometric stress concentrations. This approach captures anisotropy through a multiaxial generalization of the Ramberg-Osgood relation using a Hill's type criterion. The elastic notch response is determined using an anisotropic elastic finite element analysis (FEA) of the notch. The limitations of the simpler analytical life-modeling method are discussed in light of FEA using an anisotropic elastic-crystal viscoplastic material model. This life-modeling method provides a quick alternative to time demanding elastic-plastic FEA allowing engineers more design iterations to improve reliability and service life.

An extensive review of probabilistic techniques in fatigue analysis indicates that there is a need for new microstructure-sensitive methods in describing the effects of notches on the fatigue life reduction in cyclically loaded components. Of special interest are notched components made from polycrystalline nickel-base superalloys, which are used for high temperature applications in aircraft gas turbine engine disks. Microstructure-sensitive computational crystal plasticity is combined with novel probabilistic techniques to determine the probability of failure of notched components based on the distribution of slip within the notch root region and small crack initiation processes. The key microstructure features of two Ni-base superalloys, a fine and coarse grain IN100, are reviewed and the method in which these alloys are computationally modeled is presented. Next, the geometric model of the notched specimens and method of finite element polycrystalline reconstruction is demonstrated. Shear-based fatigue indicator parameters are used to characterize the shear-based, mode I formation and propagation of fatigue cracks. Finally, two different probabilistic approaches are described in this work including a grain-scale approach, which describes the probability of forming a crack on the order of grain size, and a transition crack length approach, which describes the probability of forming and propagating a crack to the transition crack length. These approaches are used to construct cumulative distribution functions for the probability of failure as a function of various notch root sizes and strain load amplitudes.

The topic of this thesis is the formation of microporosity in cast test bars of nickel-base superalloy Inconel 713LC during solidification. The theoretical part consists of nickel alloys, their macrostructure, the method of casting and crystallization of nickel-based superalloys. The second part is devoted to the design of casting conditions, the production of shell molds, the size and shape of grains, evaluation of microporosity by various available methods, comparing the influence of grain size and the proportion of microporosity on the resulting values of the mechanical tests. Experiments were carried out in cooperation with PBS Velká Bíteš and ÚST foundry department.

No description available.

L'alliage N19 est un superalliage base nickel, élaboré par métallurgie des poudres, qui a été développé récemment en vue d'une application pour disques de turbine aéronautique. L'objectif de cette étude est d'optimiser la microstructure de cet alliage en agissant sur les paramètres de traitements thermiques pour améliorer les propriétés mécaniques de l'alliage. Une bonne compréhension des relations entre les paramètres de traitements thermiques et la microstructure, d'une part, et, des relations entre la microstructure et les propriétés mécaniques, d'autre part, est donc nécessaire. De nombreux traitements thermiques ont été appliqués à l'alliage N19 pour évaluer l'effet de la température de mise en solution, des conditions de refroidissement et de la température de revenu sur la taille de grains, et sur la taille et la distribution des précipités gamma prime. L'observation des microstructures en microscopie électronique à balayage et en transmission a permis d'évaluer l'effet des différentes étapes du traitement thermique sur les caractéristiques microstructurales de l'alliage. L'effet de ces modifications microstructurales sur la vitesse de propagation de fissure en fatigue-fluage à 650°C a été étudié. Les résultats de ces essais de propagation de fissure en fatigue-fluage ont été analysés à l'aide d'essais de comportement en fatigue-relaxation. Une synthèse des différentes propriétés mécaniques de l'alliage en fonction des paramètres de traitements thermiques et des caractéristiques microstructurales a été proposée. / The N19 alloy is a powder metallurgy nickel-based superalloy which has recently been developed for aircraft engine turbine disks. The aim of this study is to optimize the N19 microstructure through the adjustment of the heat treatment parameters in order to enhance the mechanical properties of this alloy. A good understanding of the relationships between the heat treatment parameters and the microstructure, and between the microstructure and the mechanical properties is required. Numerous heat treatments were applied to the alloy to investigate the effect of the solutionizing temperature, the cooling path, and the ageing temperature on the grain size, and on the gamma prime precipitates size and distribution. The observation of the microstructures using scanning electron microscopy and transmission electron microscopy supports the analysis of the heat treatment parameters effects on the microstructural features of the alloy. The effect of the microstructural modifications on the creep fatigue crack growth rate at 650°C was studied. The results of the creep fatigue crack growth tests were analyzed using cyclic stress-relaxation tests. A synthesis of the various mechanical properties of the alloy according to the heat treatment parameters and microstructural features is proposed.