The titanium alloy, Ti-6Al-4V, is vastly studied and used in many applications because it has a transformation microstructure, which can be tailored for apt properties that are consistent up to 500°C. Compared to conventional steels, this alloy favours certain applications due to its high specific strength, hardenability, corrosion resistance, biocompatibility and weldability. Its weldability makes the alloy a good candidate for additive manufacturing (AM). Ti-6Al-4V parts are widely built by the AM process of electron beam melting (EBM). However, heat transfer remains crucial in EBM process. The high intensity localized, moving, electron beam heat source and the rapid self-cooling are critical, especially in thin parts/ sections. When thin sections are built by the EBM process, there will be microstructural variation in their build direction, which can lead to the variation of their mechanical properties. It is necessary to understand the microstructure and mechanical properties of thin sections when they are used as functional parts in various applications in aerospace, automotive, medical, etc. industries. The microstructure, tribological behaviour and mechanical properties of Ti-6Al-4V, as-built EBM thin plates were studied by means of various hardness, scratch and tensile testing. The hardness and scratch tests were performed on the thin plates to correlate the microstructural variation. In-situ micro tensile test was performed inside the scanning electron microscope (SEM), to see the sample’s deformation behaviour. Microstructural characterization revealed equiaxed grains in the transverse section and the longitudinal surface exhibited columnar grains elongated along the build direction. The size of the equiaxed grains are found to vary across the thickness of the plate. The indentation and scratch hardness also vary in correlation with the varying grain size across the plate’s thickness. The micro tensile results reveal that the tensile properties of the thin plate are comparable to that of its bulk Ti-6Al-4V counterpart.

This project is in collaboration with the Hydro-Quebec Research Institute (IREQ) and the Canadian Centre for Electron Microscopy (CCEM) on the mechanical test performance of miniature-scale micro-tensile specimens. The objective of the thesis project is to create an efficient and reliable fabrication route for producing micro-tensile specimens and to validate the accuracy of a newly custom-built micro-tensile bench at IREQ. The fabrication techniques developed and outlined in this thesis use the underlying fundamental physical mechanisms of secondary electron microscopy (SEM), focused-ion beam (FIB), and the femtosecond (fs)-laser machining for producing optimal quality micro-tensile specimens. The mechanical testing of the specimens is geared towards studying the localized deformation occurring in the microstructure when the size of the specimen only limits a number of grains and grain boundaries in order to target the specific detailed measurement of the mechanical behaviour of individual grains and interfaces. The goal for creating an optimal fabrication route for micro-tensile specimens is to carry out micro-mechanical testing of the primary turbine steels of 415 martensitic stainless steel used in the manufacture of Francis turbine components at Hydro-Quebec. The mechanical testing of single phase and interphase interface 415 steel micro-tensile specimens are considered building blocks to developing digital twin models of the steel microstructure. The experimental data from the mechanical tests would be fed into the crystal plasticity finite element models (CPFEM) that are currently being developed by researchers at IREQ. With the development of digital twin models, engineers at IREQ would be able to predict crack initiation at the microstructure level (prior to crack propagation into macro-scale cracks) by observing the evolution of the grain’s crystallographic orientation and morphology, as well as deformation mechanisms such as martensite formation and twinning produced from localized induced strains in the microstructure. In addition, self-organized dislocation processes such as dislocation nucleation and dislocation escape through the free surface can also be studied using the CPFEM models for size-limited mechanical deformation behaviour of miniature-scale mechanical test specimens. The fabrication routes studied in this thesis project use the combination of the fs-laser and plasma focused ion beam (PFIB) to machine the micro-tensile specimens. (100) single crystal copper was the ideal material chosen to validate the accuracy of the micro-tensile bench and quality of the fs-laser-machined tensile specimens, due to its ductile nature and well-characterized properties studied in literature. A mechanical size effect was studied for single crystal copper specimens with different gauge thicknesses. It was observed from the micro-tension testing that the strength of the specimens increased with decreasing gauge thickness