Alloy B is relatively new precipitation hardening superalloy. It´s applications are in the hot sections of the aero engines, rocket nozzles, gas turbines and in the chemical and petro-leum applications. The alloy is characterized by keeping high strength at elevated tempera-tures and high creep resistance. It´s excellent mechanical properties and corrosion resis-tance are due to the balanced amount of the coherent γ' matrix, combined with other alloy-ing elements and carbides. There are three types of carbides which can be found in nickel-based superalloys: MC, M 23C6 and M6C. Primary MC carbides act as source of carbon for the secondary carbides, which precipitate at the grain boundaries. They can have strengthening effect by hindering the movement of dislocations. In this work both simulation and experimental analysis are conducted in order to investi-gate the behaviour of the secondary carbides. JMatPro simulation is used to predict the behaviour of the material. Heat treatments are conducted at soak temperatures ranging from 920 °C to 1130 °C, with steps of 30 °C, and dwell times of 0.5, 1, 2 and 24 hours. Experimental methods included analysis at LOM, SEM, EDS, manual point counting and hardness tests. Main results show chromium rich M 23C6 carbides are stable at lower temperature compared to molybdenum rich M6C. Both appear as fine and discrete particles at the grain boundaries at 1070 °C. This morphology is believed to be beneficial for the mechanical properties of the alloy. The volume fraction varies between 0.6 and 1.3%. Hardness values are relevant in the range of 920-1010 °C. Above this range there is sudden drop of the hardness

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.

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The manufacturing of nickel-base superalloy forging for aero-engine applications requires a number of thermo-mechanical processing steps, which generate significant residual stresses that can result in distortion, uncontrolled deformation of the component during machining, or, when added to the in-service stresses, they will increase the crack initiation probability and hence reduce component life. Ageing treatments are applied to nickel-base superalloys in order to precipitate the strengthening phases that give to the alloys their remarkable strength over a wide temperature range. These isothermal treatments also result in a reduction of the residual stress level by means of thermal stress relaxation. In this work, the focus has been on the study of the thermal residual stress relaxation induced by the ageing treatment of wrought nickel-base superalloys. The aim of this work is to improve the understanding of the residual stress relaxation process and the different deformation mechanisms involved in the stress relaxation during isothermal treatments. This has been achieved by combining neutron diffraction with a new in-situ heating setup in order to track the elastic strain evolution in the centre of quenched disc shaped forgings of Inconel 718 and Udimet 720LI. This research has resulted in the implementation of an induction heating setup which was designed in order to serve in-situ residual stress analysis during isothermal treatments of large components at temperatures up to 1000°C. The in-situ analysis of residual stresses required the development of a standardised method which consists of monitoring the d-spacing evolution in stressed samples and in stress-free samples during isothermal treatments in order to determine the time dependent stress evolution. Stress calculations were performed using temperature dependent diffraction elastic constants which were determined experimentally for Inconel 718 and Udimet 720LI at high temperatures. The in-situ neutron diffraction measurements in water quenched disc shaped forgings of Inconel 718 of different thickness during isothermal treatments at 720°C revealed that the stress relaxation amplitude does not vary significantly with the initial stress distribution. However, the stress relaxation rate is strongly affected by the annealing temperature as stress relaxation through creep was observed to evolve at a diminishing rate during the isothermal treatment of Inconel 718 at 720°C and 750°C while no further stress relaxation occurred at 650°C. Most of the stress relaxation was found to occur during the heating stage as a result of a combination of plasticity and early stage creep relaxation. In-situ and ex-situ measurements exhibited good agreement on the amplitude of residual stress relaxation. It was found that a heat treatment at 750°C for 8h reduced the stresses by approximately 70% in Inconel 718 and only 20% in Udimet 720LI. For all in-situ experiments it was possible to fit the stress relaxation data by using logarithmic functions, which can now be used for validating and/or improving process models.

