The introduction of composition gradients into the already hierarchical structure of martensitic steel leads to difficulties in modeling that arise from events occurring in the material at different length scales. In this thesis we isolate the features that are important to describing the mechanical properties of martensite and constitutively couple them through their respective length scales. The idea of a representative volume element is rigorously explored in which the microstructure is represented through a Masing model as well as more advanced structures akin to a nanocomposite. As such, we are able to keep track of microscopic yielding and internal stress evolution at the smallest scales (nanoscale through microscale). With the use of representative volume elements, we are able to track events at the largest scale as well by freely being able to change scale. As such, macroscopic phenomenon such as: thermal fields, composition fields, macroscopic loads, and the associated macroscopic phase distributions and stress distributions are evaluated. We conclude by demonstrating the power of this modelling technique in the design and optimization of compositionally graded steel structures via virtual prototyping. / Dissertation / Doctor of Philosophy (PhD)

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In this study, Thermomechanical properties of Ti-50.8 and 50.7 at% Ni alloy severely deformed using Equal Channel Angular Extrusion (ECAE) are investigated. The aim of this study is to reveal the effects of severe plastic deformation on shape memory, pseudelasticity, interplay between plastic deformation via dislocation slip and twinning, and forward and reverse martensitic transformation. The samples are processed at room temperature, i.e. slightly above the austenite finish temperature, and at 450 °C, i.e. well-above the austenite finish temperature. Transformation temperatures, microstructural evolution, and thermomechanical properties of ECAE processed samples are studied before and after low temperature annealing heat treatment and compared with conventional cold drawn and precipitation hardened material. The unique findings are: 1) the observation of a mixture of heavily deformed B2 (austenite) and B19’ (martensite) phases in the samples processed at room temperature although martensite stabilization was expected, 2) the observation of highly organized, twin-related nanograins in B2 phase of the samples deformed at room temperature which was attributed to B2 to B19' via SIM, and B19' to B2 via SPD (SIM: Stress Induced Martensitic transformation, SPD: Severe Plastic Deformation) transformation sequence, 3) simultaneous observation of B2 austenite and strain induced B19’ martensite in the samples deformed at 450 °C, and 4) perfect pseudoelasticity, small pseudoelastic stress hysteresis and excellent cyclic response with no irrecoverable strain up to 1000 cycles for ECAE at 450 °C processed sample. Strain induced martensite in NiTi alloys was reported for the first time. The formation of well-organized twin-related nanograins via severe plastic deformation opens a new opportunity for twinning induced grain boundary engineering in NiTi alloys which significantly improves the matrix strength and the cyclic response against degradation of shape memory and pseudoelasticity.

In this study, Thermomechanical properties of Ti-50.8 and 50.7 at% Ni alloy severely deformed using Equal Channel Angular Extrusion (ECAE) are investigated. The aim of this study is to reveal the effects of severe plastic deformation on shape memory, pseudelasticity, interplay between plastic deformation via dislocation slip and twinning, and forward and reverse martensitic transformation. The samples are processed at room temperature, i.e. slightly above the austenite finish temperature, and at 450 °C, i.e. well-above the austenite finish temperature. Transformation temperatures, microstructural evolution, and thermomechanical properties of ECAE processed samples are studied before and after low temperature annealing heat treatment and compared with conventional cold drawn and precipitation hardened material. The unique findings are: 1) the observation of a mixture of heavily deformed B2 (austenite) and B19’ (martensite) phases in the samples processed at room temperature although martensite stabilization was expected, 2) the observation of highly organized, twin-related nanograins in B2 phase of the samples deformed at room temperature which was attributed to B2 to B19' via SIM, and B19' to B2 via SPD (SIM: Stress Induced Martensitic transformation, SPD: Severe Plastic Deformation) transformation sequence, 3) simultaneous observation of B2 austenite and strain induced B19’ martensite in the samples deformed at 450 °C, and 4) perfect pseudoelasticity, small pseudoelastic stress hysteresis and excellent cyclic response with no irrecoverable strain up to 1000 cycles for ECAE at 450 °C processed sample. Strain induced martensite in NiTi alloys was reported for the first time. The formation of well-organized twin-related nanograins via severe plastic deformation opens a new opportunity for twinning induced grain boundary engineering in NiTi alloys which significantly improves the matrix strength and the cyclic response against degradation of shape memory and pseudoelasticity.

