Understanding the precipitation reaction is very important since precipitation hardening is one of the most effective strengthening mechanisms in metallic alloys. In martensitic steels, a tempering heat treatment is often performed. During tempering various new phases are precipitated and the spatial and temporal evolution of these precipitates strongly influences the properties of the steel, such as strength/ductility, creep, fatigue and hot corrosion resistance. Therefore, the possibility of quantitative modelling of the precipitation process will provide many opportunities for advanced materials and process design and optimization as well as service life assessments. The Fe-C-Cr system forms the basis for tool steels and is consequently used in many applications such as e.g. metal forming operations. They are characterized by a high hardness and good toughness, even at elevated temperatures.In the present work, the as-quenched martensitic microstructures of four Fe-C-Cr alloys with varying Cr and C contents were characterized by Light Optical Microscopy (LOM) and Electron Microscopy. The effects of Cr and C on the morphology of martensite were investigated. It was found that Cr addition had a similar effect as C on the martensitic morphology and on the ratio of high-angle grain boundary (HAGB) to low-angle grain boundary (LAGB). However, the micro-hardness was unaffected by the Cr addition whilst it was strongly influenced by the C addition.In addition, a quantitative experimental characterization of the precipitates formed during tempering of the martensite was performed. The Langer-Schwartz theory combined with the Kampmann-Wagner-Numerical (KWN) method, as implemented in the software TC-PRISMA, was used to predict the precipitation of carbides after tempering in one of the model alloys: Fe-0.15C-4.0Cr (mass%). The microstructure characterization of the as-quenched material provided vital input parameters for the modelling work and a comparison was made between the modelling predictions and the experimental results. The effect of parameters such as dislocation density, grain size and interfacial energy on the precipitation of carbides was discussed. / <p>QC 20151105</p>

This diploma thesis studies the structure of selected austenitic stainless steels which were fatigued at room and depressed temperatures. The theoretical part is divided into three parts. The first one defines the austenitic stainless steels with accent to their chemical composition and microstructure. Second part is dedicated to deformation-induced martensite formation. The third part shows the mechanisms and stages of damage due to cyclic straining. The experimental part is focused especially on the observing of microstructural changes by methods of colour metallography and their relation to stress-strain response of material under selected conditions of fatigue loading. Series of samples of 304 and 316L steels were prepared by colour etching and their microstructures were documented using optical microscopy. Then the semi-quantitative measurements of presence and morphology of deformation-induced martensite were done and the results were discussed and correlated with fatigue damage level.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19893号 / 工博第4209号 / 新制||工||1651(附属図書館) / 32970 / 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 白井 泰治, 教授 松原 英一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No description available.

Revealing the prior austenite grain boundaries from a martensitic structure is well known to be very difficult and dependent on the chemical composition and the thermomechanical processing of the steel. In the present study, four different chemical etching reagents and additional thermal etching have been conducted for thermomechanical simulated tool steels Orvar Supreme and Stavax ESR. The etching results have been characterized using light optical microscopy and electron backscattered diffraction. The obtained results show that saturated aqueous picric acid, oxalic and sodium bisulfite based acid reveals prior austenite grain boundaries well for Orvar Supreme. For Stavax ESR, only aqueous CrO3-NaOH-picric acid provides good results in revealing the prior austenite grain boundaries. Thermal etching shows good potential and if conducted properly, thermal etching is a good alternative to the chemical reagents from a health- and environmental perspective.

No description available.

No description available.

The martensite and magnetic transformations in Ni-Fe-Ga ferromagnetic shape memory alloys are very sensitive to both alloy chemistry and thermal history. A series of Ni-Fe-Ga alloys near the prototype Heusler composition (X2YZ) were fabricated and homogenized at 1423 °K, and a Ni₅₃Fe₁₉Ga₂₈ alloy was subsequently annealed at various temperatures below and above the B2/L21 ordering temperature. Calorimetry and magnetometry were employed to measure the martensite transformation temperatures and Curie temperatures. Compositional variations of only a few atomic percent result in martensite start temperatures and Curie temperatures that differ by about 230 °K degrees and 35 °K degrees, respectively. Various one-hour anneals of the Ni₅₃Fe₁₉Ga₂₈ alloy shift the martensite start temperature and the Curie temperature by almost 70 °K degrees. Transmission electron microscopy investigations were conducted on the annealed Ni₅₃Fe₁₉Ga₂₈ alloy. The considerable variations in the martensite and magnetic transformations in these alloys are discussed in terms of microstructural differences resulting from alloy chemistry and heat treatments. The phase-field method has been successfully employed during the past ten years to simulate a wide variety of microstructural evolution in materials. Phase-field computational models describe the microstructure of a material by using a set of field variables whose evolution is governed by thermodynamic functionals and kinetic continuum equations. A two dimensional phase-field model that demonstrates the ferromagnetic shape memory effect in Ni2MnGa is presented. Free energy functionals are based on the phase-field microelasticity and micromagnetic theories; they account for energy contributions from martensite variant boundaries, elastic strain, applied stress, magnetocrystalline anisotropy, magnetic domain walls, magnetostatic potential, and applied magnetic fields. The time-dependent Ginzburg-Landau and Landau-Lifshitz kinetic continuum equations are employed to track the microstructural and magnetic responses in ferromagnetic shape memory alloys to applied stress and magnetic fields. The model results show expected microstructural responses to these applied fields and could be potentially utilized to generate quantitative predictions of the ferromagnetic shape memory effect in these alloys. / Ph. D.

