The present manuscript addresses the computational modeling of size effects in the plastic behavior of metals at small scales. It is experimentally observed that the mechanical properties of metals are strongly affected when at least one microstructural length scale is scaled down to the micro/nanometer range or when the size of the object stands in the micron or sub-micron range. In such cases from a continuum point of view, the role of plastic strain gradients is considerable in controlling hardening properties. Classical theories of plasticity cannot predict such behavior since they don’t take any intrinsic material length scale parameter into account. For such cases, strain gradient plasticity has been developed to represent size effects. This project focuses on using a phenomenological strain gradient plasticity model to represent some aspects of size effects in metals.To this end, the strain gradient viscoplastic formulation with isotropic material response, based on the formulation developed by Borg et al. (2006), was implemented. Moreover, the strain gradient crystal viscoplastic formulation, according to the development of Borg (2007b), was implemented in a finite strain 2D setting. An extension of the finite strain rate-independent isotropic formulation (Niordson and Redanz (2004)), initially implemented by Mazzoni-Leduc (2010), to plane stress was performed and exploited. As a first application of the research, the rate-independent strain gradient formulation was first used to model the material behavior of Transformation Induced Plasticity (TRIP) assisted multiphase steels. This is done by an extension of the model, developed in Mazzoni-Leduc (2010) for local phase transformation features, by applying a special averaging scheme incorporating experimentally observed transformation kinetics of the phase transformation. Results show that the model stands in a good qualitative agreement with the experiments. The model is shown to have potential for material properties optimization as a perspective. As a second application, the strain gradient viscoplasticity formulations, for both the isotropic implementation and for the crystal plasticity effects, are used to model the compression of Copper micro-pillars. Computationally, the confinement effect is modeled, and experimental data are used to validate the approach. Results show the necessity of considering orientation-dependency of the material. The experimental plastic confinement effect is captured in a qualitative manner. Extension of the model to 3D and studying the grain size effect on bi-modal polycrystals are among the future plan of the work. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

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

Industries, such as the automotive industry, aim to increase the reliability of their products to match the demands and assure the safety of their clients. The proposition of a third generation advanced high strength steel is introduced in this study. The ideas surrounding the behavior of transformation induced plasticity (TRIP) steels and particle reinforced composites are combined and investigated. A finite element model (FEM) is created to investigate the effects of dispersed ceramic particles with varying parameters throughout a TRIP steel microstructure and identify key mechanisms responsible for achieving simultaneous enhancements in strength and ductility. The ceramic material utilized and volume fraction of the ceramic particles dispersed throughout the representative volume element (RVE) are the two varying parameters investigated in this study. Through observing the equivalent plastic strain (PEEQ) distribution at different strain levels up to 100%, evidence of failure controlled by strain localization throughout the ferrite matrix is more prevalent through the softer, austenitic microstructures with 5% or less ceramic particle inclusions. On the other hand, the presence of the hard martensite constituents, or 10% volume fraction of ceramics in an austenitic structure, proposed that failure would engender due to void nucleation at the harder constituent/ferrite interfaces. These voids will then grow and coalesce throughout the microstructure, resulting in a crack. The increased addition of ceramic inclusions also illustrates a simultaneous enhancement in the ultimate tensile strength and ultimate strain in all microstructures. Tensile strength increases by a total of 18% with 10% volume ceramic particles in a TRIP steel microstructure. Between utilizing silicon carbide, cementite, zirconia and aluminum oxide ceramic particles, the microstructure that illustrated the most optimal enhanced performance in strength and ductility was the 10% volume fraction aluminum oxide particle reinforced TRIP steel composite.

Transformation induced plasticity (TRIP) steels are a class of steels with exceptional formability properties, due mainly to the presence of meta-stable retained austenite which transforms to martensite under loading, locally hardening the steel. The volume fraction and mechanical stability of the retained austenite play an important role in producing the high formabilities of TRIP steels. In this thesis, two separate morphologies of retained austenite, equiaxed versus lamellar, have been produced through thermo-mechanical processing of a single common TRIP steel chemistry. The sheet formability characteristics of these two microstructures were examined, with varying volume fractions of retained austenite, through uniaxial tensile and in-plane plane-strain (IPPS) testing. It was found that higher levels of retained austenite produced better formability properties for both microstructures and strain paths. In uniaxial tension it was seen that the the lamellar microstructure attained higher strains at maximum load, and exhibited more sustained instantaneous n values than the equiaxed structure, despite having a lower volume fraction of retained austenite. IPPS testing was performed using an optical measurement of local strain and a comparative forming limit based on differences in strain rate between a developing neck and the surrounding material. It was found that the lamellar microstructure performed better than the equiaxed microstructure for this strain path, achieving higher strains before reaching the comparative forming limit. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-01-25 16:36:07.598

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.

The diploma thesis deals with phase composition measurement of the TRIP steel, using EBSD method. The steel was delivered as thermo-mechanically treated via two different routes. The phase composition of the steel was examined during gradual plastic deformation in the range 0 to10.99%. One route of thermo-mechanical treatment exhibited good agreement with the literature in terms of measured fraction of the retained austenite (15.6%) as well as its decrease during the deformation (to 8.9% at the maximum imposed strain). The samples of the second route did not show any agreement in either of the parameters spoken.