occurring in the size-limited tensile gauges. In addition, it was determined there was negligible differences in the size effect seen between the PFIB-machined copper micro-tensile specimens and the fs-laser-machined micro-tensile specimens, demonstrating that the fs-laser is a reliable machining route for the micro-tensile specimens. X-ray computed tomography was used to validate the correct geometry of the machined gauge section produced from an innovative gauge thinning method adopted from IREQ’s research collaborator, Dr. Robert Wheeler. As well, finite-element analysis (FEA) was performed to determine the deformation behaviour under both linear-elastic and non-linear elastoplastic conditions of (100) copper and 415 steel models simulated in pure tension, prior to the fabrication of the micro-tensile specimens, respectively. Furthermore, significant progress has been made towards targeting martensite grains in the 415-steel microstructure using electron backscattered diffraction (EBSD) analysis to produce single crystal and interphase interface micro-tensile specimens. A workflow towards grain targeting using EBSD analysis has been developed, as well as for the relocation of grains using reference fiducial marks for future fabrication of the single crystal and interphase interface 415 micro-tensile specimens. / Thesis / Master of Applied Science (MASc) / Hydro-Quebec is an energy utilities company that operates the design of Francis hydro-turbines to supply hydroelectric power across the province of Quebec. The hydro-turbines have an expected service life of 70 years. Unfortunately, the turbines can get replaced by new ones prior to reaching half of its service life, due to the development of severe fatigue crack growth in the primary components of the turbines. A solution proposed by the researchers at the Hydro-Quebec Research Institute (IREQ) is to determine a linkage between the turbine’s steel’s microstructure and the mechanical behaviour of the turbine steels. Deformation of the material starts at the microstructure level, where dislocations glide through the material lattice, causing both reversible (elastic) and irreversible (plastic) deformation. Therefore, a solution was proposed by the researchers at IREQ to create computational models of the steel microstructure to predict the deformation of the steel’s microstructure. Being able to predict the deformation mechanisms through the simulation models of the microstructures allows for engineers at Hydro-Quebec to schedule regular maintenance of the turbines more efficiently and provide metallurgists the knowledge on what is occurring at the microstructure level and what can be done to improve the chemical and physical composition of the steel. To develop the digital twin models, experimental data must be collected through mechanical testing of miniature mechanical test specimens of the turbine steels. The mechanical properties of the single phases and interphase interface specimens are fed into the models as building blocks to building a microstructure map of the turbine steels. Micro-tension testing of micro-tensile specimen provides direct information about the material’s mechanical properties. In this work, a reliable and efficient fabrication route for micro-tensile specimens was developed for the purpose of extracting mechanical properties of single phase and interphase interface turbine steel specimens using focused ion beam (FIB) and femtosecond laser machining.

Pt-aluminide (PtAl) coatings form an integral part of thermal barrier coating (TBC) systems that are applied on Ni-based superalloy components operating in the hot sections of gas turbine engines. These coatings serve as a bond coat between the superalloy substrate and the ceramic yttrium stabilized zirconia (YSZ) coating in the TBC system and provide oxidation resistance to the superalloy component during service at high temperatures. The PtAl coatings are formed by the diffusion aluminizing process and form an integral part of the superalloy substrate. The microstructure of the PtAl coatings is heavily graded in composition as well as phase constitution. The matrix phase of the coating is constituted of the B2-NiAl phase. Pt, in the coating, is present as a separate PtAl2 phase as well as in solid solution in B2-NiAl. The oxidation resistance of the PtAl bond coat is derived from the B2-NiAl phase. At high temperatures, Al from the B2-NiAl phase forms a regenerative layer of alumina on the coating surface which, thereby, lowers the overall oxidation rate of the superalloy substrate. The presence of Pt is beneficial in improving the adherence of the alumina scale to the surface and thereby enhancing the oxidation resistance of the coating. However, despite its excellent oxidation resistance, the B2-NiAl being an intermetallic phase, renders the PtAl coating brittle and imparts it with a high brittle-to-ductile-transition-temperature (BDTT). The PtAl coating, therefore, remains prone to cracking during service. The