L’entraînement par application d’un champ externe s'est révélé être un moyen efficace d'améliorer la déformation induite par champ magnétique (Magnetic-Field-Induced Strain MFIS) dans les alliages Heusler de type Ni-Mn-Ga, en éliminant les variantes défavorables. Pour guider la procédure de l’entraînement, les mécanismes de l’entraînement des alliages à martensite 5M ou NM ont été étudiés, alors que ceux des alliages à martensite 7M ne sont pas entièrement clarifiés. Dans ce travail, les mécanismes de l’entraînement mécanique et thermomécanique ont été étudiés en analysant l'évolution de la microstructure et de l'orientation cristallographique au cours de ces processus. Tout d'abord, des caractérisations de microstructure et d'orientation cristallographique ont été réalisées dans l'état recuit sur l'alliage Ni50Mn30Ga20 préparé par solidification directionnelle. Cinq colonies transformées à partir d'un grain parent d'austénite ont été observées avec chaque colonie consistant en quatre variantes avec les relations d’orientation de Type-I, Type-II et composé transformation (TrF)-macle rapports. En supposant une charge de compression appliquée le long de la direction de solidification (SD), les cinq colonies pourraient être divisées en deux groupes par rapport au facteur de Schmid (Schmid Factor SF) des systèmes de démaclage de Type-I/Type-II TrF-macle des variantes dans la colonie : trois d'entre eux ont des SF élevés et désignés comme des colonies élevées de SF et les deux autres colonies de SF inférieurs. Ensuite, une compression unidirectionnelle a été effectuée sur l'alliage avec la charge appliquée le long de SD. En caractérisant l'évolution de la microstructure et le changement d'orientation cristallographique, les mécanismes de déformation ont été analysés. La déformation au stade précoce était principalement située dans certaines bandes initiées à partir des colonies de SF élevés et traversant les colonies de SF inférieurs. Le démaclage de Type-II/Type-I TrF-macle s'est produit principalement dans des colonies de SF élevés, ce qui a entraîné l'épaississement des variantes 7M favorables au détriment des variantes adjacentes. Les systèmes de maclage de Type-I/Type-II déformation (DeF)-macle et de réarrangement des variantes dans les colonies de SF inférieurs ont été activés, ce qui a entraîné la formation de nouvelles variantes 7M et NM. Les déformations correspondantes dans les colonies de SF inférieurs sont fortement coordonnées avec celles des colonies de SF élevés permettant la formation des bandes de déformation et l'accommodation de la déformation macroscopique. Au cours du stade avancé, le maclage de Type-I/Type-II DeF-macle et le réarrangement ont progressé pour coordonner la déformation macroscopique. Le processus de réarrangement inverse a été activé pour accommoder la déformation locale. Les nombres de colonies et de variantes sont considérablement réduits. Le chemin et le produit de la transformation martensitique ont également été fortement influencés par la déformation macroscopique imposée. Sous une petite charge, l'austénite transformée en martensite 5M suit à la fois l’OR Pitsch et une nouvelle OR plutôt que le martensite 7M auto-accommodée sous l’OR Pitsch. Avec l'augmentation de la charge appliquée, l'austénite s'est transformée presque simultanément en martensite 7M sous une nouvelle OR et en martensite 5M. Après la transformation martensitique, 5M s’est ensuite transformé en martensite 7M avec la diminution de la température sous la charge appliquée. La transformation martensitique a été modifiée par la contrainte externe en termes de produit de la transformation et de chemin de transformation pour accommoder la déformation macroscopique imposée. Ce travail offre de nouvelles idées sur les mécanismes de déformation des alliages Ni-Mn-Ga [...] / External field training is proven to be an effective way to improve the magnetic-field-induced strain (MFIS) in Ni-Mn-Ga Heusler type alloys by eliminating the unfavorable variants. To guide the training procedure, the training mechanisms of alloys with 5M or NM martensite have been investigated, whereas those for alloys with 7M martensite are not fully clarified. In this work, the mechanisms of mechanical and thermo-mechanical training were studied by analyzing the microstructure and crystallographic orientation evolution during these processes.Firstly, microstructure and crystallographic characterizations were performed on the as-annealed Ni50Mn30Ga20 alloy. 5 colonies transformed from one parent austenite grain were observed with each colony consisting of four variants with Type-I, Type-II and compound Transformation (TrF)-twin relations. By assuming an applied compressive load along the solidification direction (SD), 5 colonies could be divided into two groups with respect of the Schmid factor (SF) of detwinning systems of Type-I/Type-II TrF-twin of the in-colony variants: three of them have high SF and referred to as high SF colonies and the other two low SF colonies.Then unidirectional compression was performed on the alloy with the load applied along the SD. By characterizing the microstructure evolution and crystallographic orientation change, the deformation mechanisms were analyzed. The deformation in the early stage was mainly located in some band regions initiated from the high SF colonies and going through the low SF colonies. The detwinning of Type-II/Type-I TrF-twin occurred primarily in high SF colonies, resulting in the thickening of the favorable 7M variants at the expense of the adjacent variants. The twinning of Type-I/Type-II Deformation (DeF)-twin and shuffling systems of the variants in low SF colonies were activated, leading to the formation of new 7M variants and NM. The corresponding strains in the low SF colonies were highly coordinated with those in the high SF colonies allowing the formation of the deformation bands and the accommodation of the macroscopic strain. During the late stage, twinning of Type-I/Type-II DeF-twin and shuffling further progressed to coordinate the macroscopic strain. Reverse shuffling process was activated to accommodate the local deformation. The numbers of colony and variant were greatly reduced.The path and the product of martensitic transformation were also strongly affected by the imposed macroscopic deformation. Under a small load, austenite transformed to 5M martensite following both the Pitsch and a new OR rather than the self-accommodated 7M martensite under the Pitsch OR. With the increase of the applied load, austenite transformed almost simultaneously to 7M martensite under a new OR and 5M martensite. After the martensitic transformation, 5M further transformed to 7M martensite with the decrease of the temperature under the applied load. The martensitic transformation was modified by the external stress in terms of the transformation product and the transformation strain path to accommodate the imposed macroscopic deformation.This work offers new insights into the deformation mechanisms of the Ni-Mn-Ga alloys under unidirectional compression that are useful for the design of effective training procedures and provides new perspectives on further investigations of external field training on Ni-Mn-Ga alloys

