Cette thèse vise à améliorer les propriétés d'alliage à mémoire de forme et de stockage d'hydrogène du TiNi par des substitutions chimiques sur le sous-réseau de Ni. L'effet de la substitution de Ni par Pd, Cu et Co sur les propriétés structurales, la transformation martensitique et les propriétés d'hydrogénation de TiNi, a été étudiée par des techniques structurales (diffraction des Rayons-X et des neutrons sur poudre), calorimétriques, par réaction solide-gaz et par mesure électrochimique. Des calculs de type DFT ont été effectués pour mettre en évidence les modifications de la structure électronique sur les propriétés d'hydrogénation. Les trois substitutions, TiNi1-zMz (M = Pd, Cu et Co; z ≤ 0,5), conduisent à la formation de composés pseudo-binaires. La substitution par Pd et par Cu augmentent le volume de la maille de TiNi, alors que celle par Co possède un effet inverse. Les températures de transformation martensitique suivent la même tendance que les changements de volume. Ils augmentent fortement pour M = Pd et légèrement pour M = Cu, tandis qu'elles diminuent pour M = Co. Les propriétés d'hydrogénation sont très sensibles à la nature chimique des substitutions. La capacité diminue fortement avec la substitution par Pd, modérément avec le Cu et reste stable pour le M = Co. Contrairement à l'effet attendu par des considérations géométrique, la substitution par Pd et par Cu diminuent la stabilité des hydrures. Les calculs DFT montrent qu'un effet électronique, et non géométrique, régit la stabilité des hydrures pour M = Pd. La substitution par le cobalt induit une formation de plusieurs hydrures par étapes successives, observée par mesure d'isothermes pression-composition en multiple plateaux. Pour les applications, la substitution de Ni par Cu avec une faible teneur (z = 0,2) augmente la capacité de décharge électrochimique de TiNi de 150 à 300 mAh/g en raison de la déstabilisation de l'hydrure. Cela ouvre de nouvelles perspectives pour l'utilisation d'alliages TiNi comme électrodes de batteries Ni-MH. En revanche, la substitution par Pd ou Cu (avec z = 0,5) est très efficace pour réduire la réactivité de TiNi avec l'hydrogène, et est donc intéressante pour des applications à mémoire de forme sous un environnement réducteur / The PhD thesis aims to improve shape memory and hydrogen storage properties of TiNi by chemical substitutions in the Ni sub-lattice. The effect of Pd, Cu and Co substitutions on crystal structure, martensitic transformation and hydrogenation properties of TiNi has been studied by structural (X-ray and neutron powder diffraction), calorimetric, solid-gas and electrochemical means. Ab initio DFT calculations were done to highlight electronic effects on hydrogenation properties. The three substitutions, TiNi1-zMz (M = Pd, Cu and Co; z ≤ 0.5), lead to the formation of pseudobinary compounds. Substitutions by Pd and Cu increase the unit-cell volume of TiNi, whereas the reverse effect occurs for Co. Martensitic transformation temperatures correlate with volume variations. They increase strongly for M = Pd and slightly for M = Cu, whereas M = Co decreases it. Hydrogenation properties are very sensitive to chemical elements substitution. The capacity decreases strongly for M = Pd, moderately for M = Cu and remains stable for M = Co. Contrary to expected effect by geometric model, both Pd and Cu substitutions decrease the stability of hydrides. DFT calculations show that electronic rather than geometric effects govern hydride stability for M = Pd. Co substitution induces step-wise formation of hydrides with a multi-plateau behaviour in pressure-composition isotherms. As concerns applications, low amount of Cu substitution (z =0.2) increases the electrochemical discharge capacity of TiNi from 150 to 300 mAh/g due to hydride destabilization. This opens new perspectives for using TiNi-based alloys in Ni-MH batteries. In contrast, Pd and high amount of Cu substitution (z = 0.5) are effective to decrease TiNi reactivity towards hydrogen, and therefore attractive for shape memory applications under reductive environment

Os aços inoxidáveis endurecidos por precipitação vêm sendo largamente empregados na indústria aeronáutica, por combinar resistência mecânica, tenacidade à fratura e resistência à corrosão. E deste modo, são materiais que possibilitam a substituição dos aços carbonos utilizados atualmente, que necessitam de tratamento superficial adicional, como o cádmio, para melhorar a resistência à corrosão. A utilização desses revestimentos traz desvantagens como o custo, a fabricação, a susceptibilidade à fragilização por hidrogênio além dos aspectos ambientais. Neste contexto, o aço endurecido por precipitação UNS S46500, designado como Custom 465® foi avaliado considerando o efeito da temperatura de envelhecimento sobre as propriedades mecânicas e a resistência à corrosão por pite. Amostras tratadas nas condições solubilizada e envelhecida a 