Abstract This thesis considers two related problems where heat conduction is accompanied by phase transformation during the cooling of carbon steels – phenomena which are fundamental to modern thermomechanical treatment processes. In the first problem, a non-linear heat equation formulation is described which includes linkage between irreversible diffusive and diffusionless phase transformation processes and thermophysical properties. A family of numerical schemes using finite difference methods with diagonally implicit Runge-Kutta method integrators for solving this formulation is present. Sample calculations using these schemes are presented for a mild steel cooled under range of industrially relevant heat transfer conditions. These calculations illustrate the need for careful scheme design and solver selection when coupled heat conduction and microstructure evolution equations become non-linear and stiff. The second section of the thesis extends this heat conduction formulation into the realm of inverse analysis. The problem of temperature reconstruction and boundary condition estimation using temperature measurements obtained thermocouples embedded into laboratory samples undergoing phase transformation is considered. A solution method based on iterative regularization is described for solving the resulting ill-posed problem. An analysis of extremely high rate cooling experiments made on a pilot-plant descaling unit using the inverse method is presented. This analysis highlights some of the practical issues associated with embedded thermocouple temperature measurements made in this fashion. / Tiivistelmä Väitöstyössä tutkitaan kahta ongelmaa liittyen hiiliteräksen termomekaanisen käsittelyn numeeriseen mallintamiseen. Työssä tarkastellaan teräksessä tapahtuvaa lämmönsiirtoa ja faasimuutoksia jäähdytyksen aikana, eli ensisijaisen tärkeitä ilmiöitä kehittynyttä teräksen valmistusta ja prosessointia ajatellen. Ensimmäisenä ongelmana tutkitaan epälineaarisen energiayhtälön muodostaminen huomioiden teräksen termofysikaaliset materiaaliominaisuudet, diffuusion kontrolloimat ja diffuusiosta riippumattomat irreversiibelit faasimuutokset. Lisäksi huomioidaan näiden väliset kytkennät. Yhtälöt ratkaistaan numeerisesti ja käytetyt menetelmät esitellään differenssimenetelmien ja diagonaalisesti implisiittisten Runge-Kutta-menetelmien osalta. Väitöstyössä näiden käyttöä havainnollistetaan simuloimalla niukkahiilisen teräksen käyttäytymistä teollisia olosuhteita vastaavan jäähdytyksen aikana. Laskentatulokset osoittavat ratkaisumenetelmän suunnittelun ja valinnan tärkeyden erityisesti silloin, kun lämmönjohtuminen, mikrorakenteen kehittyminen ja termofysikaaliset materiaaliominaisuudet muuttavat ratkaistavan ongelman epälineaariseksi ja jäykäksi. Toisena ongelmana tutkitaan edellä esitetyn lämmönsiirtomallin käyttöä osanainversioanalyysiä. Laboratoriokokeiden, termopareilla suoritettujen lämpötilamittausten, työssä kehitetyn lämmönsiirtomallin ja suoritetun inversioanalyysin avulla rekonstruoidaan teräskappaleiden lämpötila- ja faasimuutoskäyttäytymistä sekä estimoidaan mallin reunaehtoja. Näin luotu inversio-ongelma on kuitenkin matemaattisesti tarkasteltuna ns. huonosti asetettu ongelma, ja sen ratkaisemiseksi käytetään iteratiiviseen säännöllistämiseen perustuvaa menetelmää. Väitöstyön inversioanalyysiä havainnollistetaan pilot-mittakaavan kokeiden avulla, joissa hilsepesuria käyttäen koekappaleen jäähtymisnopeus saadaan erittäin korkeaksi. Tulokset nostavat esille inversio-analyysiin liittyviä ongelmia ja rajoitteita nopeaan jäähtymiseen sekä lämpötilamittauksiin liittyen.

