京都大学 / 新制・課程博士 / 博士(工学) / 甲第23507号 / 工博第4919号 / 新制||工||1768(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 安田 秀幸, 教授 奥田 浩司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Die Arbeit widmet sich der Entwicklung und numerischen Implementierung eines nichtlokalen kontinuumsmechanischen Schädigungsmodells zur Beschreibung des duktilen Versagens eines austenitischen TRIP-Stahlgusses. Dieser weist eine martensitische Phasenumwandlung während der Verformung auf. Das Umwandlungs- und Verfestigungsverhalten des Untersuchungswerkstoffs hängt stark von Temperatur und Spannungszustand ab. Deshalb wird ein vollständig thermomechanisch gekoppeltes Viskoplastizitätsmodell zugrunde gelegt, welches die temperaturabhängige Zug-Druck-Asymmetrie von Verfestigung und verformungsinduzierter Martensitentwicklung abbildet. Bei erhöhter Dehnrate können experimentell beobachtete Kreuzungseffekte der Fließkurven vorhergesagt werden. Die Schädigungsmodellierung baut auf dem viskoplastischen Grundmodell auf, wobei das netzunabhängige Verhalten durch eine Gradientenerweiterung im Rahmen der mikromorphen Theorie erreicht wird. Im Modell können verschiedene Ansätze für Schädigungsinitiierung und -entwicklung kombiniert werden. Die Einflüsse der Modellparameter auf die Ergebnisse von Risswachstumssimulationen werden für ausgewählte Modellvarianten untersucht und bewertet. Mithilfe erarbeiteter Kalibrierungsstrategien können die qualifizierten Varianten erfolgreich an experimentelle Ergebnisse von Kerbzugversuchen und bruchmechanischen Kompaktzugproben angepasst werden. / The present thesis comprises the development and numerical implementation of a non-local damage model in order to describe ductile failure of a cast austenitic TRIP-steel. The TRIP-steel shows a martensitic phase transformation during deformation. The transformation and strain hardening behavior is strongly dependent on temperature and stress state. For this reason, a fully thermomechanically coupled viscoplasticity model is proposed, which exhibits the temperature dependent tension-compression-asymmetry of strain hardening and deformation-induced martensite evolution. Experimentally observed crossing effects of the flow curves can be predicted at increased strain rates. The damage modeling is based on the viscoplastic basic model, whereby the mesh-independent behavior is achieved by a gradient extension within the framework of micromorphic theory. Different approaches for damage initiation and evolution can be combined within the model. The influences of the model parameters on results of crack growth simulations are investigated and evaluated for selected model variants. With the help of developed calibration strategies the qualified variants can be successfully adapted to experimental results of notched tensile tests and compact tension tests.

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Formgedächtnislegierungen wie Nickel-Titan (NiTi) können sich nach einer plastischen Verformung und anschließendem Aufheizen an ihre ursprüngliche Form „erinnern“ und diese wieder einnehmen. Als meistverwendete Formgedächtnislegierung kann NiTi als Aktor, zur Dämpfung und zur elastokalorischen Kühlen verwendet werden und kommt von der Medizintechnik bis hin zur Luft- und Raumfahrt zum Einsatz. Der Formgedächtniseffekt basiert auf der martensitischen Phasenumwandlung, einer diffusionslosen Strukturänderung, bei der sich die Kristallsymmetrie ändert. Bei NiTi mit etwa 50 At.-% Ni wandelt die kubische Hochtemperaturphase (Austenit) in die monokline Tieftemperaturphase (Martensit) um. Während dieser Umwandlung entsteht eine Vielzahl an Grenzflächen, wodurch sich ein komplexes martensitisches Gefüge – eine Art dreidimensionales „Puzzle“ bildet. Um NiTi-Formgedächtnislegierungen auf verschiedene Anwendungen zuzuschneiden und deren Eigenschaften zu verbessern, ist es wichtig, das Gefüge zu verstehen. Die häufig eingesetzten polykristallinen NiTi-Schichten haben dabei den Nachteil, dass die enthaltenen Korngrenzen einen zusätzlichen Parameter darstellen, der Gefügeuntersuchungen erschwert. Dagegen werden epitaktische Schichten bereits für andere magnetische Formgedächtnislegierungen als Modellsystem eingesetzt und tragen zu einem besseren Verständnis der martensitischen Umwandlung bei. Epitaktische Schichten sind einkristallin, sodass der Einfluss von Korngrenzen ausgeklammert werden kann. Außerdem dient das Substrat, das die Orientierung der Schicht vorgibt, als festes Referenzsystem. In dieser Arbeit wurden epitaktische NiTi-Schichten mit Magnetron-Sputterdeposition hergestellt, die bei Raumtemperatur martensitisch sind. Dabei wurde der Einfluss von Parametern wie Herstellungstemperatur, chemische Zusammensetzung, Wärmebehandlungsszenarien und Pufferschichten auf das Wachstum und die Eigenschaften der Schichten untersucht. So konnten Schichten in zwei unterschiedlichen Orientierungen, (100) und (111), hergestellt werden. Die so optimierten Schichten wurden anschließend dafür genutzt, das martensitische Gefüge skalenübergreifend zu untersuchen. Mit einer Kombination von Mikroskopie- und Röntgenbeugungsmethoden wurden die auftretenden Zwillingsgrenzen, Habitusebenen und Variantenorientierungen analysiert. So lässt sich feststellen, welche Martensitcluster entstehen, wie sie nukleieren und wachsen und welche Grenzflächen auftreten. Dabei ließ sich ein hierarchischer Aufbau des martensitischen Gefüges feststellen, wobei drei Zwillingsgrenzen auf unterschiedlichen Längenskalen für die Beschreibung des Gefüges nötig sind. Die auftretenden Zwillingsgrenzen sind aus Massivmaterialien bekannt, was zeigt, dass sich die Schichten gut als Modellsystem eignen. Das identifizierte, dreidimensionale Modell des Gefüges wurde mit Röntgenmethoden global bestätigt. Dazu wurden die experimentellen Ergebnisse mit zwei unterschiedlichen Martensittheorien, der phänomenologischen Martensittheorie (PTMC) und der Korrespondenztheorie (CT) verglichen. Der hierarchische Aufbau des Gefüges lässt sich zum Großteil mit den Theorien beschreiben. Die Schichten zeigen aber auch die Limitierungen der bisherigen Theorien und bieten so eine Möglichkeit für deren Weiterentwicklung.