Transformation Induced Plasticity (TRIP) steels have both high strength and high ductility. Retained austenite in the microstructure, upon straining, transforms to martensite and this absorbs energy and improves the work hardening of the steel, giving improved elongation. The transformation can be either stress-assisted or strain-induced and the initiation and the mechanism depend on the composition of, the size and shape of, and the phases surrounding, the austenite grains. It is important to understand the relationship between these variables and the properties of the TRIP steel. The aim of this work was to determine how the microstructure of the TRIP steel affects the transformation. Four experimental microstructures were developed, containing austenite grains with different sizes, shapes, and surrounding phases. The Fine microstructure had thin elongated austenite laths between fine bainitic ferrite laths, the Coarse microstructure had elongated austenite grains between coarser bainitic ferrite laths, the Equiaxed microstructure had equiaxed austenite grains in a matrix of equiaxed ferrite and the Acicular microstructure had elongated austenite grains surrounded by recovered ferrite laths. Tensile tests were performed and detailed characterization, using neutron diffraction, was done of samples with the four microstructures. The variation in the amount of austenite during deformation was measured. The tensile tests revealed that the microstructures had different mechanical properties and different transformation behaviours. Fine had the lowest elongation and the highest strength. Acicular and Equiaxed had good elongation but lower strength. Coarse had intermediate strength and Equiaxed had sustained work hardening. The transformation in Fine and Coarse was minimal. Coarse had some slow, steady transformation, but Fine may have had none. The transformation in Equiaxed was larger. It started quickly and then slowed at higher strains. The austenite in Acicular transformed steadily. The predominant mechanism of transformation was stress-assisted transformation, with strain-induced transformation occurring only in Equiaxed. The results of this work showed that the influence of the surrounding phases on the stability of the austenite is significant. The differences in the transformation behaviour of the four microstructures seemed to be due more to the surrounding phases than the grain size or the composition, although both these factors also played a role. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2007-12-14 13:35:07.248

Understanding the changes occurring in the mechanical properties during martensite tempering is essential in the development of new industrial grades. The aim of this research was to develop new ultra-high strength steels via nanoprecipitation control, which requires an understanding of the processing-microstructure-property relationship in medium carbon (0.5-0.6 wt.%) steels throughout tempering. Much of the work has been centred in understanding the role of silicon at the precipitation level and in the recovery of martensite. By using an existing spring steel grade, the effect of interrupted ageing (IA) in tempered martensite has been studied. In IA, an intermediate step between quenching and tempering is introduced, where quenched martensite is left to rest at room temperature for a defined period of time. By allowing carbon segregation into dislocation cores, the incorporation of IA resulted in a more stable microstructure and hardness improvement. The effect of silicon in the epsilon to cementite carbide transition has also been studied. The classical nucleation theory was applied in order to model cementite formation under paraequilibrium conditions, thus incorporating silicon during nucleation. Characterisation using high energy X-rays showed the inhibiting effect of silicon in the overall cementite precipitation. The second effect of silicon was observed in the martensite recovery. A series of experiments were carried out in order to capture the various microstructural changes taking place during tempering: precipitation, grain size and dislocation density evolution. It was observed that the addition of silicon reduces the rate of martensite recovery, owing to the reduced cross-slip in the ferrite lattice. A plasticity model based on irreversible thermodynamics and EBSD characterisation was applied to identify the effective grain size. The results from these two techniques require further research. Nevertheless, based on the post-failure analysis by TEM, it appears that at relatively early tempering stages, even low angle lath boundaries can contribute to strengthening, where piled-up dislocations have been observed at lath boundaries.

Martensitic transformation is very important in austenitic stainless steels where the transformation induced plasticity phenomenon provides a combination of good mechanical properties, such as formability and strength. However, the difficulty of predicting the material behaviour is one of the major drawbacks of these steels. In order to model this behaviour it is of great importance to be able to characterize the morphology, crystallography and the amount of different types of martensite. The morphology and crystallography of thermal and deformation induced lath martensite in stainless steels were re-examined by means of optical microscopy and electron backscatter diffraction (EBSD) technique. The experiments were performed on AISI301, 304 and 204Cu austenitic stainless steels. Plastic deformation was carried out by means of uniaxial tensile tests at the strain rate of  to produce strain induced α’-martensite at a temperature ranging from 0 to 60ºC. An in-situ measurement of the martensite content was performed during the tensile testing using a Ferritescope to provide the necessary experimental values for modelling. Optical microscopy revealed the morphology of the strain induced α’-martensite as sets of thin parallel needles that go through the parent austenite grain and stop at the grain or annealing twin boundaries. Large amount of α’-martensite could be seen at the intersection of shear bands. However, considerable amount of α’-martensite was also observed when only one set of bands is activated. EBSD was successfully used to analyze the morphology and crystallography of martensite. The α’-martensite maintained the Kurdjumov-Sachs (K-S) orientation relationship with the austenite phase. Although all six possible variants did not appear within a single packet, one or two variants were often favoured out of six related to the specific {111} plane. The misorientations between the neighbouring variants were mainly <111> 60º or <110> 49.5º.

