Ni-based superalloys are used for high-temperature components of gas turbines in both industrial and aerospace applications due to their ability to maintain dimensional stability under conditions of high stress and strain. The oxidation resistance of these alloys often dictates their service lifetime. This study focuses on the isothermal oxidation behaviour of three nickel-based superalloys; namely, polycrystalline cast IN738LC, single-crystal N5 and a ternary Ni-Fe-Cr (TAS) powder metallurgy alloy. The isothermal oxidation tests were conducted at 900°C in the static air up to 1000h and the specific aspects studied were the oxidation behaviour of these chromia-forming and alumina-forming alloys that are used extensively in industry. In particular, the behaviour of oxide scale growth and subsurface changes were analysed in detail using various techniques such as SEM, EDS and AFM. From the isothermal oxidation kinetics, the oxidation rate constant, kp was calculated for each alloy and found to be; kp = 2.79 x 10-6 mg2.cm-4.s-1 for IN738LC, kp = 1.42 x 10-7 mg2.cm-4.s-1 for N5 and kp = 1.64 x 10-7 mg2.cm-4.s-1 for TAS. Based on a microstructural analysis, IN738LC exhibited a continuous dense outer scale of Cr2O3 and discontinuous inner scale of Al2O3, whereas N5 and TAS showed a dense outer scale of Al2O3 alone. The results suggested that the N5 and PM-TAS alloys are more oxidation resistant than the IN738LC under these conditions. / October 2016

The heat-treating industry is in need of heat- treatment furnace materials and fixtures that have a long service life and reduced heat capacity. Based on microstructural analysis of components that were used until failure in carburization furnace application, it was found that the primary reason for failure was the excessive carburization that leads to “metal dustingâ€� and subsequent cracking. Aluminizing is widely used to increase the high temperature oxidation and carburization resistance of nickel- based alloys. In this dissertation, RA330, RA602CA, 304L/316L, Inconel 625 alloys were selected to study their performance in an industrial carburization furnace for times up to two years. These alloys were exposed in both the as-fabricated and aluminized condition. The test samples were exposed to Cp=0.7-1.3% carburizing atmosphere at approximately 900℃ for 3 months, 6months, 12months, 18 months and 24months. The oxidation properties and oxide stability at high temperatures will be presented. In addition, the analysis of microstructural development during long term exposure experiments in an industrial carburizing furnace will be presented. These samples were characterized using optical and scanning electron microscope, EBSD, and x-ray diffraction. It was found that the aluminized alloys exhibited lower weight gain and carbon uptakes.

This research focuses on studying the microstructure of alloy 909, its susceptibility to oxidation at elevated temperatures (~700°C) and substrate coatings compatibility with high velocity oxy fuel (HVOF) sprayed oxidation resistance coatings. The characterization work involved in studying the microstructure of Incoloy 909 at three heat treated conditions namely solution treated condition (ST), commercially recommended solution heat treated and aged condition (STA), and solution treated and over aged condition (STOA) using optical microscopy, analytical scanning electron microscopy, and analytical transmission electron microscopy. The oxidation susceptibility were investigated at elevated temperatures of bare and coated alloy 909 substrates by subjecting test materials to isothermal and thermal cycle testing. The microstructure of alloy 909 in the ST condition showed only the presence of blocky Laves phase. The Laves phase in this alloy is a well known for its grain pinning effect that prevents excessive grain growth. In the STA condition, the microstructure revealed the presence of fine gamma prime, intergranular and intragranular Laves phase and occasionally gamma prime precipitates orienting in a platelet form ready to transition into the epsilon phase. In the STOA condition, the microstructure consisted of Laves phase in inter and intragranular locations, and a copious amount of Widmanstatten type epsilon phase. Incoloy 909 was observed to form oxide scales in both isothermal and cyclic thermal exposures, the oxide scale consisted of distinct outer and inner scales in the micrographs. The comparison base alloy (alloyl 718) used in this study surprisingly did not show any visible presence of oxide scale after 1000 hour exposure at ~700°C. Three coatings (CoNiCrAlY, 718 , and NiAl) were sprayed on alloy 909 and alloy 718 test coupons using the HVOF process to investigate the compatibility of the coatings with the substrate. The test results points out that all the coatings were compatible with 718 substrate and only one coating (NiAl) was found compatible with the Alloy 909 substrate, indicating that the coatings that are compatible with one substrate may not be compatible with another alloy within the same family of alloys.

