Metals and alloys with exceptional resistance to wear and corrosion, capable of withstanding a wide range of temperatures and harsh corrosive environments, are critical for diverse applications, including aerospace, automotive, energy production, and medical devices. However, achieving this dual functionality is challenging due to an inherent trade-off: strengthening phases that enhance wear resistance often increase corrosion rates through micro-galvanic coupling. Simultaneously optimizing these conflicting properties represents a significant materials design challenge, further complicated by the vast compositional space of emerging high-entropy alloys (HEAs), also known as multi-principal element alloys (MPEAs) and compositionally complex alloys (CCMs). This Ph.D. thesis seeks to address these challenges by designing robust metals and alloys with outstanding wear and corrosion resistance, spanning conditions from ambient aqueous environments to extreme high-temperature oxidative settings. The research progresses through computational, experimental, and machine learning approaches, addressing key challenges in alloy design. Initially, the effects of alloying composition on the mechanical and corrosion properties of equiatomic and non-equiatomic NiCrFeCo alloys were systematically studied using density functional theory (DFT) calculations. Specifically, the influence of chromium (Cr) concentration was evaluated for compositions containing approximately 16, 25, and 34 at.% Cr. The results revealed that Cr plays a critical role in enhancing both mechanical and corrosion properties. Alloys with higher Cr content exhibited increased Young's modulus and Poisson's ratio, as well as superior corrosion resistance. Relaxing the equiatomic constraint further enabled simultaneous improvements in these properties, leading to the identification of an optimal non-equiatomic composition, Ni22Cr34Fe22Co22, which achieves a balance between mechanical strength and corrosion resistance. Subsequently, the focus shifted to designing microstructures in NiCrFeNb-based superalloys to enhance high-temperature tribo-oxidation behavior. The carefully engineered microstructures from additive manufacturing and post-printing heat treatment promoted the formation of layered spinel-based oxides (AB2O4, where A and B include Ni, Fe, and Cr) on the tribo-oxidized surfaces. These oxides enabled sustained self-lubrication, resulting in remarkably low coefficients of friction, ranging from 0.13 to 0.34, at temperatures between 600°C and 900°C. The superior lubricity was attributed to the structural incommensurability between AO4 and BO6 layers within the oxide, as well as shear-induced phase transformations during wear, offering a novel mechanism distinct from conventional solid lubricants. This work highlights the critical role of bulk microstructure design in achieving improved tribo-oxidation performance in Ni-based alloys at high temperature oxidative environment. Finally, a study on corrosion-resistant NiCrFeCo-based HEA design was conducted integrating computational, experimental, and machine learning approaches. Designing complex alloys with exceptional corrosion resistance across a wide range of corrosive environments, from acidic solutions to chloride-containing conditions, has traditionally relied on trial-and-error methods. This conventional approach is time-consuming and often inefficient, limiting the ability to explore the vast compositional space of modern alloys, such as HEAs. To overcome these challenges, this work adopts a machine learning-driven approach to predict and optimize corrosion behavior in HEAs. A deep neural network (DNN) was developed, integrating physical principles such as the Pilling-Bedworth Ratio to enhance the prediction of corrosion rates. The model was iteratively refined by synthesizing and experimentally testing alloy compositions with high prediction uncertainty, ensuring improved accuracy and reliability. Experimental variability was accounted for using Gaussian noise and dropout techniques, while the emphasis on physical descriptors over simple compositional data enabled more universal and generalizable predictions. The framework was validated by successfully synthesizing two novel HEAs with outstanding corrosion resistance across diverse environments. Future research will focus on expanding this approach to achieve simultaneous wear and corrosion resistance in HEAs, addressing the dual demands of modern materials in harsh environments. / Doctor of Philosophy / Metals and alloys that can resist both wear and corrosion are crucial for many applications, but creating materials that excel at both has been a persistent challenge. Traditionally, making a metal more resistant to wear often makes it more susceptible to corrosion, and vice versa. This challenge becomes even more complex with newer types of alloys that mix multiple elements in roughly equal amounts, known as high-entropy alloys. This thesis explores innovative ways to develop metals that can withstand both wear and corrosion across various conditions - from everyday environments to extreme high temperatures. Using computer simulations and theoretical calculations, I discovered that moving away from equal proportions of elements in these alloys could improve both their strength and corrosion resistance. This led to the development of a new alloy composition that performs better than traditional formulations. A particularly exciting discovery was made while studying high-temperature applications. It was found that certain nickel-based alloys can be additively manufactured and heat treated to form a protective oxide layer that acts like a built-in lubricant at high temperatures, significantly reducing friction. Additionally, I developed a machine learning system that can predict how well new alloy compositions will resist corrosion, leading to the successful creation of two new corrosion-resistant alloys. This work sets the foundation for a new approach to designing better metals, combining high-speed experimental techniques with computer simulations and artificial intelligence to discover alloys that can better resist both wear and corrosion.

