L'augmentation constante de la circulation automobile a travers le monde fait des effluents gazeux un des problèmes majeurs de toutes les sociétés modernes. Tant d'un point de vue économique et écologique, chacun s'accorde sur le fait que la consommation de carburants fossiles utilisés dans le transport doit baisser, principalement en réduisant le poids des véhicules. Le développement de matériaux à hautes performances et à bas prix est donc indispensable. Pour atteindre cet objectif, cette étude visait à élucider les interactions entre la déformation et les transformations de phase dans les aciers a hautes performances qui pourraient remplir les conditions de réduction de poids. En effet, une large gamme de travaux a montrer que les transformations de phase induites mécaniquement (effet TRIP) de l'austénite peuvent être à l'origine d'une amélioration des propriétés mécaniques dans de nombreuses nuances d'acier. Les transformations de phase induites par la déformation dépendent de deux paramètres : la stabilité relative et l'énergie de fautes d'empilement de l'austénite, qui sont affectes par différents facteurs. Les interactions entre les transformations de phase et les propriétés mécaniques de différentes nuances Cr-Ni et Mn-Si-Al furent examinées sous plusieurs conditions de taille de grain, de température et d'état de chargement. Des relations particulières furent établies entre les phénomènes qui se produisent a l'échelle des grains individuels et a l'échelle macroscopique. Les mécanismes cristallographiques des transformations de phase successives (austénite - martensite e - martensite a') ont été mis en évidence. Finalement, différentes techniques de raffinage de la taille de grain furent utilisées pour produire des aciers inoxydables comportant des tailles de grain variées, et l'efficacité de ces techniques a été comparée. Pour cela, les cinétiques de retransformation, recrystallisation et croissance des grains ont été étudiées. La réduction de taille des grains par cycles de transformations de phase fut établie comme plus efficace de la méthode classique par déformation - recristallisation. / The continuously increasing use of automobiles all over the world, is making of gas effluents one of the major concerns for all modern societies. From economical and ecological points of view, everyone agrees on the fact that the consumption of fossil fuels for transport must decrease, particularly by vehicle weight reduction. Development of high performance materials at low cost is therefore needed. In order to achieve this requirement, the present work aimed at investigating the interactions between straining and phase transformations in high performance steels that could meet the weight saving requirements. Indeed, a wide range of studies has shown that mechanically-induced phase transformations (TRIP effect) of the austenite may bring about improved mechanical properties in different steel grades. Strain-induced phase transformations depend on two parameters : the relative stability and the stacking fault energy of the austenite, which are affected by different factors. The interactions between the phase transformations and the mechanical properties of different Ni-Cr and Mn-Si-Al grades were examined under various conditions of grain size, temperature or stress state. Particular relationships were clearly established between the phenomena taking place at the scale of the individual grains and at the macroscopic scale. The crystallographic mechanisms of the successive strain-induced phase transformations (austenite - e-martensite - a'-martensite) has been clarified. Finally, different techniques of grain refinement were used to process stainless steels with various grain sizes, assessing the efficiency of these techniques. Therefore, the kinetics of retransformation, recrystallisation and grain growth were studied. Grain refinement by cycles of phase transformations was found more effective than the classical deformation - recrystallisation method.

The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.

Understanding lithium-ion insertion/extraction mechanisms in battery electrode materials is of crucial importance in developing new materials with better cycling performance. In this thesis, these mechanisms are probed for two different potential cathode materials by a combination of electrochemical and single-crystal X-ray diffraction studies. The materials investigated are V6O13 and cubic LiMn2O4 spinel. Single-crystal X-ray diffraction studies of lithiated phases in the LixV6O13 system (x=2/3 and 1) exhibit superlattice phenomena and an underlying Li+ ion insertion mechanism which involves the stepwise addition of Li+ ions into a two-dimensional array of chemically equivalent sites. Each successive stage in the insertion process is accompanied by a rearrangement of the Li+ ions together with an electron redistribution associated with the reduction of specific V-atoms in the structure. This results in the formation of electrochemically active sheets in the structure. A similar mechanism occurs in the LiMn2O4 delithiation process, whereby lithium is extracted in a layered arrangement, with the Mn atoms forming charge-ordered Mn3+/Mn4+ layers. Lithium-ion insertion/extraction processes in transition-metal oxides would thus seem to occur through an ordered two-dimensional arrangement of lithium ions extending throughout the structure. The lithium ions and the host structure rearrange cooperatively to form superlattices through lithium and transition-metal ion charge-ordering. A picture begins to emerge of a universal two-dimensional lithium-ion insertion/extraction mechanism analogous to the familiar staging sequence in graphite.