Process irregularities in remelting furnace operation have an impact on the ingot solidification structure which depends on the local thermal conditions and the alloy concerned. In this work, a laboratory investigation into the structures resulting from interruptions in solid growth is presented in order to understand the cause and effect relationship between the solidification interruption and banding. The results demonstrate the range of structure, segregation and precipitation changes which are experienced by a range of alloys including Inconel 718, Nimonic 80A, Waspaloy andM50. In the case of alloys which form primary precipitates, the interruption period is shown to give rise to structure changes. Segregations are found due to the interruption. We also comment on the differences observed between industrial examples of banding and the laboratory samples which may be due to the absence of liquid movement in the latter technique. It is concluded that the interruption in solidification condition during a directionally solidification itself can produce banding through its effects on the morphology of precipitates.

Process irregularities in remelting furnace operation have an impact on the ingot solidification structure which depends on the local thermal conditions and the alloy concerned. In this work, a laboratory investigation into the structures resulting from interruptions in solid growth is presented in order to understand the cause and effect relationship between the solidification interruption and banding. The results demonstrate the range of structure, segregation and precipitation changes which are experienced by a range of alloys including Inconel 718, Nimonic 80A, Waspaloy andM50. In the case of alloys which form primary precipitates, the interruption period is shown to give rise to structure changes. Segregations are found due to the interruption. We also comment on the differences observed between industrial examples of banding and the laboratory samples which may be due to the absence of liquid movement in the latter technique. It is concluded that the interruption in solidification condition during a directionally solidification itself can produce banding through its effects on the morphology of precipitates.

Nickel-based Superalloys are widely used in the steam turbine power generation and aerospace industries. They possess the desirable qualities of high-temperature strength, oxidation and corrosion resistance and can operate in some of the highest temperature ranges of the structural metals. The oxidation resistance of a Superalloy is achieved primarily through the formation of a dense alumina and/or chromia oxide layer(s) including spinels. This resistance has been further improved in wrought and cast alloys through the addition of reactive elements such as silicon, yttrium and lanthanum, although the exact effects of these elements have not been well defined. This project concentrated on a powder metallurgy ternary master alloy consisting of Ni-12Cr-9Fe (w/o) with additions of 6w/o aluminum, 0.5w/o Si, and 0.1w/o Y, in various combinations. Specifically, the primary goal was to produce and characterize a PM manufactured nickel-based Superalloy with minor additions of reactive elements and to assess the effectiveness of the Si and/or Y in improving the oxidation resistance. JMatPro modeling software was first used to help determine temperatures at which various events would occur in the alloys such as solutionizing and liquation temperatures. Subsequently green compacts were produced by a press (uni-axially) and sinter route to create transverse rupture strength bars (TRS bars). These bars were then thermomechanically deformed using a Gleeble tester to reduce porosity followed by a heat treatment to restore a microstructure better suited for high temperature oxidation. Sectioned TRS bars were then oxidized (static) 900?C in air for times up to 1000h and the influence of the Si/Y additions on oxidation resistance was determined via a combination of weight gain data and microstructural examination. Whereas JMatPro predicted solutionizing temperature of the compositions studied (1010°C quaternary; 1020°C quaternary + Si, respectively) these values were slightly lower than the results observed through DSC experiments (1045°C quaternary; 1065°C quaternary + Si, respectively). A w/o ?’ of approximately 25% was predicted by the modeling tool, but values of 58.3% to 61.7% were determined using a point count method. Finally, the addition of 0.5w/o Si to the quaternary Ni-Cr-Fe-Al PM system provided a measureable improvement in the oxidation resistance both in terms of thickness of oxide layer and in overall weight gain. Conversely, 0.1w/o Y provided little benefit, and was shown to be detrimental to alloys not containing Si.

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.

Process irregularities in remelting furnace operation have an impact on the ingot solidification structure which depends on the local thermal conditions and the alloy concerned. In this work, a laboratory investigation into the structures resulting from interruptions in solid growth is presented in order to understand the cause and effect relationship between the solidification interruption and banding. The results demonstrate the range of structure, segregation and precipitation changes which are experienced by a range of alloys including Inconel 718, Nimonic 80A, Waspaloy andM50. In the case of alloys which form primary precipitates, the interruption period is shown to give rise to structure changes. Segregations are found due to the interruption. We also comment on the differences observed between industrial examples of banding and the laboratory samples which may be due to the absence of liquid movement in the latter technique. It is concluded that the interruption in solidification condition during a directionally solidification itself can produce banding through its effects on the morphology of precipitates. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate