Aços inoxidáveis dúplex apresentam uma estrutura composta por ferrita e austenita. O fato de a austenita ser estável à temperatura ambiente possibilita que esses aços sejam usados no estudo da reação de formação da austenita a partir da ferrita, podendo os resultados ser aplicados a aços de baixa liga, em que a austenita se transforma em martensita no resfriamento brusco, e a outros sistemas com transformações entre fases cúbicas de corpo centrado (CCC) e cúbicas de face centrada (CFC). Foram realizados estudos em um aço inoxidável dúplex do tipo DIN W.Nr. 1.4462 (UNS 31803). As amostras foram solubilizadas no campo ferrítico a 1325°C e resfriadas em água. As amostras foram ainda tratadas isotermicamente em temperaturas entre 700°C e 1100°C por tempos entre 5.000 e 30.000 segundos. Deste modo, a formação de austenita a partir da ferrita foi estudada em seu estágio inicial e em estágios avançados da reação, com relação aos seus aspectos morfológicos e cristalográficos. As morfologias encontradas apresentaram variações dependentes dos segmentos de contorno de grão em que as partículas se formam. As partículas nucleadas nos contornos de grão podem ser adequadamente descritas pela classificação morfológica de Dubé. Além disso, essas partículas apresentaram, em geral, relações de orientação do tipo Kurdjumov-Sachs e Nishyiama-Wassermann com pelo menos um dos dois grãos, podendo ser encontradas relações intermediárias entre essas duas. . Em alguns casos, as partículas mantêm relações de orientação com os dois grãos adjacentes, apresentando pequenos desvios das relações exatas relatadas na literatura. As partículas de austenita intragranulares apresentam desvios em relação à relações exatas maiores do que os encontrados nas partículas de contornos de grão. Em alguns casos, as partículas intragranulares aparentam não apresentar relações de orientação com a matriz ferrítica. / Duplex Stainless Steels have a structure composed by ferrite and austenite. The fact that austenite, in these steels, is stable at low temperatures, allows the use of these steels in studies of austenite formation from ferrite, in such way that the results can be applied to low alloy steels, in which austenite transforms to martensite upon rapid cooling, and to other systems containing transformations between body-centred cubic (BCC) and face-centred cubic (FCC) phases. Studies were performed on a DIN W.Nr. 1.4462 (UNS 31803) duplex stainless steel. The samples were solution treated in the ferrite region at 1325°C and water cooled. Samples were then submitted to isothermal treatments at temperatures between 700°C and 1100°C for up to 30.000 seconds. In this way, austenite formation from ferrite was studied on initial and advanced stages of the reaction, concerning morphological and crystallographic aspects. The morphologies observed varied with the grain boundary segment in which the particles were formed. The grain boundary particles may be adequately described by the Dubé classification. Moreover, these particles presented Kurdjumov-Sachs and Nishyiama-Wassermann orientation relationships with at least one of the adjacent grains, with possibilities of occurrence of intermediate relationships between K-S and N-W relationships. In some cases, the particles show orientation relationships with both adjacent grains, with small deviations form the exact relationships reported in literature. Intragranular austenite particles show higher deviations from the exact relationships than grain boundary particles. In some cases, intragranular particles have no apparent orientation relationships with the ferrite matrix.

The orientation relationship and interfaces of PtSn4 and PtSn with the Pt (001) and (111) surfaces have been studied with transmission electron microscopy. Pt was evaporated onto the NaCl (001) and (111) surfaces to form epitaxial Pt thin films and Sn was evaporated onto the Pt films at different temperature to form PtSn4 and PtSn. Pt was evaporated onto the NaCl (001) and (111) surfaces at 350 ¢J to form epitaxial Pt thin films of [001] and [111] zone axes, respectively. Some grains are in random orientation and other as ring pattern. The grain size was at about 10-20 nm. Sn was evaporated onto the Pt surface at 150 ¢J to form PtSn4 and at 200 ¢J to form PtSn. No good orientation relationships were formed on both the PtSn4/ Sn and the PtSn/ Sn interfaces. Heterogeneous nucleation theory, predicts that PtSn should form before PtSn4, but PtSn4 was observed to the first to form. The possible reasons were discussed. Keywords: PtSn4/ Sn interface, PtSn/ Sn interface, orientation relationships, thin films, evaporator, transmission electron microscopy