This thesis reveals the tribological mechanisms behind the intrinsic low friction potential of the Co-based family of alloys called Stellites. Although being an established and important group of materials, a satisfactory explanation to why they exhibit low-friction properties under severe sliding conditions has not previously been found in the literature. The main part of this thesis is dedicated to the clarification of the tribological performance of Stellites in highly loaded sliding contact. The results should assist the development of Co-free alternatives, suitable for replacing Stellites in nuclear applications. Owing to their beneficial properties they are today the most commonly used material in the sealing surfaces on gate valves in the primary circuits of boiling water reactors (BWR). The underlying reason for the replacement in the nuclear applications is an undesired contribution to the background radiation level, originating from the Co in the Stellite surfaces. The Stellites mainly consist of Cr-rich carbides in a solid solution dominated by Co. The commonly used Stellite 6 and Stellite 21 were chosen as primary test materials and applied by laser cladding, providing a metallically bonded clad layer with a fine dendritic microstructure. By combining information from a series of dedicated tribological tests and modern high-resolution analysis instruments (e.g. SEM, XRD and TEM) available at the Ångström Laboratory at Uppsala University, the following conclusions can be made regarding the tribological performance of Stellites under high load sliding. Mechanisms. The (tested) Stellites form a thick deformation hardened layer, topped with a superficial easily sheared layer of hcp basal planes aligned parallel to the worn surface. The easy-shear layer is continually regenerated, replacing worn off material. Technical benefits. The Stellites offer low-friction properties thanks to their easily sheared surface layers. The risk of severe galling is also avoided by restricting shear and adhesive transfer to very thin superficial layers. In closed sliding contacts, self-generated protective layers formed by re-deposition of wear fragments are also offered.

We study the plasticity mechanisms of diffusionless martensite phase transformation in Nickel-Titanium, one of the most widely used shape memory alloys. The research here involves four thrusts focusing on different length and time scales: (I) Molecular statics and dynamics simulations are applied to study the nanotwin structures and temperature-driven B2 → B19′ phase transitions. (II) Molecular dynamics simulations are performed to explore the stress-driven martensitic phase transformation governing the pseudoelasticity and shape memory effects in NiTi nanopillars. (III) Monte Carlo simulations are conducted to characterize the temperature- driven B2 → B19 phase transition and the patterning of martensitic nanotwins in NiTi thin films. (IV) Phase field simulations are performed to predict the formation and evolution of complex martensitic microstructures, including the detailed analysis of twin compatibility under complex loading conditions. We also study the nucleation-controlled plasticity mechanisms in different metals of Cu, Al and Ni. Our work focuses on understanding how dislocations nucleate in single crystals. Interatomic potential finite element method is applied to determine when, where and how dislocations nucleate during nanoindentation in metals such as Cu, Al and Ni.

Double layer capacitors owe their large capacitance to high specific surface area carbon-based electrode materials adhered to a current collector via an adhesive. However, recent studies attribute greater electrical energy storage capacity to transition metal oxides/nitrides: a new generation of electrode materials for use in super-capacitors with mixed double-layer and pseudo-capacitive properties. Solution precursor plasma spray deposition is a technique that allows coatings to be fabricated with fine grain sizes, high porosity levels, and high surface area; characteristics ideal for application as super-capacitor electrodes. This investigation established conditions for deposition of porous, high specific surface area α-MoO3. It further identified a two-step temperature-programmed reaction for topotactic phase transformation of the α-MoO3 deposits into high specific surface area molybdenum nitrides of higher conductivity and higher electrochemical stability window. The electrochemical behavior of molybdenum oxide/nitride deposits was also studied in order to assess their potential for use in super-capacitors.

Double layer capacitors owe their large capacitance to high specific surface area carbon-based electrode materials adhered to a current collector via an adhesive. However, recent studies attribute greater electrical energy storage capacity to transition metal oxides/nitrides: a new generation of electrode materials for use in super-capacitors with mixed double-layer and pseudo-capacitive properties. Solution precursor plasma spray deposition is a technique that allows coatings to be fabricated with fine grain sizes, high porosity levels, and high surface area; characteristics ideal for application as super-capacitor electrodes. This investigation established conditions for deposition of porous, high specific surface area α-MoO3. It further identified a two-step temperature-programmed reaction for topotactic phase transformation of the α-MoO3 deposits into high specific surface area molybdenum nitrides of higher conductivity and higher electrochemical stability window. The electrochemical behavior of molybdenum oxide/nitride deposits was also studied in order to assess their potential for use in super-capacitors.