La présente thèse s’inscrit dans un contexte d’optimisation des procédés de forgeage et de traitements thermiques des alliages de titane pour les pièces de moteurs Snecma.Les sociétés TIMET, élaborateur, et SNECMA, motoriste, ont identifié des enjeux très analogues concernant le forgeage des billettes en alliage de titane destinées à la fabrication de disques moteurs. Il est nécessaire d'acquérir une maîtrise complète de la gamme de transformation, de la coulée au produit final. Ce traitement thermomécanique comporte une alternance d’étapes de forgeage dans les domaines bêta, à haute température, et alpha + bêta, à basse température.L’objectif est de déterminer expérimentalement et de modéliser le comportement mécanique ainsi que les évolutions de microstructure de l’alliage Ti17 au cours d'une déformation dans le domaine alpha + bêta suivie d’un maintien en bêta.En amont, ces travaux mènent à une meilleure compréhension des facteurs qui influencent la taille de grain dans le domaine bêta : la taille de grain bêta initiale, les orientations des aiguilles alpha dans le grain, la déformation et la désorientation subies lors du forgeage en alpha + bêta, etc.En ce qui concerne les applications, ils s’intègrent à un post-processeur métallurgique dédié au forgeage des alliages de titane, qui utilise les histoires thermomécaniques issues d’un calcul par éléments finis. / The thesis is part of a broader optimization program related to the forging and the heat treatments of SNECMA-engine components in titanium alloys.TIMET (producer) and SNECMA (engine manufacturer) companies have identified very similar issues concerning the forging of billets in titanium alloy for manufacturing engine discs. They need thus to gain complete control over the transformation range, from the casting to the final product. This thermomechanical treatment involves alternating forging steps in the beta domain, at high temperature, and in the alpha + beta field, at lower temperatures.The aim is to simulate the forging step in the two-phase, alpha + beta, range followed by recrystallization in the beta field during which microstructure evolves. This simulation involves the following phenomena: grain deformation, recrystallization and grain growth of the beta matrix and disorientation/fragmentation of the alpha-Widmanstätten platelets. More specifically, the effects of deformation in the alpha + beta field on the subsequent beta-grain growth during the heat treatment at higher temperatures have been analyzed in the case of Ti17 titanium alloy.Upstream, the work leads to a better understanding of the factors that influence grain size in the beta domain: the initial beta-grain size, the orientation of alpha needles in the grains, the distortion and disorientation experienced during the alpha + beta forging, etc. With regard to the applications, the above results will be implemented into a metallurgical post-processor dedicated to the forging process of titanium alloys, using the thermomechanical history resulting from a finite-element calculation.

Orientador: Prof. Dr. Sydney Ferreira Santos / Tese (doutorado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, Santo André, 2018. / Molas automotivas são componentes de segurança veicular que demandam um controle rigoroso de suas características. Por este motivo, necessitam ser confeccionadas a partir de aços de alta qualidade, tanto em termos microestruturais quanto de propriedades mecânicas. A fabricação de tais molas envolve diversas etapas de processamento, destacando-se as de tratamentos térmicos e termomecânicos, que são críticas na determinação do desempenho da mola. No presente trabalho, foi investigada a influência dos parâmetros de tratamentos térmicos e termomecânicos na microestrutura e microdureza do aço mola SAE 9254. Como primeira etapa deste trabalho, foi avaliada a influência dos parâmetros de tratamentos térmicos na microestrutura do aço SAE 9254. Este estudo foi realizado com amostras tratadas em forno convencional e também simulando os ciclos térmicos destes tratamentos num dilatômetro de têmpera, que permitiu determinar a sequência de transformações de fases durante a têmpera, para taxas de resfriamento selecionadas. Analisou-se também a influência dos tempos e temperaturas de austenitização empregados nas temperaturas de transformações de fase. Observou-se que a evolução do tamanho médio dos grãos austeníticos prévios apresentou comportamento cinético esperado em função das variáveis adotadas. Foi também possível determinar as mudanças nas temperaturas de transformações de fase (Ar1, Ar3, Ac1, Ac3 e Ms) em função das taxas de resfriamento investigadas. Com base nas informações de dilatometria, foi determinado o diagrama de transformação em resfriamento contínuo (TRC) do aço SAE 9254. Na segunda etapa desta tese, amostras do mesmo aço foram tratadas termomecanicamente em um simulador físico Gleeble. No tratamento termomecânico, diversos fenômenos metalúrgicos ocorrem concomitantemente, tais como transformações de fases, encruamento, e fenômenos de amaciamento. Por simulação física, é possível identificar diversas informações relativas a estes fenômenos em função dos parâmetros de processamento empregados. Demonstrou-se a forte influência da taxa de deformação em diversos parâmetros das curvas de escoamento plástico, o que foi explicado com base na evolução microestrutural do aço, bem como aspectos relevantes de metalurgia física da liga. / Automotive springs are safety components of the vehicles requiring a rigorous control of all their main features. Therefore, they must be produced from high-quality steels with controlled microstructural features and mechanical properties. The production of such components encompass a number of processing steps, with special attention for heat treatments and thermomechanical treatments which are critical steps to determine the spring performance. In this thesis, the influence of heat and thermomechanical treatment parameters on the microstructure and microhardness of the SAE 9254 steel were investigated. In a first step of this Thesis work, the influence of heat treatment parameters on the steel microstructure was investigated using conventional furnace and quenching dilatometer. This later approach allowed accessing the phase transformations under gone by this steel during quenching for several cooling rates. The influence of asutenitizing times and temperatures on these phase transformations was also accessed. The grain growth of the (previous) austenite phase showed the expected kinetic behavior. Changes in phase transformation temperatures (Ar1, Ar3, Ac1, Ac3, and Ms) were also determined as function of the investigated quenching rates. Based on the information obtained by dilatometry, the continuous cooling transformation (CCT) diagram of the SAE 9254 steel was determined. In the second step, the SAE steel was thermomechanically treated in a Gleeble simulator. During thermomechanical treatments, several metallurgical phenomena take place simultaneously, such as phase transformations, work hardening, and softening. Performing the thermomechanical treatments in a Gleeble machine, some important information can be accessed allowing understanding the metallurgical phenomena taking place as function of the processing parameters investigated. The strong influence of the deformation rate in some relevant aspects of the plastic flow curves could be demonstrated which could be explained based on the microstructural characterization of the steels and fundamental aspects of physical metallurgy.

