The prime design considerations for modern nickel based superalloys for use in aero gas-turbine engines, are those of mechanical performance, namely good resistance to creep and fatigue with good toughness and microstructural phase stability. Design of the current generation of superalloys has attained these properties at the expense of environmental resistance. This design philosophy has lead to the widespread use of surface coatings technology to protect hot-section componentry from the harsh operating environment. The ongoing drive towards higher operating temperatures has lead to an interest, over the last few years, in thermal barrier coatings (TBCs). TBCs are duplex coating systems consisting of a thin, insulating, ceramic layer over a metallic bondcoat. The bondcoat provides both environmental protection and the necessary adhesive interface to maintain the adherence of the ceramic during the rigours of operation. Central to the performance of a TBC system is the integrity and adherence of the alumina scale promoted by the bondcoat. This study aimed to design and optimise a novel bondcoat system that was capable of out-performing the current generation of bondcoats and progress the resultant coating into a production ready status. This was achieved by comparing the performance of a range of bondcoats of both novel and standard compositions, using the modified scratch test in conjunction with hot isothermal and cyclic furnace tests. The down selected system was then analysed using a range of techniques including optical and electron microscopy, XRD, WDS and SIMS in order to understand the failure mechanisms. The results of the testing programme lead to bondcoat chemistry changes and processing improvements that enabled better performance to be achieved. The bondcoat was optimised and taken to a production standard by using the Taguchi Method of fractional factorial experimental design. The resultant coating system offered a higher TBC/bondcoat interface temperature capability and extended the life of the system at more moderate temperatures, beyond that offered by systems currently available. The coating system has subsequently been run as a bondcoat for EB-PVD TBCs and has successfully completed the duty cycles on a number of development and test engines.

No description available.

Platinum alumnide coatings have been produced by first plating a thin layer of platinum usinq a fused salt platinum plating technique and then pack aluminizing using powder packs containingAl, NH4 C1 and Al 2 0 3 or Ni 2A1 3 , NH4 C1 and Al 203 . The chemistry and morphology of these coatings on IN-738 superalloy both in the ascoated and in the subsequently heat treated condition have been studied. The coating morphology and chemistry are highly dependent upon the thickness of the platinum layer, pack activity and time of processing. A relatively thick platinum layer (l0 pm) produced a coating with an outer Pt2A13 layer above other narrow layers. The Pt concentration decreases towards zero as the diffusion zone is approached. A second type, usually formed with a thin (5 pm) Pt layer is characterised by a marked interaction with the substrate. An outer Pt/U 2 layer is followed by a layer of NiAl containing fine precipitates of a chromiumtungsten rich phase. A lamella-like layer hiqh in chromium and other refractory elements exists at the coating/substrate interface in most of the as-coated samples. A third type of coating has been produced by a post-platinising heat treatment process prior to aluminizing. This type of coating is characterised by an outer duplex layer of PtAl 2 and Ni/U. Heat treatment of the as-formed coating results in interdiffusion between Al , Ni and Pt to produce an overall thickening of the coating layer and a decrease in the coating Al concentration. Thus a (Pt,Ni) Al or (Ni,Pt) Al outer layer may develop after heat treating these types of coatings at 1000°C for up to 1200 hours. In addition to this Widmanstatten sigma phase plates extending into the substrate are normally found beneath the outer layer after several hours' of heat treatment. Diffusion paths on pseudo-ternary phase diagrams are made to represent the phase constitution of the as-formed coatings. Isothermal oxidation tests in an oxygen atmosphere between 800 - 1000°C of different Pt-Al surfaces have been studied and the result of tests showed that the incorporation of Pt into the aluminide coatings enhance the oxidation resistance (particularly at 1000°C). Furthermore, thermal cyclic oxidation tests showed a remarkable improvement in oxide adherence over the simple aluminides.

No description available.