Metastable austenitic and duplex stainless steels are widely used materials in industrial anddomestic applications, owing to their attractive characteristics such as good corrosion resistanceand favorable mechanical properties. Both types of steel experience enhanced mechanicalproperties during plastic deformation due to the formation of the martensite phase from theparent austenite phase, this is called deformation-induced martensitic transformation (DIMT).It is therefore of technical interest to study the transformation mechanism and its impact onmechanical properties for a better understanding and ultimately for developing new materialswith improved performance in certain applications. In the present thesis, two austenitic stainless steels (201Cu, HyTens® 301) and two duplexstainless steels (FDX25®, FDX27®) were investigated. Samples were tensile tested during insitusynchrotron radiation experiments performed at the Cornell High Energy SynchrotronSource (CHESS), Ithaca, USA. Tests were performed at both room temperature and at elevatedtemperatures. The collected diffraction data were then processed by software such as Fit2D andMATLAB. Quantitative phase fraction analysis based on the direct comparison method wasperformed successfully. Microstructural analysis of samples before deformation and after thefull tensile testing was also performed using electron microscopy. The deformation induced martensitic transformation took place in HyTens 301, FDX25 andFDX27, but in 201Cu the austenite was stable during the tensile tests conducted here. The a’-martensite formed in a significantly higher fraction than the ε-martensite in all alloys. At roomtemperature, the critical stress levels for martensitic transformation were 490 MPa, 700 MPaand 700MPa for HyTens 301, FDX25 and FDX27, respectively.

Polycrystalline NiTi shape memory alloys have the ability to recover their original, pre-deformed shape in the presence of external loads when heated through a solid-solid phase transformation from a lower-symmetry B19' martensite phase to a higher-symmetry B2 austenite phase. The strain associated with a shape memory alloy in an actuator application typically has thermal, elastic and inelastic contributions. The objective of this work was to investigate the aforementioned strains by recourse to in situ neutron diffraction experiments during selected combinations of heating, cooling and/or mechanical loading. The primary studies were conducted on polycrystalline Ni49.9Ti50.1 specimens on the Spectrometer for MAterials Research at Temperature and Stress (SMARTS) at Los Alamos National Laboratory. Quantitative information on the phase-specific strain, texture and phase fraction evolution was obtained from the neutron data using Rietveld refinement and single-peak analyses, and compared with macroscopic data from extensometry. First, the lattice strain evolution during heating and cooling in an unloaded sample (i.e., free-recovery experiment) was studied. The lattice strain evolution remained linear with temperature and was not influenced by intergranular stresses, enabling the determination of a thermal expansion tensor that quantified the associated anisotropy due to the symmetry of B19' NiTi. The tensor thus determined was subsequently used to obtain an average coefficient of thermal expansion that was consistent with macroscopic dilatometric measurements and a 30,000 grain polycrystalline self-consistent model. The accommodative nature of B19' NiTi was found to account for macroscopic shape changes lagging (with temperature) the start and finish of the transformation. Second, the elastic response of B19' martensitic NiTi variants during monotonic loading was studied. Emphasis was placed on capturing and quantifying the strain anisotropy which arises from the symmetry of monoclinic martensite and internal stresses resulting from intergranular constraints between individual variants and load re-distribution among variants as the texture evolved during variant reorientation and detwinning. The methodology adopted took into account both tensile and compressive loading given the asymmetric response in the texture evolution. Plane specific elastic moduli were determined from neutron measurements and compared with those determined using a self-consistent polycrystalline deformation model and from recently reported elastic stiffness constants determined via ab initio calculations. The comparison among the three approaches further helped understand the influence of elastic anisotropy, intergranular constraint, and texture evolution on the deformation behavior of polycrystalline B19' NiTi. Connections were additionally made between the assessed elastic properties of martensitic NiTi single crystals (i.e., the single crystal stiffness tensor) and the overall macroscopic response in bulk polycrystalline form. Lastly, the role of upper-cycle temperature, i.e., the maximum temperature reached during thermal cycling, was investigated during load-biased thermal cycling of NiTi shape memory alloys at selected combinations of stress and temperature. Results showed that the upper-cycle temperature, under isobaric conditions, significantly affected the amount of transformation strain and thus the work output available for actuation. With the objective of investigating the underlying