Metal alloy honeycomb structures were fabricated using a paste extrusion technique and characterized for potential application as interconnects in solid oxide fuel cells. Thermal expansion characteristics of Fe-Cr, Fe-Ni, Ni-Cr, Fe-Ni-Cr, and similar alloys containing an oxide dispersion were determined and compared with the thermal expansion behavior of yttria-stabilized zirconia (YSZ). A method was developed to calculate thermal expansion mismatch between two materials under a variety of heating and cooling conditions. It was shown that Fe 20 wt% Cr and Fe 47.5 wt% Ni alloys have low expansion mismatch with YSZ under a wide range of heating and cooling conditions. Oxidation experiments showed that Fe-Cr alloys have superior oxidation resistance in air at 700℃compared with Fe-Ni-Cr alloys with similar chromium contents. The inclusion of oxide dispersions (Y₂O₃ and CaO) into an alloy honeycomb was shown to improve oxidation resistance without affecting thermal expansion behavior. The honeycomb extrusion process provides a method by which experimental alloys can be produced and characterized rapidly to develop an alloy suitable for use as an interconnect in a solid oxide fuel cell.

The purpose of this work is to optimize the chemical composition as well as the heat treatment for improving the mechanical performance of the TRIP steel by employing the theoretical models. TRIP steel consists of the microstructure with ferrite, bainite, retained austenite and minor martensite. Austenite contributes directly to the TRIP effect as its transformation to martensite under the external stress. In order to stabilize austenite against the martensitic transformation through the heat treatment, the two-step heat treatment is broadly applied to enrich the carbon and stabilize the austenite. During the first step of the heat treatment, intercritical annealing (IA), a dual phase structure (ferrite+austenite) is achieved. The austenite can be initially stabilized because of the low carbon solubility of ferrite. The bainite isothermal treatment (BIT) leads to the further carbon enrichment of IA-austenite by the formation of carbon-free ferrite. Comparing to the experiments, the thermodynamic and kinetic models are the lower and upper bounds of the carbon content of retained austenite. The mechanical properties are predicted using the swift model based on the predicted microstructure. In this work, a theoretical approach is coupled to a Genetic Algorithm-based optimization procedure to design (1) the heat treated temperatures to maximize the volume fraction of retained austenite in a Fe-0.32C-1.42Mn-1.56Si alloy and the chemical composition of (2) Fe-C-Mn-Si and (3) Fe-C-Mn-Si-Al-Cr-Ni alloy. The results recommend the optimum conditions of chemical composition and the heat treatment for maximizing the TRIP effect. Comparing to the experimental results, this designing strategy can be utilized to explore the potential materials of the novel alloys.

<p>Ni-based superalloys have been widely used for high temperature applications such as turbine blades for jet propulsion and power plants due to their excellent creep, fatigue, and corrosion resistance. But as the demand for higher temperature capability and strength increases, there remains a need to better understand high temperature deformation mechanisms and improve and strengthen superalloys at these elevated temperatures. Recently, a correlation has been observed between solute segregation at planar defects (stacking faults, antiphase boundaries, etc) and enhanced high temperature creep properties – known colloquially as phase transformation strengthening. Experimentally, regardless of alloy composition, strong Co segregation at planar defects along with Cr has been observed. In addition, it has been suggested by density functional theory work that Co would promote Cr concentration at stacking faults by forming strong Cr-Co bonds. Based on these findings, it was hypothesized the presence of Co provides a significant thermodynamic driving force for segregation to planar defects. </p><p>In order to further investigate the correlation between solute segregation and deformation mechanisms the fabrication of a planar front single crystal Ni-based superalloy and its microstructure, alloy composition, and microhardness properties of the as-zone melted and solution heat treated states were investigated and compared to the directionally-solidified state to study the effect of microsegregation on these alloy characteristics. Next, new Co-containing, Cr-free alloys are designed to optimize g-g’ volume fraction, size, and morphology to mimic microstructures observed in single crystal superalloys. The general alloy design strategy and approach are outlined, and the composition, microstructure, phase transformation temperatures, and mechanical properties of new Cr-free and Co-containing alloys are reported. A new set of Cr-free alloys have thus been designed, with modifications of Nb, Ta, and Ti additions ranging from 3 to 7 at.% to investigate the role of these elements on the phase transformation strengthening mechanism at elevated temperatures.</p><p></p>