nicialmente descreve-se o Sistema Cu-A1 e as fases que ocorrem do lado rico em cobre; em seguida faz-se uma revisão sobre as reações de decomposição da fase ? de alta temperatura no resfriamento e sobre as reações de decomposição da martensita ?\'? no revenido. Faz-se também a revisão de alguns conceitos teóricos sobre interfaces e estabilidade das microestruturas. Estudou-se, na parte experimental, a decomposição isotérmica (revenido) da martensita ?\'? de duas ligas hipoeutetóides em temperaturas que correspondem aos campos ? + ?? (520 ºC) e ? + ? (650 ºC) do diagrama Cu-A1. Realizaram-se ainda experiências de solubilização no campo ? + ? a 576 ºC da estrutura ? + ?? obtida pelo revenido por 90 minutos a 520 ºC. No revenido a 520 ºC da martensita ?\'? de ambas as ligas observou-se inicialmente a precipitação de alotriomorfos de fase ? em contornos de grão, a reversão parcial da martensita para fase ?? , e transformação in situ da martensita em fase ? supersaturada, que gradualmente passa a fase ? de equilíbrio. Com tempos maiores, observou-se a decomposição da fase ?? em ?? e? . A fase ? tem a forma de plaquetas que mantém planos de hábito herdados da martensita. A fase ?? precipita como glóbulos que coalescem sem formar contornos de grão, formando lamelas de interface lisas entre as plaquetas de ? . A estrutura ? + ?? não coalesce nem esferoidiza para os tempos estudados, o que pode ser explicado pela existência de interfaces ? / ?? , e ? / ?? de baixa energia. Para se estudar a existência de interfaces de baixa energia entre a fase ? da transformação in situ e a fase ? da reversão da martensita, realizou-se a decomposição da martensita no campo ? + ? .Observou-se que a fase ? resultante de transformação in situ mantém os planos de hábito da martensita; as plaquetas de fase ? coalescem sem esferoidizar, mantendo interfaces retas com a fase ? e os mesmos planos de hábito que a martensita obtida pelo resfriamento da fase ? de reversão. Cada grão de ? original dá origem a apenas um grão de ? , com a mesma orientação cristalográfica. Para estudar a existência de interfaces de baixa energia entre as fases ?? e ?? e de ambas com fase ? , realizou-se a solubilização no campo ? + ? da estrutura ? + ?? obtida por revenido. Verificou-se que a fase ? precipita no interior da fase ?? na forma de glóbulos, que depois crescem dissolvendo parcialmente a fase ? . As interfaces ? / ? mantém-se planas e as plaquetas de ? mantém os planos de hábito herdados da martensita. Cada antigo grão de ? dá origem a um grão de ? com a mesma orientação. Discutiu-se os resultados sob o ponto de vista da relação entre morfologia e a natureza das interfaces. Concluiu-se que os resultados podem ser explicados pela existência de interfaces ? / ?? , ? / ?? e ? / ? de baixa energia em algumas direções e interfaces ?? / ?? e ?? / ? em todas as direções. Discutiu-se ainda a origem de relações de orientação entre os reticulados que correspondem a interfaces de baixa energia. / The Cu-Al system, in the copper-rich side, is described. The reactions that occur during decomposition of the high temperature ? phase, as well as the decomposition of the ?\'? martensite during tempering, are reviewed. Some theoretical concepts about interfaces and microstructural stability are also reviewed. In the experimental part of this dissertation, the isothermal decomposition (tempering) of the ?\'? martensite is studied in two hipoeutectoid alloys, at temperature in the ? + ?? field (520 ºC) and in the ? + ? field (650 ºC) of the Cu-Al equilibrium diagram. Experiments on the solubilization of the ? + ?? structure obtained by tempering, in the ? + ? field of the equilibrium diagram at 576 ºC, are also made. During tempering at 520 ºC of the ?\'? martensite, for both alloys, it was initially observed the precipitation of ? phase alotriomorphs in grain boundaries, the partial reversion of the martensite ?? phase, and the transformation \"in situ\" of the martensite to supersaturated ? phase, which gradually becomes equilibrium ? phase. Later it was observed the decomposition of the ?? phase to ?? and ? phases. The \"in situ\" formed ? phase is plate-like, and maintain the habit plane of the martensite. The ?? phase precipitates as globules, which coalesce without leaving grain boundaries, forming lamellas between ? plates, with smoth inbterfaces. The ? + ?? structure neither coarsen nor spheroidizes for the studied periods of times. The results can be explained by the existence of low energy ? / ?? and ? / ?? interfaces. Experiments on the decomposition of the martensite in the ? + ? field were made to check the existence of low energy interfaces between the \"in situ\" transformed ? phase and the ? phase from the martensite reversion. It was observed that ? phase plates maintain the martensite habit planes. The plates coarsen without spheroidizing, but maintaining straight interfaces with the ? phase, and with the same habit plane of the martensite that results from the quenching of the ? phase from the reversion. Each ? grain originates only one new ? grain, with the same crystallographic orientation. Experiments on the solubilization in the ? + ? field of the ? + ?? structures obtained by tempering were made in order to study the existence of low energy interfaces between the ?? and ?? phases, and between both and the ? phase. It was observed that the ? phase precipitates inside the ?? as globules which later grow, dissolving partially the ? phase. The ? / ? interfaces stay straight and the ? plates maintain their habit planes. Each original ? grain originates one new ? grain with the same orientation. The results were discussed by relating the morphologies to the nature of the interfaces. It was concluded that the results could be explained by the existence of low energy ? / ?? , ? / ?? and ? / ? interfaces on some directions, and ?? / ?? and ?? / ? interfaces in all directions. The origins of the lattices orientation relationships that gives low energy interfaces are discussed.

nicialmente descreve-se o Sistema Cu-A1 e as fases que ocorrem do lado rico em cobre; em seguida faz-se uma revisão sobre as reações de decomposição da fase ? de alta temperatura no resfriamento e sobre as reações de decomposição da martensita ?\'? no revenido. Faz-se também a revisão de alguns conceitos teóricos sobre interfaces e estabilidade das microestruturas. Estudou-se, na parte experimental, a decomposição isotérmica (revenido) da martensita ?\'? de duas ligas hipoeutetóides em temperaturas que correspondem aos campos ? + ?? (520 ºC) e ? + ? (650 ºC) do diagrama Cu-A1. Realizaram-se ainda experiências de solubilização no campo ? + ? a 576 ºC da estrutura ? + ?? obtida pelo revenido por 90 minutos a 520 ºC. No revenido a 520 ºC da martensita ?\'? de ambas as ligas observou-se inicialmente a precipitação de alotriomorfos de fase ? em contornos de grão, a reversão parcial da martensita para fase ?? , e transformação in situ da martensita em fase ? supersaturada, que gradualmente passa a fase ? de equilíbrio. Com tempos maiores, observou-se a decomposição da fase ?? em ?? e? . A fase ? tem a forma de plaquetas que mantém planos de hábito herdados da martensita. A fase ?? precipita como glóbulos que coalescem sem formar contornos de grão, formando lamelas de interface lisas entre as plaquetas de ? . A estrutura ? + ?? não coalesce nem esferoidiza para os tempos estudados, o que pode ser explicado pela existência de interfaces ? / ?? , e ? / ?? de baixa energia. Para se estudar a existência de interfaces de baixa energia entre a fase ? da transformação in situ e a fase ? da reversão da martensita, realizou-se a decomposição da martensita no campo ? + ? .Observou-se que a fase ? resultante de transformação in situ mantém os planos de hábito da martensita; as plaquetas de fase ? coalescem sem esferoidizar, mantendo interfaces retas com a fase ? e os mesmos planos de hábito que a martensita obtida pelo resfriamento da fase ? de reversão. Cada grão de ? original dá origem a apenas um grão de ? , com a mesma orientação cristalográfica. Para estudar a existência de interfaces de baixa energia entre as fases ?? e ?? e de ambas com fase ? , realizou-se a solubilização no campo ? + ? da estrutura ? + ?? obtida por revenido. Verificou-se que a fase ? precipita no interior da fase ?? na forma de glóbulos, que depois crescem dissolvendo parcialmente a fase ? . As interfaces ? / ? mantém-se planas e as plaquetas de ? mantém os planos de hábito herdados da martensita. Cada antigo grão de ? dá origem a um grão de ? com a mesma orientação. Discutiu-se os resultados sob o ponto de vista da relação entre morfologia e a natureza das interfaces. Concluiu-se que os resultados podem ser explicados pela existência de interfaces ? / ?? , ? / ?? e ? / ? de baixa energia em algumas direções e interfaces ?? / ?? e ?? / ? em todas as direções. Discutiu-se ainda a origem de relações de orientação entre os reticulados que correspondem a interfaces de baixa energia. / The Cu-Al system, in the copper-rich side, is described. The reactions that occur during decomposition of the high temperature ? phase, as well as the decomposition of the ?\'? martensite during tempering, are reviewed. Some theoretical concepts about interfaces and microstructural stability are also reviewed. In the experimental part of this dissertation, the isothermal decomposition (tempering) of the ?