Actuellement, l’industrie automobile est à la recherche d’une meilleure solution pour l’allégement de la structure de véhicule afin de diminuer la consommation de carburant et par conséquent diminuer les émissions nocives de CO2. Les aciers à très haute résistance (THR) mécanique permettent d’obtenir les tôles d’acier à section diminué avec les mêmes ou meilleurs propriétés fonctionnels. Les aciers Dual-Phase (DP), constitués majoritairement d’une phase ductile, la ferrite, et d’une phase dure, la martensite, occupent une place importante en tant que matériaux de structure destinés au challenge préoccupant l’industrie automobile. Une bonne résistance à l’endommagement est exigée pour leur utilisation en tant que des pièces de structures et de renfort pour l’automobile. Il a été bien établi que la résistance à l’endommagement des ces aciers Dual-Phase est contrôlée par leur microstructure. Ce travail de thèse s’est inscrit dans une logique de compréhension des mécanismes d’endommagement d’un acier Dual-Phase modèle, le DP 780, en fonction de différents paramètres microstructuraux. Deux mécanismes d’endommagement ont été identifiés pour l’acier DP 780 : la décohésion de l’interface ferrite/martensite et la formation de cavités autour des carbures, dans la martensite revenue. Un modèle qualitatif de mécanisme d’endommagement a été développé afin de pouvoir prédire l’endommagement de l’acier DP 780. Ce modèle qualitatif, développé pour l’acier DP 780, servira de base d’approfondissement de modèles plus élaborés et quantitatifs permettant la compréhension et la prédiction de l’endommagement des aciers Dual-Phase, de façon générale / In the automotive industry current environmental concerns require that the vehicle fuel consumption and CO2 emissions should be reduced as much as possible. It is therefore advantageous to reduce the weight of body in white components by replacing existing parts with higher strength, thinner gauge alternatives with equivalent or improved functional properties. Dual Phase (DP) steels are a class of high-strength low-alloy steels characterized by a microstructure consisting of martensite and ferrite. Dual Phase steels combine high strength levels with good ductility. Thus, DP steels are potentially very attractive for the automobile industry. In addition to the required high strength and ductility, DP steel has to be cold formed into complex shapes. It appears that DP steel damage behaviour is very complex and cannot be predicted using existing models based on standard mechanical properties. This work is concerned with the study of microstructural evolution and investigation of the relation between the microstructure and damage mechanisms in a reference DP 780 steel. Two damage mechanisms have been identified in this DP steel: ferrite/martensite interface decohesion and void formation at tempered carbides. A simple modeling for qualitative description of the observed damage formation mechanisms is proposed. This modeling permits a basic understanding of the experimentally observed trends and could be used as the starting point for a more detailed analysis in future