penetration of these cracks into the substrate is known to degrade the strain tolerance of the components. Evaluation of the mechanical behavior of these coatings, therefore, becomes important from the point of views of scientific understanding as well as application of these coatings in gas turbine engine components. Studies on the mechanical behavior of coatings have been mostly carried on coated bulk superalloy specimens. However, since the coating is brittle and the superalloy substrate more ductile when compared to the coating, the results obtained from these studies may not be representative of the coating. Therefore, it is imperative that the mechanical behavior of the coating in stand-alone condition, i.e. the free-standing coating specimen without any substrate attached to it, be evaluated for ascertaining the true mechanical response of the coating. Study of stand-alone bond coats involves complex specimen preparation techniques and challenging testing procedures. Therefore, reports on the evaluation of mechanical properties of stand-alone coatings are limited in open literature. Further, no systematic effort has so far been made to examine important aspects such as the effect of temperature and strain rate on the tensile behavior of these coatings. The deformation mechanisms associated with these bond coats have also not been reported in the literature. In light of the above, the present research study aims at evaluating the tensile behavior of free-standing PtAl coatings by the micro-tensile testing technique. The micro-tensile testing method was chosen for property evaluation because of its inherent ability to generate uniform strain in the specimen while testing, which makes the results easy to interpret. Further, since the technique offers the feasibility to test the entire graded PtAl coating in-situ, the results remain representative of the coating. Using the above testing technique, the tensile behavior of the PtAl coating has been evaluated at various temperatures and strain rates. The effect of strain rate on the BDTT of the coating has been ascertained. Further, the effect of Pt content on the tensile behavior of these coatings has also been evaluated. Attempts have been made to identify the mechanisms associated with tensile deformation and fracture in these coatings. The thesis is divided into nine chapters. Chapter 1 presents a brief introduction on the operating environment in gas turbine engines and the materials that are used in the hot sections of gas turbine engines. The degradation mechanisms taking place in the superalloy in gas turbine environments and the need for application of coatings has also been highlighted. The basic architecture of a typical thermal barrier coating (TBC) system applied on gas turbine engine components has been presented. The constituents of the TBC system, i.e. the ceramic YSZ coating, MCrAlY overlay as well as diffusion aluminide bond coats and, the various techniques adopted for the deposition of these coatings have been described in brief. Chapter 2 presents an overview of the literature relevant to this study. This chapter is divided into four sub-chapters. The formation of diffusion aluminide coatings on Ni-based superalloys has been described in the first sub-chapter. Emphasis has been laid on pack cementation process for the formation of the coatings. The fundamentals of pack aluminizing process, including the thermodynamic and kinetic aspects, have been mentioned in brief. The microstructural aspects of high activity and low activity plain aluminide and Pt-aluminide coatings have also been illustrated. The techniques applied for the mechanical testing of bond coats have been discussed in the second sub-chapter. The macro-scale testing techniques have been mentioned in brief. The small scale testing methods such as indentation, bend tests and micro-tensile testing have also been discussed in the context of evaluation of mechanical properties of bond coats. Since the matrix in the aluminide bond coats is constituted of the B2-NiAl phase, a description of the crystal structure and deformation characteristics of this phase including the flow behavior, ductility and fracture behavior has been mentioned in the third sub-chapter. In the fourth sub-chapter, reported literature on the tensile behavior and brittle-to-ductile-transition-temperature (BDTT) of diffusion aluminide bond coats has been discussed. In Chapter 3, details on experiments carried out for the formation of various coatings used in the present study and, their microstructural characterization, are provided. The method for extraction of stand-alone coating specimens and their testing is discussed. The microstructure and composition of the various coatings used in the present study are discussed in detail in Chapter 4. Unlike in case of bulk tensile testing, for which standards on the design of specimens exist, there are no standards available for the design of micro-tensile specimens. Therefore, as part of the present research