The recent emphasis on magnesium alloys can be appreciated by following the research push from several agencies, universities and editorial efforts. With a density equal to two-thirds of Al and one-thirds of steel, Mg provides the best opportunity for lightweighting of metallic components. However, one key bottleneck restricting its insertion into industrial applications is low strength values. In this respect, Mg-Y-Nd alloys have been promising due to their ability to form strengthening precipitates on the prismatic plane. However, if the strength is compared to Al alloys, these alloys are not attractive. The primary reason for low structural performance in Mg is related to low alloying and microstructural efficiency. In this dissertation, these terminologies are discussed in detail. A simple calculation showed that the microstructural efficiency in Mg-4Y-3Nd alloy is 30% of its maximum potential. Guided by the definitions of alloying and microstructural efficiency, the two prime objectives of this thesis were to: (i) to use thermomechanical processing routes to tailor the microstructure and achieve high strength in an Mg-4Y-3Nd alloy, and (ii) optimize the alloy chemistry of the Mg-rare earth alloy and design a novel rare—earth free Mg alloy by Calphad approach to achieve a strength of 500 MPa. Experimental, theoretical and computational approaches have been used to establish the process-structure-property relationships in an Mg-4Y-3Nd alloy. For example, increase in strength was observed after post aging of the friction stir processed/additive manufactured microstructure. This was attributed to the dissolution of Mg2Y particles which increased the alloying and microstructural efficiency. Further quantification by numerical modeling showed that the effective diffusivity during friction stir processing and friction stir welding is 60 times faster than in the absence of concurrent deformation leading to the dissolution of thermally stable particles. In addition, the investigation on the interaction between dislocations and strengthening precipitate revealed that, specific defects like the I1 fault aid in the accelerated precipitation of the strengthening precipitate in an Mg-4Y-3Nd alloy. Also, the effect of external field (ultrasonic waves) was studied in detail and showed accelerated age hardening response in Mg-4Y-3Nd alloy by a factor of 24. As the bottleneck of low strength is addressed, the answers to the following questions are discussed in this dissertation: What are the fundamental micro-mechanisms governing second phase evolution in an Mg-4Y-3Nd alloy? What is the mechanical response of different microstructural states obtained by hot rolling, friction stir processing and friction stir additive manufacturing? Is defect engineering critical to achieve high strength Mg alloys? Can application of an external field influence the age hardening response in an Mg-4Y-3Nd alloy? Can a combination of innovative processing for tailoring microstructures and computational alloy design lead to new and effective paths for application of magnesium alloys?

<p>Structural metallic materials exposed to energetic particle bombardments often experience various types of irradiation-induced microstructural damage, thus degrading the mechanical properties of the materials in form of irradiation hardening and embrittlement. Nanostructured materials have shown better radiation resistance than their coarse-grained (CG) counterparts due to the existence of abundant defect sinks, such as grain boundaries, twin boundaries, and phase boundaries. However, recently developed nanocrystalline (NC) steels show limited room-temperature tensile ductility (< 1%), which may become a concern for their future application for nuclear reactors. The focus of this thesis is to explore the strength-ductility dilemma in modified 9Cr1Mo (T91) ferritic/martensitic (F/M) steel processed by thermomechanical treatment (TMT) and surface severe plastic deformation (SSPD) with an attempt to fabricate strong, ductile and radiation resistant F/M steels. </p> <p><b>Carbon partitioning</b> between the quenched martensite and the other phases (bainitic ferrite or retained austenite) is critical for enhancing the strength and ductility of T91 steel. The tensile properties of partially tempered (PT) T91 steel can be tailored through introducing bainitic ferrite with high-density nanoscale transition carbides and refined lath martensite. In addition, retained austenite was introduced by increasing the carbon concentration of T91 steel to 0.6 wt.%. The carbon-modified steel processed by quenching partitioning (Q-P) treatment exhibits an ultrahigh strength, ~ 2 GPa, with a uniform strain of ~ 5% due to the existence of coherent carbides, ultrafine martensite and retained austenite. </p> <p>Meanwhile, surface mechanical grinding treatment (SMGT) on T91 steel reveals that introducing <b>gradient structures</b> on the sample surface contributes to a higher strength and an improved plasticity than its homogeneously structured counterpart. The deformation mechanism of the gradient structures was investigated with the assistance of quasi <i>in situ</i> crystal orientation analyses. Furthermore, <i>ex situ</i> He ion irradiation on the gradient T91 steel indicates that radiation-induced damage, such as bubble-induced swelling and irradiation hardening, were gradually mitigated by grain refinement from the sample surface to the center, resulting in superior radiation resistance. The results obtained from this thesis may facilitate the design and fabrication of strong, ductile and radiation-tolerant F/M steels.</p>