For the induction heating process of bi-metallic tubes, the inner tube of alloy-layer is much easier to cause a lot of defects of cavities due to the fact that heating power, maximum temperature value and the time frame of temperature retention were chosen improperly. This research focuses on the effect of maximum temperature value and the time frame of temperature retention on the micro-structure and defects of cavities of the Nickel-based alloy-layer. The experiments of this study are divided into two parts. In the domain of the experiment in simulation fashion, Nickel-based alloy powders were put into the specimens of AISI 4140 steel. Radio Frequency (RF) oven were used to smelt Nickel-based alloy powders in the vacuum conditions over the maximum temperature range of 920~1180¢Jrespectively. After that, the time frame of temperature retention was conducted from 0 to 10 minutes. Then, the furnace-cooling went down to 700¢J then air-cooling down to the room temperature. Nickel-based alloy-layer, microstructure, component analysis, defects of cavities of the interface between Nickel-based alloy-layer and steels, and diffusion of interfaces were analyzed using optical microscopes (OM) and scanning electron microscopes (SEM). From the experiments, it was found that Nickel-based alloy-layer consisted of £^-Ni¡BCrB¡BCr7C3 over the maximum temperature range of 920~1050¢Jwhether temperature retention is performed or not. According to the findings of metallographic observation, the increase of coarsening and the reduction of the capacity of CrB and Cr7C3 become more obvious as maximum temperature value and the time frame of temperature retention become large. In addition, the whitening layer (diffusion zone) formed between the interface of alloy-layer and steels become much wider as maximum temperature value and the time frame of temperature retention become large. Secondly, the field experiment method was also applied in this paper. The tube rich in Nickel-based alloy powders was heating to analyze induction coil in various conditions: heating power (200~285KW), maximum temperature value (1020~1040¢J), the time frame of temperature retention (10, 30, 50sec), and the rotating speed (1000~1300rpm). The results of the experiments indicated that the surface of the alloy-layer cause defects of vermicular cavities since the volume of liquid cannot fill out the crack of cavities completely due to lower temperature when there is insufficient time; too long periods of the time frame of temperature retention lead to the tough and huge dendrites to obstacle the flowing of liquid and the solidification of shrinkage cavity. According to the observation of the microstructure, the larger the maximum temperature value and the time frame of temperature retention were, the more the dendrites formed. The formation of dendrites causes not only the uneven distribution of hardening phase of CrB and Cr7C3 of the alloy-layer but also the reduction of hardness of the alloy-layer. The dendrites are typically formed from the interface to the surface of the alloy-layer. Besides that, the alloy-layer mainly consists of £^-Ni, Ni3B, Ni3Si, CrB, and Cr7C3 via X-ray Diffraction (XRD). Among them, the main hardening phases are CrB and Cr7C3 which is the main reason that the alloy-layer has high-level hardness. As maximum temperature value and the time frame of temperature retention become large, the whitening layer (diffusion zone) was formed between the interface of alloy-layer become much wider because the faster the elements of the based materials (tube) diffused and the wider the intermetallic compound formed among the interfaces. After heated for 800 seconds over the temperature range of 750~1030¢J, iron element was diffused all over the alloy-layer. The increase of coarsening and the reduction of the capacity near interface and interface become more obvious as maximum temperature value and the time frame of temperature retention become large.

Haynes 230 is a nickel based, solid-solution strengthened alloy that is used for high-temperature applications in the aero-engine and power generation industries. The alloy composition is balanced to avoid precipitation of undesirable topologically closedpacked (TCP) intermetallic phases, such as Sigma, Mu, or Laves-type, that are detrimental to mechanical and corrosion properties. This material is currently being used for the NASA's J2X upper stage rocket nozzle extension. Current fabrication procedures use fusion welding processes to join blanks that are subsequently formed. Cracks have been noted to occur in the fusion welded region during the forming operations. Use of solid state joining processes, such as friction stir welding are being proposed to eliminate the fusion weld cracks. Of interest is a modified friction stir welding process called thermal stir welding. Three welding process: Gas Metal Arc Welding (GMAW), Electron Beam Welding (EBW), and Thermal Stir Welding (TSWing) are compared in this study.