work, a finite element method (FEM)-based study was carried out for ascertaining the dimensions of the specimens. The simulation studies predicted that failure of the specimens within the gage length can be ensured only when certain correlations between the dimensional parameters are satisfied. Further, the predictions from the simulation study were validated experimentally by carrying out actual testing of specimens of various dimensions. Details on the above mentioned aspects of specimen design are provided in Chapter 5. The PtAl coatings undergo brittle fracture at lower temperatures while ductile fracture occurs at higher temperatures. Further, the coatings exhibit a scatter in the yielding behavior at temperatures in the vicinity of BDTT. Therefore, the BDTT, determined as the temperature at which yielding is first observed in the stress-strain curves, may not be representative of the PtAl coatings. In Chapter 6, a method for the precise determination of BDTT of aluminide bond coats, based on the variation in the plastic strain to fracture with temperature, has been demonstrated. The BDTT determined by the above method correlated well with the variation in fracture surface features of the coating and was found representative of these coatings. In Chapter 7, the effect of temperature and strain rate on the tensile properties of a PtAl bond coat has been evaluated. The temperature and strain rate was varied between room temperature (RT)-1100°C and 10-5 s-1-10-1 s-1, respectively. The effect of strain rate on the BDTT of the PtAl bond coat has been examined. Further, the variation in fracture surface features and mechanism of fracture with temperature and strain rate are illustrated. The micro-mechanisms of deformation and fracture in the coating at different temperature regimes have also been discussed. The coating exhibited brittle-to-ductile transition with increase in temperature at all strain rates. The BDTT was strain rate sensitive and increased significantly at higher strain rates. Above BDTT, YS and UTS of the coating decreased and its ductility increased with increase in the test temperature at all strain rates. Brittle behavior occurring in the coating at temperatures below the BDTT has been attributed to the lack of operative slip systems in the B2-NiAl phase of the coating. The onset of ductility in the coating in the vicinity of BDTT has been ascribed to generation of additional slip systems caused by climb of dislocations onto high index planes. The coating exhibited two distinct mechanisms for plastic deformation as the temperature was increased from BDTT to 1100°C. For temperatures in the range BDTT to about 100°C above it, deformation was controlled by dislocations overcoming the Peierls-Nabarro barrier. Above this temperature range, non-conservative motion of jogs by jog dragging mechanism controlled the deformation. The transition temperature for change of deformation mechanism also increased with increase in strain rate. For all strain rates, fracture in the coating at test temperatures below the BDTT, occurred by initiation of cracks in the intermediate single phase B2-NiAl layer of the coating and subsequent inside-out propagation of the cracks across the coating thickness. Ductile fracture in the coating above the BDTT was associated with micro-void formation throughout the coating. The effect of Pt content on the tensile behavior of PtAl coating, evaluated at various temperatures ranging from room temperature (RT) to 1100°C and at a nominal strain rate of 10-3 s-1, is presented in Chapter 8. Irrespective of Pt content in the coating, the variation in tensile behavior of the coating with temperature remained similar. At temperatures below BDTT, the coatings exhibited linear stress-strain response (brittle behavior) while yielding (ductile behavior) was observed at temperatures above BDTT. At any given temperature, the elastic modulus decreased while the strength increased with increase in Pt content in the coating. On the other hand, the ductility of the coating remained unaffected with Pt content. The BDTT of the coating also increased with increase in Pt content in the coating. Addition of Pt did not affect the fracture mechanism in the coating. Fracture at temperatures below BDTT was caused by nucleation of cracks at the intermediate layer and their subsequent inside-out propagation. At high temperatures, fracture occurred in a ductile manner comprising void formation, void linkage and subsequent joining with cracks. The deformation sub-structure of the coating did not get affected with Pt incorporation. Short straight dislocations were observed at temperatures below BDTT, while, curved dislocations marked by jog formation were observed at temperatures above BDTT. The factors controlling fracture stress and strength in the PtAl coatings at various temperatures have also been assessed. The overall summary of the present research study and recommendations for future studies are presented in the last chapter, i.e. Chapter 9.