510ºC (H950) e 538ºC (H1000) foram submetidas ao ensaio de tração, caracterização microestrutural e ensaios de polarização potenciodinâmica (PP) para determinar a resistência à corrosão por pite. Os exames microestruturais foram realizados com auxílio de microscopia óptica (MO), microscopia eletrônica de varredura (MEV), espectroscopia de energia dispersiva (EDS) e difração de raios X. Também foram realizadas análises utilizando o software Thermo-Calc. A resistência à corrosão por pite foi avaliada em solução 0,6M NaCl com adições crescentes de Na2SO4. Os resultados obtidos nos ensaios de PP nas duas condições de tratamento térmico foram comparados entre si e com resultados disponíveis na literatura (CALDERÓN-HERNANDEZ, 2012) para o aço inoxidável UNS S30403 (304L). Os exames e análises da microestrutura revelaram que o aço Custom 465® envelhecido apresenta uma matriz martensítica, precipitados de fase chi, austenita e precipitados Ni3Ti. O tratamento H950 apresentou maior resistência mecânica e menor alongamento do que o tratamento H1000. Tal comportamento foi devido à produção de maior porcentagem de fase chi e menor porcentagem de austenita nesse tratamento de envelhecimento. Os diferentes tratamentos térmicos, condição solubilizada, H950 e H1000 apresentaram praticamente a mesma resistência à corrosão por pite. Por outro lado, o aço Custom 465® apresentou ótima resposta à inibição da nucleação de pite com adições crescentes de sulfato em meio de 0,6M NaCl, sendo que a condição H1000 se sobressai sobre a H950 nessa questão. Além disso, através da adição de sulfato foi 7 possível obter maior resistência a corrosão por pite do aço Custom 465® comparativamente ao aço 304L. Tal comportamento foi discutido em termos da afinidade química entre níquel, cloreto e sulfato, levando a maior resistência à corrosão por pite quando o aço contém maior teor de níquel (que é o caso do aço Custom 465®). Este trabalho indicou que o critério na escolha do tratamento de envelhecimento do aço Custom 465® deve ser o das propriedades mecânicas almejadas, uma vez que a resistência à corrosão por pite mostrou-se praticamente independente do tratamento térmico. / The precipitation hardened stainless steels have been widely used in the aircraft industry to combine mechanical strength, fracture toughness and corrosion resistance. And therefore, are materials that enable replacement of the carbon steels used today, which require additional surface treatment, such as cadmium plating, to improve the corrosion resistance. The use of such coatings brings disadvantages such as cost, manufacturing, susceptibility to hydrogen embrittlement beyond environmental aspects. In this context, the precipitation hardened steel UNS S46500, known as Custom 465® were evaluated for the effect of aging temperature on the mechanical properties and the resistance to pitting corrosion .Treated samples in solubilized and aged condition at 510°C (H950) and 538ºC (H1000) were subjected to tensile strength test, microstructural characterization and potentiodynamic polarization (PP) tests to determine the pitting corrosion resistance. The microstructural studies were performed with the aid of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. Analyzes were also performed using the Thermo -Calc software. The resistance to pitting corrosion was evaluated at 0.6M NaCl solution with increasing additions of Na2SO4. The results obtained from tests of PP in both heat treatment conditions were compared with each other and with results available in the literature (CALDERÓN HERNANDEZ, 2012) to stainless steel UNS S30403 (304L). The analysis of the microstructure revealed that the steel aged Custom 465 ® presents a martensitic matrix, chi precipitates, Ni3Ti precipitates and austenite phase. The H950 age treatment had higher mechanical strength and lower elongation than the treatment H1000. Such behavior was due to the production of higher percentage of chi and a lower percentage of austenite phase in aging treatment. The different heat treatments, solubilized condition, H950 and H1000 showed almost the same pitting corrosion resistance. On the other hand, the Custom 465® showed good response to pitting inhibition with increasing nucleating additions of sulfate in 0.6M NaCl, with enhanced result for H1000 condition. Furthermore, by adding sulfate was possible to obtain greater pitting corrosion resistance of Custom 465® compared to the 304L steel. This behavior has been discussed in terms of the chemical affinity between nickel, chloride and sulphate, leading to higher pitting corrosion resistance when the steel contains a higher nickel 9 content (which is the case of steel Custom 465®). This study indicates that the criterion in selecting the aging heat treatment of Custom 465® steel must be the desired mechanical properties, since the pitting corrosion resistance was found to be substantially independent of heat treatment.