This research focuses on studying the microstructure of alloy 909, its susceptibility to oxidation at elevated temperatures (~700°C) and substrate coatings compatibility with high velocity oxy fuel (HVOF) sprayed oxidation resistance coatings. The characterization work involved in studying the microstructure of Incoloy 909 at three heat treated conditions namely solution treated condition (ST), commercially recommended solution heat treated and aged condition (STA), and solution treated and over aged condition (STOA) using optical microscopy, analytical scanning electron microscopy, and analytical transmission electron microscopy. The oxidation susceptibility were investigated at elevated temperatures of bare and coated alloy 909 substrates by subjecting test materials to isothermal and thermal cycle testing. The microstructure of alloy 909 in the ST condition showed only the presence of blocky Laves phase. The Laves phase in this alloy is a well known for its grain pinning effect that prevents excessive grain growth. In the STA condition, the microstructure revealed the presence of fine gamma prime, intergranular and intragranular Laves phase and occasionally gamma prime precipitates orienting in a platelet form ready to transition into the epsilon phase. In the STOA condition, the microstructure consisted of Laves phase in inter and intragranular locations, and a copious amount of Widmanstatten type epsilon phase. Incoloy 909 was observed to form oxide scales in both isothermal and cyclic thermal exposures, the oxide scale consisted of distinct outer and inner scales in the micrographs. The comparison base alloy (alloyl 718) used in this study surprisingly did not show any visible presence of oxide scale after 1000 hour exposure at ~700°C. Three coatings (CoNiCrAlY, 718 , and NiAl) were sprayed on alloy 909 and alloy 718 test coupons using the HVOF process to investigate the compatibility of the coatings with the substrate. The test results points out that all the coatings were compatible with 718 substrate and only one coating (NiAl) was found compatible with the Alloy 909 substrate, indicating that the coatings that are compatible with one substrate may not be compatible with another alloy within the same family of alloys.

No description available.

An experimental investigation to study the influence of base metal composition on the time required to achieve complete isothermal solidification (tf) during TLP bonding of three Ni-base superalloys was performed. Alloys IN 738, DS Rene80 and DS IC 6 show similar behaviour when bonded at 1100 oC, with comparable tf. However, at higher temperatures, IN 738 requires extended period of time (as compared to DS Rene80 and DS IC 6) to achieve complete isothermal solidification. The prolonged tf in IN 738 appears to be caused by a more pronounced reduction in concentration gradient of the diffusing solute within the material during bonding. In contrast, the shorter complete isothermal solidification time experienced by alloy DS IC6 is attributable to its capability to better accommodate the diffusing solute, through the formation of densely packed second-phase precipitates in the diffusion affected zone (DAZ).

An experimental investigation to study the influence of base metal composition on the time required to achieve complete isothermal solidification (tf) during TLP bonding of three Ni-base superalloys was performed. Alloys IN 738, DS Rene80 and DS IC 6 show similar behaviour when bonded at 1100 oC, with comparable tf. However, at higher temperatures, IN 738 requires extended period of time (as compared to DS Rene80 and DS IC 6) to achieve complete isothermal solidification. The prolonged tf in IN 738 appears to be caused by a more pronounced reduction in concentration gradient of the diffusing solute within the material during bonding. In contrast, the shorter complete isothermal solidification time experienced by alloy DS IC6 is attributable to its capability to better accommodate the diffusing solute, through the formation of densely packed second-phase precipitates in the diffusion affected zone (DAZ).

Compte tenu de l’évolution des températures d’entrée de turbines, le principal enjeu pour les métallurgistes est d’élaborer des alliages capables de présenter d’excellentes propriétés mécaniques à des températures de plus en plus élevées. Dans ce contexte, connaitre les relations entre les différents paramètres microstructuraux et les propriétés mécaniques à haute température (700°C et plus) est capital.La durabilité en fluage et en fatigue-temps de maintien ainsi que les processus d’endommagement de l’alliage AD730™, un nouveau superalliage pour disques de turbines, ont été analysés. Plusieurs paramètres ont été étudiés, qu’ils soient microstructuraux (joints de grains, taille de grains, taille et distribution des précités γ′), ou expérimentaux (température, environnement, contrainte appliquée, temps de maintien). L’utilisation de monograins de composition chimique identique à l’alliage de l’étude a permis de mettre en évidence le fait qu’une microstructure monogranulaire ne présente pas nécessairement de meilleures propriétés en fluage qu’une microstructure polycristalline. Ceci est attribué au rôle durcissant des joints de grains. Il a de plus été montré qu’à 700°C, la taille et la distribution des précipités γ′ est le paramètre microstructural pilotant les propriétés viscoplastiques à l’ordre 1.En fatigue avec temps de maintien, un comportement original a été observé pour les longs temps de maintien en fonction de la contrainte appliquée. Ce phénomène est attribué à un effet « type Bauschinger » apparaissant lors des phases de décharges. / In view of the turbine entry temperature evolution, the main challenge for metallurgists is to elaborate new alloys able to withstand higher temperatures while keeping great mechanical properties. Therefore, knowing the relationships between microstructural parameters and mechanical properties at high temperatures (700°C and more) is mandatory.The creep and dwell-fatigue durability as well as the damage mechanisms of AD730™, a new nickel base superalloy developed for turbine disks, have been analyzed. Several microstructural parameters were studied (grain boundaries, grain size, size and distribution of γ′ precipitates) as well as experimental parameters (temperature, environment, applied stress or dwell period). By using single crystalline specimens having the same chemical composition of the studied alloy, it has been shown unambiguously that single crystalline microstructures do not necessarily present better creep properties compared to polycristalline ones. This result is supposed to be caused by a grain boundary strengthening mechanism. Moreover, in creep at 700°C, it has been shown that the main viscoplasticity controlling parameters are the size and distribution of γ′ precipitates.An unexpected dwell-fatigue behavior has been observed for long hold times and in a specific applied stress window. This phenomena is attributed to a “Bauschinger type” effect, occurring during unloading phases.

