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A new pre-weld heat treatment was developed recently for IN 738 superalloy. Therefore, the research described in this master’s thesis was to study the laser weldability improvement of the alloy which had undergone the new heat treatment, FUMT and the results were compared to an existing pre-weld heat treatment, solution heat treatment (SHT). Also, thermomechanical fatigue (TMF) behavior of the welded alloy was investigated. The results showed that IN-738 welded in FUMT condition had about 70% reduction in weld cracking compared to SHT treated alloy. TMF tests showed that at higher strain ranges, the TMF life of the samples of the alloy welded in FUMT condition is comparable to unwelded samples, while at lower strain ranges, welded samples of the alloy in FUMT condition has a higher TMF life than unwelded samples. This shows that welding IN 738 superalloy in FUMT does not degrade the TMF performance of the alloy.

Commercial superalloys like Rene88DT are used in high temperature applications like turbine disk in aircraft jet engines due to their excellent high temperature properties, including strength, ductility, improved fracture toughness, fatigue resistance, enhanced creep and oxidation resistance. Typically this alloy's microstructure has L12-ordered precipitates dispersed in disordered face-centered cubic γ matrix. A typical industrially relevant heat-treatment often leads to the formation of multiple size ranges of γ¢ precipitates presumably arising from multiple nucleation bursts during the continuous cooling process. The morphology and distribution of these γ′ precipitates inside γ matrix influences the mechanical properties of these materials. Therefore, the study of thermodynamic and kinetic factors influencing the evolution of these precipitates and subsequent effects is both relevant for commercial applications as well as for a fundamental understanding of the underlying phase transformations. The present research is primarily focused on understanding the mechanism of formation of different generations of γ′ precipitates during continuous cooling by coupling scanning electron microscopy (SEM), energy filtered TEM and atom probe tomography (APT). In addition, the phase transformations leading to nucleation of γ′ phase has been a topic of controversy for decades. The present work, for the first time, gives a novel insight into the mechanism of order-disorder transformations and associated phase separation processes at atomistic length scales, by coupling high angle annular dark field (HAADF) - STEM imaging and APT. The results indicate that multiple competing mechanisms can operate during a single continuous cooling process leading to different generations of γ′ including a non-classical mechanism, operative at large undercoolings.

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SUMMARY GTD-111 DS is a directionally solidified superalloy currently used in turbine engines. To accurately predict the life of engine components it is essential to examine and characterize the microstructural evolution of the material and its effects on material properties. The as-cast microstructure of GTD-111 is highly inhomogeneous as a result of coring. The current post-casting heat treatments do not effectively eliminate the inhomogeneity. This inhomogeneity affects properties including tensile strength, fracture toughness, fracture path, and corrosion behavior, primarily in terms of the number of grains per specimen. The goal of this work was to link microstructural features to these properties. Quantitative fractography was used to determine that the path of cracks during failure of tensile specimens is influenced by the presence of carbides, which are located in the interdendritic regions of the material as dictated by segregation. The solvus temperature of the precipitate phase, Ni3(Al, Ti), was determined to be 1200C using traditional metallography, differential thermal analysis, and dilatometry. A heat-treatment was designed to homogenize the microstructure for tensile testing that isolates the carbide by dissolving all of the eutectic Ni3(Al, Ti) precipitate phase, which is also found in the interdendritic areas. High temperature oxidation/sulfidation tests were conducted to investigate the corrosion processes involved when GTD-111 DS is utilized in steam and gas combustion turbine engines. The kinetics of corrosion in both oxidizing and sulfidizing atmospheres were determined using thermogravimetric analysis. Additionally, metallography of these samples after TGA revealed a correlation between the presence of grain boundaries and sulfur attack, which led to catastrophic failure of the material under stress-free conditions in a sulfur bearing environment. In summary, this work correlates the inhomogeneous microstructure of GTD-111 DS to tensile fracture, and the corrosion process in turbine engines.

New methods in the fabrication of top coat and bond coat have been introduced to improve the efficiency and performance of advanced thermal barrier coatings (TBCs).i. Top coat. Thick yttria-stabilized-zirconia (YSZ) coatings (300-400 μm) have been fabricated by using electrophoretic deposition (EPD) method. The EPD coatings have more favorable microstructures with uniformly distributed porosity and stronger bonding, in comparison with conventional air-plasma spray (APS) coatings. ii. Bond coat. Pt-diffused single γ’-phase bond coat has been fabricated by applying selective etching prior to the electroplating of Pt on CMSX-4 single crystal superalloys. The concern on the compromised scale adhesion caused by the depletion of Pt is effectively avoided, as Pt remains stable in a coherent γ’-phase layer after long-term diffusion and oxidation. Considerable cost of Pt could also be reduced. Commercial TBCs, comprising an electron beam physical vapour deposition (EBPVD) top coat, a Pt-enriched intermetallic bond coat and a CMSX-4 single crystal superalloy, have also been investigated focusing on the failures that typically occurred at the scale/alloy interface. Advanced characterization techniques have been used to study the chemical factors (Al, Pt, S, Hf, etc.) that determine the durability of TBCs. Mechanisms have been discussed that control the TBCs behaviours of diffusion, oxidation, and adhesion. i. Diffusion. A depletion of Pt near the scale/alloy interface inevitably occurs at high temperatures, which significantly weakens the scale adhesion. Mechanisms controlling the diffusion of Pt in Ni-based single crystal superalloys at high temperatures have been investigated focusing on the evolution of phase, microstructure, and composition. It was found that Pt has negative chemical interactions with Al, Ti and Ta, all of which could stabilize Pt in β- and γ’-phases, and therefore avoid the depletion of Pt. ii. Oxidation. Selective oxidation behaviour of Ni-based superalloys has been studied by using thermodynamic calculations, which is mainly affected by alloy compositions, oxygen partial pressures and temperatures. It was found that the formation of a protective α-Al2O3 scale is more favoured under lower oxygen partial pressures and higher temperatures. The additions of Al and Pt in Ni-based superalloys could also promote the formation of Al2O3 and the exclusion of NiO and spinel. The additions of reactive elements (RE), however, are less effective and may even cause severe internal oxidations due to a competitive oxidation between Al and RE.iii. Adhesion. Sulphur effect in TBCs mainly refers to a segregation of sulphur at the scale/alloy interface, which significantly deteriorates the scale adhesion to alloys. High resolution secondary ion mass spectrometry (Nano-SIMS) was employed to trace sulphur in commercial TBCs. The undesired “sulphur effect” on scale adhesion was suggested to be caused by the formation of residual sulphides beneath the scale with weaker ionic bonding to alloy cations, rather than a segregation of sulphur atoms. Possible solutions have been suggested to alleviate the sulphur effect in TBCs.

Effects of short-term overtemperature on Rene 80 DS were studied using Gleeble thermo-mechanical simulation apparatus. Volume fraction of gamma prime to gamma matrix was quantified and used to assess the effects of time, temperature and stress. Volume fraction was found to decrease with increasing temperature and time. Dissolution was found to occur through both solid and liquid state dissolution. Application of tensile stress was found to influence dissolution behaviour of gamma prime. / May 2017

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