A geração de novas fontes de energia limpa, segura e renovável por meio da fusão nuclear envolve importantes desafios tecnológicos, dentre eles a pesquisa, caracterização e a fabricação de materiais avançados para os futuros reatores de fusão nuclear. Os aços ferrítico-martensíticos de reduzida atividade radioativa, em especial a liga Eurofer-97, destacam-se por apresentar uma combinação única de propriedades para esta aplicação. O objetivo desta Tese de Doutorado é avaliar a estabilidade microestrutural deste aço recozido numa ampla faixa de temperaturas. Cálculos termodinâmicos e testes de dilatometria foram usados para determinar as temperaturas de transformação de fase. A estabilidade microestrutural foi estudada por meio de recozimentos isotérmicos entre 200 e 1350oC após laminação a frio com reduções de 40, 70, 80 e 90%. A avaliação da estabilidade mecânica do aço Eurofer-97 foi realizada por meio de medidas de dureza Vickers. As principais técnicas utilizadas para caracterização microestrutural foram microscopias eletrônica de varredura e de transmissão, tomografia por sonda atômica e medidas de magnetização DC. Tanto a textura como a microtextura foram determinadas por meio de medidas de difração de raios X e de elétrons retroespalhados (EBSD). Recuperação, recristalização primária e crescimento anormal de grão ocorrem neste material recozido abaixo de 800oC. Acima desta temperatura, a transformação martensítica ocorre alterando bastante a micoroestrutura. A cinética de crescimento anormal de grão é alterada pela quantidade de redução a frio previamente aplicada. A hipótese proposta para explicar o crescimento anormal de grãos neste material baseia-se principalmente na vantagem de tamanho adquirida pelos núcleos de recristalização primária com diferenças de orientação médias superiores a 45º em relação aos vizinhos. Neste caso, o crescimento anormal de grão é responsável por fortalecer as componentes {111} e {111}, {001} e {110}. Acima de 800oC a transformação martensítica prevalece elevando a dureza Vickers e randomizando a textura deste aço. As características do produto transformado dependem tanto da temperatura de austenitização quanto do tamanho incial do grão ferrítico. Os dados de composição química das partículas estáveis após recozimento em temperaturas inferiores a 800oC foram usados para validar os resultados dos cálculos termodinâmicos obtidos via Thermo-Calc. / Clean, safe, and renewable energy sources such as nuclear fusion comprise important technological challenges, including research, characterization and manufacture of advanced materials for future fusion reactors. Modified ferritic-martensitic steels with reduced radioactive activity (RAFM), especially Eurofer-97 steel, are among worldwide references in the nuclear field for their unique properties. The scope of this Thesis is to evaluate the microstructural (thermal) stability in ferritic-martensitic Eurofer-97 after annealing within a wide range of temperatures. Themodinamic calculations as well as dilatometric tests were used to determine the main phase transformation temperatures. The microstructural stability of this steel was followed by isothermal annealing between 200 and 1350°C after cold rolling to 40, 70, 80 and 90% reductions in thickness. The mechanical stability in the Eurofer-97 was assessed by Vickers microhardness measurements. Representative samples for each metallurgical condition were characterized by scanning electron microscopy, transmission electron microscopy, atom probe tomography, and DC-magnetization tests. Both texture and microtexture were evaluated by X-ray diffraction and electron backscattered diffraction (EBSD) techniques. Recovery, primary recrystallization, and abnormal grain growth (secondary recrystallization) processes have been observed at temperatures below 800°C. The amount of abnormally grown grains depends on the amount of previous cold rolling. The hypothesis for the most probable mechanism responsible for abnormal grain growth is based on the advantage size acquired by nuclei with misorientations above 45º surrounding their neighboring grains, even in regions where primary recrystallization was incomplete. The texture developed after abnormal grain growth has components belonging to ?- and ?-fibers with predominance of {111}, {111}, {100} e {110} components. The martensite transformation takes place when this steel is annealed above 800°C causing an increase of hardness, significant changes in microstructure, and texture weakening. The martensitic sructure depends very much on both austenitization temperature and initial austenitic grain size. The results of chemical analyses of stable particles present in samples annealed below 800oC were used to validate the thermodynamic calculations provided by Thermo-Calc.