\'? martensite is studied in two hipoeutectoid alloys, at temperature in the ? + ?? field (520 ºC) and in the ? + ? field (650 ºC) of the Cu-Al equilibrium diagram. Experiments on the solubilization of the ? + ?? structure obtained by tempering, in the ? + ? field of the equilibrium diagram at 576 ºC, are also made. During tempering at 520 ºC of the ?\'? martensite, for both alloys, it was initially observed the precipitation of ? phase alotriomorphs in grain boundaries, the partial reversion of the martensite ?? phase, and the transformation \"in situ\" of the martensite to supersaturated ? phase, which gradually becomes equilibrium ? phase. Later it was observed the decomposition of the ?? phase to ?? and ? phases. The \"in situ\" formed ? phase is plate-like, and maintain the habit plane of the martensite. The ?? phase precipitates as globules, which coalesce without leaving grain boundaries, forming lamellas between ? plates, with smoth inbterfaces. The ? + ?? structure neither coarsen nor spheroidizes for the studied periods of times. The results can be explained by the existence of low energy ? / ?? and ? / ?? interfaces. Experiments on the decomposition of the martensite in the ? + ? field were made to check the existence of low energy interfaces between the \"in situ\" transformed ? phase and the ? phase from the martensite reversion. It was observed that ? phase plates maintain the martensite habit planes. The plates coarsen without spheroidizing, but maintaining straight interfaces with the ? phase, and with the same habit plane of the martensite that results from the quenching of the ? phase from the reversion. Each ? grain originates only one new ? grain, with the same crystallographic orientation. Experiments on the solubilization in the ? + ? field of the ? + ?? structures obtained by tempering were made in order to study the existence of low energy interfaces between the ?? and ?? phases, and between both and the ? phase. It was observed that the ? phase precipitates inside the ?? as globules which later grow, dissolving partially the ? phase. The ? / ? interfaces stay straight and the ? plates maintain their habit planes. Each original ? grain originates one new ? grain with the same orientation. The results were discussed by relating the morphologies to the nature of the interfaces. It was concluded that the results could be explained by the existence of low energy ? / ?? , ? / ?? and ? / ? interfaces on some directions, and ?? / ?? and ?? / ? interfaces in all directions. The origins of the lattices orientation relationships that gives low energy interfaces are discussed.

<p>Advanced high strength steels, such as dual-phase and martensitic steels, are increasingly being used by automakers to decrease the thickness of steel sheet used in parts without sacrificing part strength. When welded, the martensite within the dual-phase and martensitic steel microstructures tempers, reducing the heat-affected zone (HAZ) hardness compared to the base material, locally reducing strength. This process is known as HAZ softening. HAZ softening has been well studied; however, the kinetics of this process has not been quantified and the processes responsible for HAZ softening have not been examined. This thesis investigated both of these topics.</p> <p>HAZ softening was modelled using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. As the thermal profile during welding is non-isothermal, the effects of temperature and time on steel tempering kinetics could not be separated by examining post-welded properties. The effects of tempering temperature and time were separated through a series rapid isothermal tempering experiments. Hardness data from these experiments allowed the HAZ softening rate to be empirically quantified through fitting the JMAK equation. This material model was then validated by predicting HAZ softening in laser and resistance spot welds. Although the fitted JMAK constants could be used to predict post-weld HAZ hardness, they did not agree with the classic literature values associated with martensite tempering.</p> <p>To understand why the JMAK coefficients did not match those of the classic martensite tempering literature, the softening data from one of the martensitic steels was re-examined. This study revealed that the softening process was a combination of two processes: carbide nucleation and carbide coarsening. The activation energies calculated for each process matched the classic literature values. Carbide coarsening dominated during tempering, which had a non-linear relation with change in hardness. The relationship between carbide coarsening and hardness was responsible for the softening kinetics measured from the rapid tempering experiments.</p> / Doctor of Philosophy (PhD)