No description available.

No description available.

La compréhension des effets du grenaillage ultrasonore sur l’intégrité de surface des composants métalliques représente un enjeu industriel important. Dans le cadre de cette thèse, une modélisation physique multi-échelle de la plasticité cristalline à la DEM (Discrete Element Model) a été développée. Le grenaillage ultrasonore consiste à mettre en mouvement des billes à l’intérieur d’une enceinte par vibration de la sonotrode. Les impacts répétés sur le matériau entraînent un écrouissage en surface, l’établissement de contraintes résiduelles de compression et la formation d’une couche nanostructurée. L’objectif de cette thèse est d’obtenir une meilleure compréhension des mécanismes conduisant à ces modifications sur un alliage à base nickel. Le mouvement des billes obtenu par DEM est relié aux modifications de la microstructure sous impacts via un modèle éléments finis utilisant une loi de plasticité cristalline. Après validation de chaque étape par des mesures expérimentales, le modèle a permis d’étudier l’effet de la quantité de billes utilisées dans le procédé. Ainsi, un nombre croissant de billes induit une augmentation d’impacts en biais de faible vitesse permettant de concentrer les contraintes résiduelles de compression en extrême surface. De plus, les simulations multi-impacts utilisant une loi de plasticité cristalline ont montré que ces impacts en biais engendraient une densité totale de dislocations et un niveau de désorientations élevées pouvant expliquer la fragmentation des grains et la nanostructuration de la surface. Enfin, ces modifications de la microstructure, visibles jusqu’à 300 µm de profondeur, sont en accord avec les profils de dureté obtenus par nano-indentation et les profils de désorientation issus de l’analyse EBSD (Electron BackScatter Diffraction). / Ultrasonic shot peening is widely used to improve mechanical properties of metallic components. Mastering the effects of this surface treatment is a major industrial issue. A physical multi-scale modelling based on crystal plasticity and DEM (Discrete Element Model) was developed in this PhD thesis. This process is performed in a closed chamber where spherical balls are moved by sonotrode vibration. Thousands of impacts induce hardening, residual compressive stress and microstructure modification leading to a nanostructured layer. The aim of this work was to improve our understanding of the mechanisms occurring during this process on nickel-based alloys. Ball motion was computed by DEM and linked to microstructure modifications induced by impacts through a crystal plasticity finite element model. Experimental analyses were performed in order to validate each step of the multi-scale modelling. Then the model was applied to investigate ball quantity effects on peened surface modification. Increasing the number of balls created a larger quantity of low speed oblique impacts which concentrated the residual compressive stress near the surface. Furthermore multi-impacts performed with a crystal plasticity law showed oblique impacts enhanced dislocation storage and disorientations within grains, which could explain the nanostructuration of the peened surface. Moreover, the numerical microstructure modification, observed up to 300 µm in depth, was in agreement with hardness profiles obtained by nanoindentation and disorientation profiles measured by EBSD (Electron BackScatter Diffraction) analysis.

IN-738 LC, a precipitation hardened nickel based superalloy, containing substantial amounts of Al + Ti contents, is very difficult to weld due to its high susceptibility to HAZ cracking, during welding via conventional fusion welding techniques and subsequent PWHT. The cracking is mainly intergranular in nature and associated with liquation of secondary solidification products (such as MC carbides,  -  eutectic phases), and solid state reaction products ( precipitate particles) that are present along the grain boundaries in the pre-weld material. The current research was to find effective ways to improve weldability of IN-738 LC. Laser welds were produced autogenously on conventionally solidified (CS) and directionally solidified (DS) polycrystalline, and single crystal (SC) IN-738 LC subjected to two preweld heat treatments. Weldability was assessed by measuring the total crack lengths in HAZ. The influence of grain boundaries and heat treatment on extent of cracking in IN-738 LC was studied.