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2019-02-08T11:08:03Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Willian Ricardo.pdf: 8465496 bytes, checksum: a01921f9da37abb9909a79e5b398408c (MD5) / Made available in DSpace on 2019-02-08T11:08:03Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Willian Ricardo.pdf: 8465496 bytes, checksum: a01921f9da37abb9909a79e5b398408c (MD5) Previous issue date: 2018-08-24 / Turbinas hidráulicas usadas em usinas hidrelétricas podem apresentar desgastes por cavitação e trincas em sua operação, as quais em alguns casos reparadas por soldagem a arco elétrico, por ser um processo de menor custo em comparação à troca da turbina. O aço inoxidável martensítico A743 grau CA6NM é empregado em serviços que requerem boa resistência mecânica associada a resistência à corrosão, principalmente em equipamento para extração de petróleo nos rotores e componentes de turbinas hidráulicas. Este trabalho tem por escopo analisar a microestrutura formada após processo de soldagem TIG (Tungsten Inert Gas) de um aço ASTM A743 CA6NM, usando material de adição similar, avaliando a presença da estrutura ferrita delta, austenita retida e carbonetos. Variando as espessuras das amostras e parâmetros de soldagem como passe único de soldagem e multipasse, mantendo temperatura de pré-aquecimento e interpasse de150ºC. Estimando a influência dos parâmetros na microestrutura com a dureza dos cordões e da ZTA. Os experimentos revelaram que um único passe de solda é suficiente para a formação de ferrita delta na forma de rede ao longo do cordão de solda e nas regiões expostas a altas temperaturas na zona termicamente afetada (ZTA), assim como o perfil de durezas da seção transversal do cordão de solda apontam durezas que variam desde 344 ± 6 HV para a região fundida, passando por um pico de dureza de 379 ± 4 HV na ZTA e terminando no metal base com 290 ± 5 HV, na solda multipasse a origem das diferentes morfologias de ferrita é múltipla, havendo uma diferença da taxa de resfriamento nos cordões,sendo que entre a base e o topo da solda pode ser percebido acumulo de ferrita delta na região interpasse e no topo do ultimo cordão de solda e a variação de largura da ZTA pelos passes subsequentes, a morfologia da ferrita delta zona termicamente afetada (ZTA) na condição multipasse, assim como o perfil de durezas da seção transversal dos cordões de solda apontam durezas que variam desde 311 ± 24 HV para a região fundida, passando por um pico de dureza de 326 ± 33 HV na ZTA. / Hydraulic turbines used in hydroelectric plants may present wear by cavitation and cracks in their operation, and in some cases repaired by electric arc welding, being a process of lower cost compared to the turbine exchange. A743 grade martensitic stainless steel CA6NM is used in services that require good mechanical resistance associated with corrosion resistance, especially in oil extraction equipment, rotors and hydraulic turbine components. This work aims to analyze the microstructure formed after TIG (Tungsten Inert Gas) welding of an ASTM A743 CA6NM steel, using similar addition material, evaluating the presence of the delta ferrite, retained austenite and carbides structure. By varying the sample thicknesses and welding conditions as single pass welding and multipass, maintaining preheating temperature and interpass of 150ºC. Estimating the influence of the parameters in the microstructure with the hardness of the strands and the ZTA. The experiments revealed that a single soldering pass is sufficient for the formation of delta ferrite in the form of a net along the weld bead and in the regions exposed to high temperatures in the thermally affected zone (ZTA), as well as the hardness profile of the section cross section of the weld beads indicate hardness ranging from 344 ± 6 HV to the molten region, passing through a hardness peak of 379 ± 4 HV in the ZTA and ending in the base metal with 290 ± 5 HV, in the multipass weld the origin of the different In the present study, the ferrite morphologies are multiple, with a difference in the cooling rate in the cords, between the base and the top of the weld it is possible to detect the accumulation of delta ferrite in the interpass region and at the top of the last weld bead and the ZTA The thermally affected zone delta ferrite (ZTA) morphology in the multipass condition, as well as the cross section hardness profile of the weld beads, point to hardnesses that vary from 311 ± 24 HV for the molten region, passing through a hardness peak of 326 ± 33 HV in the ZTA.