Aços inoxidáveis dúplex apresentam uma estrutura composta por ferrita e austenita. O fato de a austenita ser estável à temperatura ambiente possibilita que esses aços sejam usados no estudo da reação de formação da austenita a partir da ferrita, podendo os resultados ser aplicados a aços de baixa liga, em que a austenita se transforma em martensita no resfriamento brusco, e a outros sistemas com transformações entre fases cúbicas de corpo centrado (CCC) e cúbicas de face centrada (CFC). Foram realizados estudos em um aço inoxidável dúplex do tipo DIN W.Nr. 1.4462 (UNS 31803). As amostras foram solubilizadas no campo ferrítico a 1325°C e resfriadas em água. As amostras foram ainda tratadas isotermicamente em temperaturas entre 700°C e 1100°C por tempos entre 5.000 e 30.000 segundos. Deste modo, a formação de austenita a partir da ferrita foi estudada em seu estágio inicial e em estágios avançados da reação, com relação aos seus aspectos morfológicos e cristalográficos. As morfologias encontradas apresentaram variações dependentes dos segmentos de contorno de grão em que as partículas se formam. As partículas nucleadas nos contornos de grão podem ser adequadamente descritas pela classificação morfológica de Dubé. Além disso, essas partículas apresentaram, em geral, relações de orientação do tipo Kurdjumov-Sachs e Nishyiama-Wassermann com pelo menos um dos dois grãos, podendo ser encontradas relações intermediárias entre essas duas. . Em alguns casos, as partículas mantêm relações de orientação com os dois grãos adjacentes, apresentando pequenos desvios das relações exatas relatadas na literatura. As partículas de austenita intragranulares apresentam desvios em relação à relações exatas maiores do que os encontrados nas partículas de contornos de grão. Em alguns casos, as partículas intragranulares aparentam não apresentar relações de orientação com a matriz ferrítica. / Duplex Stainless Steels have a structure composed by ferrite and austenite. The fact that austenite, in these steels, is stable at low temperatures, allows the use of these steels in studies of austenite formation from ferrite, in such way that the results can be applied to low alloy steels, in which austenite transforms to martensite upon rapid cooling, and to other systems containing transformations between body-centred cubic (BCC) and face-centred cubic (FCC) phases. Studies were performed on a DIN W.Nr. 1.4462 (UNS 31803) duplex stainless steel. The samples were solution treated in the ferrite region at 1325°C and water cooled. Samples were then submitted to isothermal treatments at temperatures between 700°C and 1100°C for up to 30.000 seconds. In this way, austenite formation from ferrite was studied on initial and advanced stages of the reaction, concerning morphological and crystallographic aspects. The morphologies observed varied with the grain boundary segment in which the particles were formed. The grain boundary particles may be adequately described by the Dubé classification. Moreover, these particles presented Kurdjumov-Sachs and Nishyiama-Wassermann orientation relationships with at least one of the adjacent grains, with possibilities of occurrence of intermediate relationships between K-S and N-W relationships. In some cases, the particles show orientation relationships with both adjacent grains, with small deviations form the exact relationships reported in literature. Intragranular austenite particles show higher deviations from the exact relationships than grain boundary particles. In some cases, intragranular particles have no apparent orientation relationships with the ferrite matrix.

Методом ориентационной микроскопии (EBSD) исследованы текстурные состояния сплава Ti-6Al-4V, изготовленного методом селективного электронно-лучевого плавления (СЭЛС). Установлено, что текстура β-фазы представлена текстурой кристаллизации, при которой направление <100>β параллельно направлению синтеза изделия. Текстура α-фазы является многокомпонентной и наследует текстуру β-фазы через ОС Бюргерса. Показано, что при последующем охлаждении в сплаве Ti-6Al-4V выделяется вторичная βII-фаза, кристаллографически отличная от высокотемпературной β-фазы. Предложена схема протекающих фазовых превращений. / The textural states of the Ti-6Al-4V alloy fabricated by a method of electron beam melting (EBM) were studied by orientation microscopy (EBSD). It was found that the texture of the β-phase is represented by the texture of crystallization, where the direction <100>β is parallel to the building direction. The texture of the α-phase is multicomponent and inherits the texture of the β-phase following the Burgers OR. It was shown that upon subsequent cooling in the Ti-6Al-4V alloy, a secondary βII-phase is precipitated, which is crystallographically different from the high-temperature β-phase. A scheme of the ongoing phase transformations is proposed.

Hypoeutectic Fe-C and Fe-C-Si model alloys were produced at different solidification conditions. Copper mold casting yields low cooling rates promoting the formation of a eutectic microstructure characterized by two morphologies: elongated cementite plates and a rod structure growing perpendicular to the plates, i.e. austenite rods in a cementite matrix. Electron beam surface remelting generates a mainly plate-like eutectic due to rapid solidification. The microstructures were characterized by light-optical microscopy and electron backscatter diffraction (EBSD). The latter allows for a spatially resolved investigation of the growth crystallography of the eutectic phases. Thereby, a possible existence of crystallographic orientations relationships between cementite and austenite within the plate-like eutectic was assessed experimentally. The eutectic phases were found to grow largely crystallographically independently. Moreover, ferrite and eutectic cementite within the decomposed eutectic microstructure comply frequently with the Bagaryatsky or the Pitsch-Petch orientation relationship. Complementary X-ray diffraction (XRD) analysis reveals a pronounced cementite {002} texture in the microstructure produced by mold casting. Characteristic changes in the lattice parameters indicate that as-cast cementite is non-stoichiometric.