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No. of bitstreams: 1 DissTPL.pdf: 6513464 bytes, checksum: 6fb12fd61a0888fcb4dff3aa607285ad (MD5) Previous issue date: 2017-03-23 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Stainless steels are outstanding candidates for applications in the oil exploitation at the pre-salt layer in Brazil since they have an excellent combination of mechanical and corrosion properties. Nevertheless, they have questionable wear resistance in case of more severe application conditions. Recent alloy developments from DEMa-UFSCar adopted a strategy based on boron additions to stainless steel processed by spray forming (SF). Boron additions lead to the formation of hard metallic borides and, in turn, improve the wear resistance. However, boride formation also induces other microstructural changes that, up to now, have not been systematically investigated. Additionally, recently proposed solidification models of spray-formed Fe-based alloys indicate that solidification in the deposition zone tends to follow the thermodynamic equilibrium. Consequently, this present work proposes a methodology employing a computational thermodynamic tool with the ultimate goal of developing Fe-based alloys with boron additions prepared by spray forming. This work focused on spray-formed duplex stainless steels modified with boron (AIDM) containing a hypoeutectic composition with a duplex matrix. The thermodynamic software (Thermo-Calc® 4.0) with the TCFe7 database was used. Firstly, experimental results of the superduplex and ferritic modified stainless steels with boron already described in the literature were used for validation of the computational tool. After this step, a methodology for the thermodynamic simulation was proposed aiming at finding a chemical composition that should lead to specific pre-established microstructural features in the AIDM. As means of experimental validation for the proposed methodology, the selected chemical composition was prepared by spray forming and the microstructure investigated. The results showed that all pre-established microstructural features were indeed achieved thus supporting the proposed methodology for alloy design. Therefore the application of computational thermodynamics seems to be an excellent resource for designing spray-forming boron-modified stainless steels. / Os aços inoxidáveis são potenciais candidatos para atuação na exploração de petróleo na camada de pré-sal, pois apresentam uma excelente combinação entre propriedades mecânicas e resistência à corrosão. No entanto, não possuem uma boa resistência ao desgaste. Trabalhos recentes realizados no DEMa-UFSCar adotaram como estratégia a adição de boro em ligas de aços inoxidáveis processadas por meio da conformação por spray (CS). Os resultados apontaram a formação de boretos que aumentaram significativamente a resistência ao desgaste, mas também provocaram mudanças microestruturais na matriz, que não foram investigadas. Modelos de solidificação de ligas a base de ferro conformadas por spray recentemente propostos na literatura indicam que a solidificação na zona de deposição tende a seguir o caminho descrito pelo equilíbrio termodinâmico. Nesse contexto, o presente trabalho propõe uma metodologia que utiliza uma ferramenta computacional termodinâmica com intuito de desenvolver ligas a base de Fe modificadas com boro conformadas por spray. Este trabalho teve como foco o desenvolvimento de um aço inoxidável duplex modificado com boro (AIDM) conformado por spray, com composição hipoeutética (matriz duplex) por meio do programa termodinâmico Thermo-Calc® versão 4.0, com a base de dados TCFe7. Resultados experimentais dos aços inoxidáveis superduplex e ferrítico modificados com boro já descritos na literatura foram utilizados inicialmente para validação da ferramenta computacional. Após validação, foi desenvolvida uma metodologia baseada em simulação termodinâmica visando projetar uma composição química que permitisse a obtenção de um AIDM com características microestruturais pré-estabelecidas. Para validação experimental da metodologia proposta, a composição química selecionada foi conformada por spray e o depósito teve sua microestrutura caracterizada por diferentes técnicas. Os resultados mostraram que todas as características microestruturais pré-estabelecidas foram atingidas, validando a metodologia proposta. A ferramenta computacional se mostrou um excelente recurso para projetar ligas inoxidáveis modificadas com boro conformadas por spray.

Les aciers Dual-Phase sont largement utilisés dans le secteur de l’automobile enraison de leurs propriétés mécaniques remarquables et du bon compromis résistanceductilité qui lui donne d’intéressante potentialités comme absorbeur d’énergiemécanique. Cependant, la recherche de bons compromis entre les propriétésmécaniques en traction et celles de formabilité nécessite une optimisation desparamètres microstructuraux. Ce travail de thèse s’inscrit dans cet optique. Dans unepremière partie, l’étude bibliographique proposée permet de mieux cerner lesparamètres influençant la formation des microstructures ainsi que les propriétés desaciers DP. Dans une seconde partie, nous proposons un travail expérimental originalpermettant de mieux comprendre la formation des microstructures des aciers DP etde découpler l’effet de certains paramètres microstructuraux sur les propriétés deces aciers. Enfin, la modélisation micromécanique proposée permet de compléter etd’interpréter les données expérimentales acquises. Ce travail ouvre des voiesintéressantes de « design » des microstructures des aciers DP en vue de développerdes aciers de nouvelles générations possédant des propriétés optimisées. / Ferrite-martensite dual-phase (DP) steels have been widely used in automotiveindustry due to their excellent mechanical properties, such as high work-hardeningrate and a good compromise between strength and ductility allowing high energyabsorbing performance. In order to fully exploit the potential of DP steels and extendthe application, the dual-phase microstructure has to be optimized for bettercombination of strength and formability that is characterized by uniform strainand/or fracture strain. As a starting point, detailed literature review is made on themicrostructure development and mechanical properties of DP steels, and the keyfactors controlling microstructural features and determining mechanical propertiesare identified. Through experimental investigation, microstructures are developed inorder to decouple the effects of various microstructural features, and themicrostructure—mechanical properties relationship is systematically studied.Micromechanical modeling is used to further understand the experimental resultswithin a quantitative framework, and to provide a support for microstructurerefinement of DP steels by parametric study. Strategies of designing DP steels tofulfill specific forming operation have been proposed, and the concept of DP steelswith graded martensite islands has been discussed with FEM analysis as a possibilityof improving strength—formability trade-off.

科学研究費補助金 研究種目:研究(A)(1) 課題番号:07555500 研究代表者:森永 正彦 研究期間:1995-1996年度