Este trabalho tem por objetivo avaliar os efeitos do hidrogênio em três diferentes tipos de aços de alta resistência mecânica. São descritos os fenômenos de introdução, difusão e aprisionamento de hidrogênio (H) dentro dos metais, em conjunto com os diferentes tipos de danos provocados devido à presença do hidrogênio. Os materiais de estudo são aços da família Advanced High Strength Steels (AHSS): aços Dual Phase (DP 1000 e DP 1200) e aço Martensítico (M 190). A introdução de hidrogênio nos materiais foi realizada através de carregamento catódico, o qual é representativo para as condições industriais a que se destinam. De modo a avaliar a influência do H nas propriedades mecânicas dos aços, os seguintes ensaios foram propostos: ensaio de carregamento com H, para determinar o conteúdo total de H (saturação) e conteúdo de H difusível (suscetibilidade a fragilização); ensaio de tração ao ar, para determinar a tensão no final da região elástica e resistência à tração na região do entalhe e ensaio de tração com carga constante em ambiente hidrogenado, para avaliar os efeitos provocados pela presença do H e determinar o patamar abaixo do qual o H não apresenta efeito crítico sobre o material. Os efeitos provocados pela aplicação de diferentes densidades de correntes (0,2 – 1,0 mA/cm²) foram avaliados nos ensaios de quantificação de H difusível. Conforme os resultados obtidos todos os aços apresentaram perdas na resistência mecânica à tração quando em ambiente hidrogenado, ou seja, sofreram fragilização por H. Os aços DP 1200 e M 190 (de microestrutura predominantemente martensítica) foram fortemente afetados, conforme evidenciado pela notável queda nos valores de tensão necessários para provocar a falha. Por outro lado, o aço DP 1000, de menor resistência mecânica, demonstrou menor suscetibilidade à fragilização, o que é atribuído a menor permeabilidade do H na microestrutura austenítica. / This work aims to evaluate the effects of hydrogen in three high- strength steel grades. The phenomena of hydrogen (H) entry, transport and trapping inside the metals, together with the different types of damages due to the presence of hydrogen are presented. The study materials are a range of AHSS steel grades: Dual Phase Steel (DP 1000 and DP 1200) and Martensitic Steel (M 190). The hydrogen entry was performed by cathodic charging, which is suitable for industrial applications. In order to evaluate the influence of H on the steel mechanical properties, the following tests were done: H charging, to measure total H content (saturation point) and diffusible H content (embrittlement susceptibility); uniaxial tensile test of uncharged samples to determine notched tensile strength values and the strength levels at the end of elastic region and constant load tensile testing carried out in hydrogen environment, to determine the threshold values where hydrogen has an effect on the material. DP 1200 and M 190 were strongly affected by H pre-charging, as shown by the significant drop in stress required to break them. On the other hand, DP 1000 showed a lower embrittlement susceptibility, which is attributed to its lower mechanical strength. The current densities effects (0.2 up to 1.0 mA/cm²) were evaluated during H charging to measure diffusible H content. All steels showed a drop in the tensile strength i.e. experienced hydrogen embrittlement. Steels with higher tensile strength, as DP 1200 and M 190, showed a much bigger drop that is related to the favorable characteristics of martensitic microstructure regarding to the hydrogen permeability and diffusivity.