L’industrie automobile cherche constamment à augmenter la part des pièces fabriquées à partir de tôles minces en aciers plus résistants et à plus bas coût. Le parallèle entre les diagrammes de phases Fe-N et Fe-C montre qu’il est possible de développer des aciers similaires dans le système Fe-N. Les objectifs de cette étude étaient l’élaboration des aciers binaires Fe-N et le développement des structures équivalentes à celles dans les aciers au carbone. Les approches envisagées s’articulent autour de : Elaboration : la nitruration gazeuse en phase austénitique suivie de traitements d’homogénéisation ont permis de charger des tôles minces de fer pur en concentrations importantes d’azote. Une simulation de la diffusion de l’azote a été proposée. Genèse des microstructures : Le refroidissement lent de l’austénite Fe-N conduit à des structures perlitiques lamellaires et globulaires, constituées de ferrite et du nitrure non-stoechiométrique Fe4N. Une structure aciculaire particulière a été identifiée. Il s’agit de la ferrite se développant dans le nitrure Fe4N. Le refroidissement lent de la ferrite Fe-N sursaturée conduit à la précipitation des nitrures stables Fe4N et métastable Fe16N2. Des microstructures multiphasées [alpha+alpha'+gamma] ont été obtenues par des maintiens dans le domaine intercritique suivis de trempe. Le domaine intercritique a été réexaminé en utilisant le modèle des sous-réseaux. Des essais in-situ en MET ont relevé l’évolution des précipités de Fe16N2 dans la ferrite au cours du vieillissement à 85 °C. Des proportions importantes de l’austénite résiduelle ont été relevées, ce qui serait à la base du développement des aciers TRIP à l’azote / Car designers are seeking ways to increase the proportion of parts made of sheet steels of higher strength and lower cost. The parallel between the Fe-N and Fe-C phase diagrams shows that it is possible to develop similar steels in the Fe-N system. The objective of this study was to prepare binary Fe-N steels and to develop structures equivalent to those in carbon steels. Approaches to meet the objectives are considered: Preparations of Fe-N steels: gas nitriding in austenite domain followed by homogenization treatments allowed to introduce high amount of nitrogen in pure iron sheet. A simulation of the nitrogen diffusion was proposed to describe the weight increase during nitriding. Genesis of microstructures: The slow cooling of the Fe-N austenite led to lamellar and globular pearlitic structures composed of ferrite and nitrideFe4N. An acicular microstructure, which is the consequence of the precipitation of the ferrite in the nitride Fe4N, was also identified. The slow cooling of the supersaturated Fe-N ferrite led to the precipitation of the stable nitride Fe4N and the metastable nitride Fe16N2, which witnessed a rapid diffusion of nitrogen in ferrite at low temperature, comparable to that of carbon. The multiphase microstructures [alpha+alpha'+gamma] were obtained by intercritical treatments followed by quenching. The intercritical domain was reviewed using the sub-lattice model. In-situ TEM investigations have identified the precipitation of Fe16N2 nitride in the ferrite during the aging at 85 °C. High amount of the residual austenite have been identified, which would be the basis for development of TRIP nitrogen steels

This thesis concerns the mathematical modelling of phase transformations with a special emphasis on martensitic phase transformations and their application to the modelling of steels. In Chapter 1, we develop a framework that determines the optimal transformation strain between any two Bravais lattices and use it to give a rigorous proof of a conjecture by E.C. Bain in 1924 on the optimality of the so-called Bain strain. In Chapter 2, we review the Ball-James model and related concepts. We present some simplification of existing results. In Chapter 3, we pose a conjecture for the explicit form of the quasiconvex hull of the three tetragonal wells, known as the three-well problem. We present a new approach to finding inner and outer bounds. In Chapter 4, we focus on highly compatible, so called self-accommodating, martensitic structures and present new results on their fine properties such as estimates on their minimum complexity and bounds on the relative proportion of each martensitic variant in them. In Chapter 5, we investigate the contrary situation when self-accommodating microstructures do not exist. We determine, whether in this situation, it is still energetically favourable to nucleate martensite within austenite. By constructing different types of inclusions, we find that the optimal shape of an inclusion is flat and thin which is in agreement with experimental observation. In Chapter 6, we introduce a mechanism that identifies transformation strains with orientation relationships. This mechanism allows us to develop a simpler, strain-based approach to phase transformation models in steels. One novelty of this approach is the derivation of an explicit dependence of the orientation relationships on the ratio of tetragonality of the product phase. In Chapter 7, we establish a correspondence between common phenomenological models for steels and the Ball-James model. This correspondence is then used to develop a new theory for the (5 5 7) lath transformation in low-carbon steels. Compared to existing theories, this new approach requires a significantly smaller number of input parameters. Furthermore, it predicts a microstructure morphology which differs from what is conventionally believed.