microstructural and micromechanical changes due to the influence of the upper-cycle temperature, the texture evolution was systematically analyzed. While the changes in transformation strain were closely related to the evolution in texture of the room temperature martensite, retained martensite in the austenite state could additionally affect the transformation strain. Additionally, multiple thermal cycles were performed under load-biased conditions in both NiTi and NiTiPd alloys, to further assess and understand the role of retained martensite. Dimensional and thermal stabilities of these alloys were correlated with the volume fraction and texture of retained martensite, and the internal strain evolution in these alloys. The role of symmetry, i.e., B19' monoclinic martensite vs. B19 orthorhombic martensite in these alloys was also assessed. This work not only established a methodology to study the thermal and elastic properties of the low symmetry B19' monoclinic martensite, but also provided valuable insight into quantitative micromechanical and microstructural changes responsible for the thermomechanical response of NiTi shape memory alloys. It has immediate implications for optimizing shape memory behavior in the alloys investigated, with extension to high temperature shape memory alloys with ternary and quaternary elemental additions, such as Pd, Pt and Hf. This work was supported by funding from NASAÂ s Fundamental Aeronautics Program, Supersonics Project (NNX08AB51A) and NSF (CAREER DMR-0239512). It benefited additionally from the use of the Lujan Neutron Scattering Center at Los Alamos National Laboratory, which is funded by the Office of Basic Energy Sciences (Department of Energy) and is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.

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O aço maraging é conhecido pela sua alta resistência mecânica proveniente da formação de precipitados intermetálicos como Ni3Ti e Fe2Mo durante o envelhecimento, porém com subsequente perda de tenacidade à fratura. Existe uma vasta literatura sobre este tipo de aço, o qual é submetido a diversos tratamentos térmicos. Normalmente dois tratamentos térmicos são aplicados: uma homogeneização inicial para solubilização dos precipitados, sendo a recomendação geral 820 °C/1 h, e posteriormente o tratamento de envelhecimento, variando a temperatura de 455 °C a 510 °C e o tempo de 3 a 12 h. No entanto, pode haver variações no tratamento, como aquecimento após homogeneização numa faixa de temperatura intercrítica ou abaixo da Ac1 visando a reversão da martensita em austenita e o refino de grão. Até o presente momento, não está bem definida a influência da temperatura/tempo em tratamentos de temperaturas inter ou subcríticas que visam o refinamento da estrutura e o aumento da austenita retida/reversa. A literatura mostra tratamentos na liga maraging 300 por meio de estudos mais antigos, da década de 70 e 80, utilizando aquecimentos curtos e cíclicos para reversão e estabilização da austenita reversa. Estudos mais recentes de outras ligas utilizaram tratamentos isotérmicos para difusão e reversão da martensita em austenita. Nesta proposta serão pesquisadas as duas rotas de tratamento, cíclica e isotérmica, para avaliar a reversão martensita-austenita. Os tratamentos cíclicos foram caracterizados por EBSD, MEV e difração de raio-X ex-situ. Os tratamentos isotérmicos foram caracterizados por EBSD e difração de raio-X de fonte sincrotron in-situ, ou seja, medida em tempo real durante o tratamento. Ambas as condições foram avaliadas mecanicamente por meio de ensaios de flexão de 3 pontos. / Maraging steel is known for its high mechanical strength resulting from the formation of intermetallic precipitates such as Ni3Ti and Fe2Mo during aging heat treatment, with subsequent loss of fracture toughness. There is a vast literature on this type of steel, which is subjected to various thermal treatments. Normally two heat treatments are applied: an initial homogenization for solubilization of the precipitates, the general recommendation being 820 ° C/1 h, and later the aging treatment, varying temperature from 455 ° C to 510 ° C and time of treatment from 3 to 12 hours. However, there may be applied other heat treatments, such as heating after homogenization in an intercritical temperature range or below Ac1 for the reversion of martensite-to-austenite and grain refinement. To date, the influence of temperature and time on inter- or subcritical temperature treatments aiming at grain refinement and martensite-to-austenite reversion is not well defined. The literature shows treatments in the maraging 300 alloy, from the 70s and 80s, using short and cyclic heat treatment for reversion and stabilization of austenite. Recent studies of other alloys have used isothermal treatments for diffusion and martensite-to-austenite revresion. In this study, two heat treatment routes, cyclical and isothermal, were investigated to evaluate the martensite-to-austenite reversion. Cyclic treatments were characterized by EBSD, SEM and ex-situ X-ray diffraction. The isothermal treatments were characterized by EBSD and X-ray diffraction of synchrotron source in-situ, that is, measured in real time during the treatment. Both conditions were mechanically evaluated by 3-point-bending tests.