The utilization of ferromagnetic shape memory alloys (FSMAs) in small scale devices has attracted considerable attention within the last decade. However, the lack of sufficient studies on their reversible shape change mechanisms, i.e, superelasticity, magnetic field-induced martensite variant reorientation and martensitic phase transformation, at the micron and submicron length scales prevent the further development and the use of FSMAs in small scale devices. Therefore, investigating the size effects in these mechanisms has both scientific and technological relevance. Superelastic behavior of Ni54Fe19Ga27 shape memory alloy single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters ranging between 200 nm and 10 µm were cut on a single crystalline bulk sample oriented along the [110] direction in the compression axis and with fully reversible two-stage martensitic transformation. The results revealed size dependent two-stage martensitic transformation which was suppressed for pillar sizes of 1 µm and below. We also demonstrated that the reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation in small scales. Size effects in the magnetic field-induced martensite variant reorientation were investigated in the Ni50Mn28.3Ga21.7 single crystals oriented along the [100] direction of the austenite phase. Single crystalline compression pillars were fabricated on the martensite twins between the sizes of 630 nm and 20 µm. It was found that the stress-induced and magnetic field-induced martensite variant reorientation are size dependent and became more difficult with the reduction in sample size. Surprisingly, it was still possible to magnetically activate the shape change in the micropillars which indicates the fact that magnetocrystalline anisotropy energy increases with the reduction in sample dimensions. Ni45Mn36.6Co5In13.4 pillars between the 600 nm and 10 µm diameters were investigated along the [100] direction of the austenite to study the size effects in the magnetic field-induced phase transformation (MFIPT). MFIPT was obtained down to 5 µm size in these pillars with reasonable magnetic field levels similar to their bulk counterparts.

High strength secondary hardening lath martensitic steel is a strong candidate for high performance and reliable transmission systems in aircraft and automotives. The fatigue resistance of this material depends both on intrinsic microstructure attributes, such as fine scale (M2C) precipitates, and extrinsic attributes such as nonmetallic primary inclusions. Additionally, the aforementioned attributes are affected by processing history. The objective of this research is to develop a computational framework to quantify the influence of both extrinsic (primary inclusions and residual stresses) and intrinsic (martensite laths and carbides) microstructure attributes on fatigue crack formation and the early stage of microstructurally small crack (MSC) growth that dominate high cycle fatigue (HCF) lifetime. To model the fatigue response at various microstructure scales, a hierarchical approach is adopted. A simplified scheme is developed to simulate processing effects such as shot peening that is suitable to introduce representative residual stresses prior to conducting fatigue calculations. Novel strategies are developed to couple process route (residual stresses) and microstructure scale response for comprehensive analysis of fatigue potency at critical life-limiting primary inclusions in gear steels. Relevant microstructure-scale response descriptors that permit relative assessment of fatigue resistance are identified. Fatigue crack formation and early growth is highly heterogeneous at the grain scale. Hence, a scheme for physically-based constitutive models that is suitable to investigate crack formation and early growth in martensitic steel is introduced and implemented. An extreme value statistical/probabilistic framework to assess the influence of variability of various microstructure attributes such as size and spatial distribution of primary inclusions on minimum fatigue crack formation life is devised. Understanding is sought regarding the relative role of microstructure attributes in the HCF process, thereby providing a basis to modify process route and/or composition to enhance fatigue resistance. Parametric studies are conducted to assess the effect of hot isostatic pressing and introduction of compliant coatings at debonded inclusion-matrix interface on enhancement of fatigue resistance. A comprehensive set of 3D computational tools and algorithms for hierarchical microstructure-sensitive fatigue analysis of martensitic gear steels is developed as an outcome of this research; such tools and methodologies will lend quantitative and qualitative support to designing improved, fatigue-resistant materials and accelerating insertion of new or improved materials into service.