Transient Liquid phase bonding (TLP) has evolved as a viable method of joining difficult-to-weld superalloys with potential of producing joints with comparable mechanical properties to the base material. Although the high temperature properties of aerospace superalloys have been studied extensively, there is little information on the corrosion behaviour of these special class of materials that had been subjected to TLP bonding. In this work, electrochemical assessment of the corrosion behaviour of TLP bonded nickel-based superalloy was performed. Microstructural evaluation of the TLP bonded joint revealed the presence of a centreline eutectic when isothermal solidification was not completed and the corrosion resistance increased with a decrease in this eutectic width. The use of a composite interlayer produces TLP joints with smaller eutectic size and results in complete isothermal solidification in shorter processing time. Complete isothermal solidification, achieved with the composite interlayer, results in a uniform chromium distribution in the joint centre and produced a corrosion performance similar to the as-received cast base metal. It was found that aside from the mere presence of chromium, which is widely recognised as necessary for corrosion resistance, its uniform distribution within the joint region is imperative for achieving adequate corrosion resistance in TLP joints. / May 2016

Requirements for increased operating efficiencies mean that future generations of aero-engines will need to operate at temperatures beyond the capabilities of current nickel-base superalloys. As a result, new alloy compositions for turbine disc applications are being developed. Optimising these alloy compositions requires balancing directly competing requirements. Increased Cr contents are needed to provide environmental resistance and increased concentrations of other refractory metals to improve solid solution strengthening. However, these elements compromise the alloyâs long-term microstructural stability by promoting the formation of topologically close-packed (TCP) phases, which are deleterious to alloy performance. High $\gamma^\prime$ volume fractions, which are needed to provide high-temperature strength, exacerbate the problem by increasing the concentration of these elements in the $\gamma$ phase. Therefore, an understanding of TCP formation and the compositional limits of stability is vital in the design of new alloys. This thesis presents a combination of fundamental studies of TCP phase formation in model alloys and microstructural assessment of the thermal stability of developmental alloy compositions. Knowledge of the effect of individual elements on thermal stability is important to enable the development of optimised alloy compositions. As a result, the first fundamental study investigated the effect of Co content on thermal stability. An unexpected transition in $\sigma$ precipitation behaviour after 500 hours at 800°C was observed between 12 and 16 at.\% Co. It is proposed that this behaviour may be due to the effect of Co on the $\gamma$/$\gamma^\prime$ partitioning behaviour of other elements. Preliminary results from further fundamental studies investigating the effect of the Mo/W ratio and B content on thermal stability are also presented. Decreasing the Mo/W ratio was found to reduce the quantity of $\sigma$ precipitation and promote the precipitation of a W-rich phase. B additions were found to promote the precipitation of the M$_3$B$_2$ phase. Thermodynamic predictions are frequently used to inform alloy design as an alternative to time-consuming and costly experiments. However, the accuracy of solvus temperature predictions for TCP phases has not been thoroughly considered. In this work, it was found that differential scanning calorimetry could be used as a means of measuring $\sigma$ solvus temperature in a series of alloys designed to be sufficiently unstable with respect to $\sigma$ precipitation. Comparison of experimental results with thermodynamic solvus temperature predictions revealed a significant underprediction of the $\sigma$ solvus temperatures for all of the studied alloys. This can inform our use of such predictions during alloy design. The ability to quantify the amount of TCP precipitation that occurs is extremely important when assessing the thermal stability of alloys. A new method was applied to the problem of TCP quantification, involving synchrotron X-ray diffraction of solid aged samples. This was an attempt to avoid some of the problems identified with the commonly used quantification method, which involves electrolytic extraction of minor phases, and assess the accuracy of the results produced by this method. Samples of a currently used commercial alloy, RR1000, were investigated following ageing for up to 5000 hours at 800°C, revealing the evolution of phases at this temperature. The presence of extremely low quantities of minor phases was successfully detected in the solid samples using this method. However, these quantities were too low for this to be a reliable method of quantification for commercial alloys. In parallel with these fundamental and technique-based studies, the thermal stability of a number of candidate alloys, which were developed during the design of a next-generation disc alloy by Rolls-Royce, was assessed. The alloys were characterised following a variety of thermal exposure temperatures and durations, which were determined by industrial needs at the time. Various minor phases were identified depending on the alloy compositions, including the TCP phases, $\sigma$ and $\mu$, as well as MC and M$_{23}$C$_6$ carbides and M$_3$B$_2$ borides.