This work is a thermodynamic and kinetic study of the Fe-Cr-Ni system as the core of stainless steels. The Fe-Cr, Fe-Ni and Cr-Ni systems were studied intensively using both computational and experimental techniques, including CALPHAD (CALculation of PHAse Diagrams), phase field simulation, ab initio modeling, calorimetry, and atom probe tomography. The purpose of this thesis is to reveal the complexity of the phase transformations in the Fe-Cr-Ni system via the integrated techniques. Due to the importance of the binary Fe-Cr system, it was fully reassessed using the CALPHAD technique by incorporating an updated description of the lattice stability for Fe down to zero kelvin. The improved thermodynamic description was later adopted in a phase field simulation for studying the spinodal decomposition in a series of Fe-Cr binary alloys. Using atom probe tomography and phase field simulation, a new approach to analyze the composition amplitude of the spinodal decomposition was proposed by constructing an amplitude density spectrum. The magnetic phase diagram of the Fe-Ni system was reconstructed according to the results from both ab initio calculations and reported experiments. Based on the Inden-Hillert-Jarl magnetic model, the thermodynamic reassessment of the Fe-Ni system demonstrated the importance of magnetism in thermodynamic and kinetic investigations. Following this, the current magnetic model adopted in the CALPHAD community was further improved. Case studies were performed showing the advantages of the improved magnetic model. Additionally, the phase equilibria of the Fe-Cr-Ni ternary were discussed briefly showing the need of thermodynamic and kinetic studies at low temperatures. The “low temperature CALPHAD” concept was proposed and elucidated in this work showing the importance of low temperature thermodynamics and kinetics for designing the new generation of stainless steels. / <p>QC 20120612</p> / Hero-m

The exciting thermomechanical properties of nickel-titanium shape memory alloys have sparked significant research efforts seeking to exploit their exotic capabilities. Until recently, the performance capabilities of nickel-titanium devices have been inhibited by the retention of only one thermomechanical characteristic. However, laser processing technology promises to deliver enhanced material offerings which are capable of multiple functional responses. Presented in this thesis, is an investigation of the effects of laser processing on the thermomechanical behaviour of nickel-titanium shape memory alloys. In the context of this work, laser processing refers to removal of alloy constituents, as in the case of laser ablation, or alternatively, addition of elements through laser alloying. The effects of laser ablation on the composition, crystallography and phase transformation temperatures of a nickel-titanium strip have been studied. Application of laser energy was shown to ablate nickel constituents, induce an austenite-martensite phase change and cause an increase in phase transformation onset temperatures, which correlated well with reported findings. Laser processing of a nickel-titanium wire was shown to locally embed an additional thermomechanical response which manifested as unique shape memory and pseudoelastic properties. Localized alloying of ternary species via laser processing of nickel-titanium strip was investigated. Synthesis of a ternary shape memory intermetallic within the laser processing region was achieved through melting of copper foils. Results from thermoanalytical testing indicated that the ternary compound possessed a higher phase transformation temperature and reduced transformation hysteresis in comparison to the reference alloy. Indentation testing was used to demonstrate the augmented thermomechanical characteristics of the laser processed shape memory alloy. In order to demonstrate the enhanced functionality of laser processed nickel-titanium shape memory alloys, a self-positioning nickel-titanium microgripper was fabricated. The microgripper was designed to actuate through four different positions, corresponding to activation of three embedded shape memory characteristics. Thermoanalytical and tensile testing instrumentations were used to characterize the thermomechanical performance of the laser processed nickel-titanium microgripper. Results indicated that each of the laser processed microgripper components possessed unique mechanical and shape memory recovery properties.

The research objectives of this thesis are as follows: (1) Investigate the effects of carbon content, alloying, and heat treatment of steels on white layer formation, (2) Prove/disprove that the temperature for phase transformation in machining is the same as the nominal phase transformation temperature of the steel, (3) Quantify the contributions of thermal and mechanical effects to white layer generation in machining, (4) Develop a semi-empirical procedure for prediction of white layer formation that accounts for both thermal and mechanical effects. These research objectives are realized through experimental and modeling efforts on steels. Depth and hardness measurements of the white layers formed in steels show the importance of heat treatment and carbon content on white layer formation. Measurements of workpiece surface temperature and X-Ray Diffraction characterization of the machined surfaces show that phase transformation occurs below the nominal As temperature suggesting that mechanical effects play an important role in white layer formation. The maximum workpiece surface temperature, the effective stress, and plastic strain on the workpiece surface are measured and/or calculated and shown to affect the white layer depth and amount of retained austenite. A semi-empirical procedure is developed by correlating the maximum workpiece temperature and the unit thrust force increase with white layer formation.