Ou study focuses on the effects of phase transformations on crack propagation. We want to understand the changes of fracture toughness during welding. In this work, fracture toughness is expressed by J-integral. There are many experimental methods to obtain the critical toughness JIC but they are impractical for our investigation during phase transformation. That is the reason why we have proposed a method coupling mechanical tests, digital image correlation and finite element simulation. The fracture tests are implemented on pre-cracked single edge notched plate sample which is easy for machining and heat conduct during phase transformation. The tests are conducted at different temperatures until rupture. Digital image correlation gives us the displacement information on every sample. Each test is then simulated by finite element where the fracture toughness is evaluated by the method G-Theta at the crack propagation starting moment found by potential drop method and digital image correlation technical. Two materials have been studied, 15Cr-5Ni martensitic precipitation hardening stainless steel and 16MND5 ferritic low carbon steel. For these two materials, different test temperatures were chosen before, during and after phase transformation for testing and failure characterization of the mechanical behavior. Investigation result shows that metallurgical phase transformation has an influence on fracture toughness and further crack propagation. For 15-5PH, the result of J1C shows that the as received 15-5PH has higher fracture toughness than the one at 200°C. The toughness is also higher than the original material after one cycle heat treatment probably due to some residual austenite. Meanwhile, pure austenite 15-5PH at 200°C has higher fracture toughness than pure martensitic 15-5PH at 200°C. For 16MND5, the result also proves that the phase transformation affects fracture toughness. The as received material has bigger J1C than the situation where it was heated to 600°C. On the other hand, the material at 600°C just before isothermal bainite transformation after the austenitization during cooling process also has higher fracture toughness than the one at 600°C before austenitization. These two conclusions are consistent well with the result of 15-5PH. But the final situation of 16MND5 after one cycle heat treatment has a slightly smaller J1C than the receiving situation. It means that one cycle heat treatment hasn't an significant influence on 16MND5fracture toughness. Conclusions show that one should pay attention to the heating period before austenitization of the substrate material when people do the welding as the higher temperature will bring the lower fracture toughness during this process. While during cooling period, the fracture toughness doesn't change a lot during, before or after the cooling induced phase transformation. Even for 15-5PH, it has a better fracture toughness after the martensite transformation than before.

The need to develop light-weight and high strength materials for car frames which improve fuel efficiency and provide increased passenger safety during dynamic events such as automobile crashes has been the focus of the steel and automobile industries for the past 30 years. In recent years, the development of high strength steels such as multi-phase TRIP (Transformation-Induced Plasticity)-aided steels have shown great promise due to their excellent combination of high strength and ductility. The savings in automobile weight is provided by the inherent strength of TRIP steels which allows for the use of thinner sections. The TRIP effect is characterized by the phenomenon known as strain-induced martensitic transformation (SIMT) which enhances the work hardenability of such steels as the austenite phase transforms to the much harder martensite phase during plastic straining. This results in a resistance to local necking which subsequently enhances the strength, ductility, and formability of such steels. However, various factors exist which affect the mechanical behaviour of TRIP steels. This study will aim, through the use of finite element models, to investigate the role and influence of each of these factors on the TRIP effect in type 304 austenitic and multi-phase TRIP steels. These factors include the rate at which the martensitic transformation proceeds, the state of stress to which the material is subjected to, the interaction between the surrounding matrix and embedded retained austenite islands in multi-phase TRIP steels, and the volume fraction and morphology of the retained austenite islands. Investigation of these factors will provide further insight on each of their contributions to the TRIP effect in order to exploit the potential benefits offered by these steels.

In the course of this PhD thesis metastable Cu50Zr50-xTix (0≤ x ≤ 10) and (Cu0.5Zr0.5)100-xAlx (5 ≤ x ≤ 8) alloys were prepared and characterised in terms of phase formation, thermal behaviour, crystallisation kinetics and most importantly in terms of mechanical properties. The addition of Al clearly enhances the glass-forming ability although it does not affect the phase formation. This means that the Cu-Zr-Al system follows the characteristics of the binary Cu-Zr phase diagram, at least for Al additions up to 8 at.%. Conversely, the presence of at least 6 at.% Ti changes the crystallisation sequence of Cu50Zr50-xTix metallic glasses and a metastable C15 CuZrTi Laves phase (Fd-3m) precipitates prior to the equilibrium phases, Cu10Zr7 and CuZr2. A structurally related phase, i.e. the “big cube” phase (Cu4(Zr,Ti)2O, Fd-3m), crystallises in a first step when a significant amount of oxygen, on the order of several thousands of mass-ppm (parts per million), is added. Both phases, the C15 Laves as well as the big cube phase, contain pronounced icosahedral coordination and their formation might be related to an icosahedral-like short-range order of the as-cast glass. However, when the metallic glasses obey the phase formation as established in the binary Cu-Zr phase diagram, the short-range order seems to more closely resemble the coordination of the high-temperature equilibrium phase, B2 CuZr. During the tensile deformation of (Cu0.5Zr0.5)100-xAlx bulk metallic glasses where B2 CuZr nanocrystals precipitate polymorphically in the bulk and some of them undergo twinning, which is due to the shape memory effect inherent in B2 CuZr. Qualitatively, this unique deformation process can be understood in the framework of the potential energy landscape (PEL) model. The shear stress, applied by mechanically loading the material, softens the shear modulus, thus biasing structural rearrangements towards the more stable, crystalline state. One major prerequisite in this process is believed to be a B2-like short-range order of the glass in the as-cast state, which could account for the polymorphic precipitation of the B2 nanocrystals at a comparatively small amount of shear. Diffraction experiments using high-energy X-rays suggest that there might be a correlation between the B2 phase and the glass structure on a length-scale less than 4 Å. Additional corroboration for this finding comes from the fact that the interatomic distances of a Cu50Zr47.5Ti2.5 metallic glass are reduced by cold-rolling. Instead of experiencing shear-induced dilation, the atoms become more closely packed, indicating that the metallic glass is driven towards the more densely packed state associated with the more stable, crystalline state. It is noteworthy, that two Cu-Zr intermetallic compounds were identified to be plastically deformable. Cubic B2 CuZr undergoes a deformation-induced martensitic phase transformation to monoclinic B19’and B33 structures, resulting in transformation-induced plasticity (TRIP effect). On the other hand, tetragonal CuZr2 can also be deformed in compression up to a strain of 15%, yet, exhibiting a dislocation-borne deformation mechanism. The shear-induced nanocrystallisation and twinning seem to be competitive phenomena regarding shear band generation and propagation, which is why very few shear offsets, due to shear banding, can be observed at the surface of the bulk metallic glasses tested in quasistatic tension. The average distance between the crystalline precipitates is on the order of the typical shear band thickness (10 - 50 nm) meaning that an efficient interaction between nanocrystals and shear bands becomes feasible. Macroscopically, these microscopic processes reflect as an appreciable plastic strain combined with work hardening. When the same CuZr-based BMGs are tested in tension at room temperature and at high strain rate (10-2 s-1) there seems to be a “strain rate sensitivity”, which could be related to a crossover of the experimental time-scale and the time-scale of the intrinsic deformation processes (nanocrystallisation, twinning, shear band generation and propagation). However, further work is required to investigate the reasons for the varying slope in the elastic regime. As B2 CuZr is the phase, that competes with vitrification, it precipitates in a glassy matrix if the cooling rate is not sufficient to freeze the structure of the liquid completely. The pronounced work hardening and the plasticity of the B2 phase, which are a result of the deformation-induced martensitic transformation, leave their footprints in the stress-strain curves of these bulk metallic glass matrix composites. The behaviour of the yield strength as a function of the crystalline volume fraction can be captured by the rule of mixtures at low crystalline volume fractions and by the load bearing model at high crystalline volume fractions. In between both of these regions there is a transition caused by percolation (impingement) of the B2 crystals. Furthermore, the fracture strain can be modelled as a function of the crystalline volume fraction by a three-microstructural-element body and the results imply that the interface between B2 crystals and glassy matrix determines the plastic strain of the composites. The combination of shape memory crystals and a glassy matrix leads to a material with a markedly high yield strength and an enhanced plastic strain. In the CuZr-based metastable alloys investigated, there is an intimate relationship between the microstructure and the mechanical properties. The insights gained here should prove useful regarding the optimisation of the mechanical properties of bulk metallic glasses and bulk metallic glass composites.