Formable Advanced High-Strength Steels (AHSS) have a unique combination of strength and ductility, making them ideal in the effort to lightweight vehicles. The AHSS in this study, Quenched and Partitioned 1180, rely on the Transformation Induced Plasticity (TRIP) effect, in which retained austenite (RA) grains transform to martensite during plastic deformation, providing extra ductility via the transformation event. Understanding the factors involved in RA transformation, such as local strain and grain attributes, is therefore key to optimizing the microstructure of these steels. This research seeks to increase understanding of those attributes and the correlations between microstructure and RA transformation in TRIP steels. To measure local strain, the viability of using forescatter detector (FSD) images as the basis for DIC study is investigated. Standard FSD techniques, along with an integrated EBSD / FSD approach (Pattern Region of Interest Analysis System), are both analyzed. Simultaneous strain and microstructure maps are obtained for tensile deformation up to around 6% strain. The method does not give sub-grain resolution, and surface feature evolution prevents DIC analysis across large strain steps; however, the data is easy to obtain and provides a natural set of complementary information for the EBSD analysis. In-situ tensile tests combined with EBSD allow RA grain and neighboring attributes to be characterized and corresponding transformation data to be obtained. However, pseudo-symmetry of the ferrite (BCC) and martensite (BCT) phases prevents EBSD from accurately identifying all phases. Measuring the relative distortion of the crystal lattice, tetragonality, is one approach to identifying the phases. Unfortunately, small errors in the pattern center can cause significant errors in tetragonality measurement. Therefore, this research utilizes a new approach for accurate pattern center determination using a strain minimization routine and applies it to tetragonality maps for phase identification. Tetragonality maps based on dynamically simulated patterns result in the most accurate maps and can also be used to predict approximate local carbon content. Machine learning is then used on the collected data to isolate key attributes of RA grains and provide a decision tree model to predict transformation based on those attributes. Among the most relevant attributes found, RA grain area, RA grain shape aspect ratio, a “hardness” factor, and major axis orientation are included. Possible correlations between these factors and transformation improve understanding of relevant attributes and show the advantage that machine learning can have in unravelling complex material behavior.

Medium Mn TRIP steels are amongst the most widely researched third generation advanced high strength steels (3G-AHSSs) as they are ideal candidates for automotive light-weighting applications owing to their superior strength and ductility balance. However, the thermal processing cycles of these steels need to be compatible with the industrial continuous galvanizing line (CGL) in order to successfully employ them in the automotive manufacturing industry. The main objective of the present research was to develop a CGL compatible thermal processing cycle for a prototype medium Mn steel that would produce significant volume fractions of chemically stable retained austenite and exhibit mechanical properties consistent with established 3G-AHSS targets. In that regard, the effects of intercritical annealing (IA) time and temperature and starting microstructure were determined in the first part of this research. The as-received tempered martensite (S-TM) and heat treated martensite (S-M) were the two different starting microstructures studied in this research. In this case, the overaging temperature (OT) treatment (460°C for 20s) was kept constant. It was found that high volume fractions (≥ 0.30) of retained austenite were achieved for S-M samples intercritically annealed at 675°C for shorter times (i.e. 60 to 120s) compared to S-TM samples. TEM analysis of the S-M samples showed that most of the retained austenite was present in a film type morphology, which is known to be more stable chemically and mechanically compared to the block type morphology. The tensile test results showed that although both the S-TM and S-M samples exhibited a high strength/ductility balance, the S-M samples, particularly the S-M 675°C + 120s samples, showed more potential in terms of CGL compatibility and achieving 3G-AHSS target mechanical properties. The effect of OT holding time was determined in the second part of this research. In that regard, the OT holding time was varied form 20s to 120s for selected S-TM and S-M samples. The S-TM 710°C samples with increased OT holding times (60s and 120s) had a significant increase in retained austenite volume fraction compared to the baseline 20s OT samples. However, the retained austenite volume fractions did not change for S-M samples regardless of OT holding time. It was also found that the mechanical properties of the annealed S-TM and S-M steels depended on the OT holding time. For the S-TM samples with > 120s IA holding times, longer OT holding times (60s and 120s) produced chemically unstable retained austenite which transformed rapidly at low strain resulting in low UTS × TE products. However, although longer OT holding times significantly increased the yield strength of the annealed S-M samples, the UTS × TE product decreased significantly owing to decreased retained austenite stability. Finally, based on the results of this research, it was concluded that the prototype medium Mn TRIP steel can achieve 3G-AHSS target mechanical properties using CGL-compatible thermal processing cycles. Moreover, depending on successful reactive wetting, it may be possible to perform both thermal processing and galvanizing of this steel in the industrial CGL. / Thesis / Master of Applied Science (MASc)

QC 20100927

Die neuen TRIP-Matrix-Composite-Werkstoffe - verstärkt durch mit MgO teilstabilisiertem ZrO2 - gestatten es, durch die Besonderheiten der beteiligten Phasen eine gute Eigenschaftskombination hinsichtlich hoher Festigkeits- und Dehnungswerte zu erzielen. Aus diesem Grund ist die vorliegende Arbeit der Erforschung wissenschaftlicher Grundlagen zur Herstellung von TRIP-Matrix-Compositen sowie zur Analyse deren Eigenschaften in Abhängigkeit von den unterschiedlichen pulvermetallurgischen Herstellungsverfahren, wie konventionelles und konduktives Sintern sowie Heißpressen gewidmet worden. Als Ergebnis ist ein tieferes Verständnis der Kinetik von Sinterprozessen mit dem Aufbau eines physikalisch-mathematischen Modells festzuhalten. Ferner wurden mit weiterführenden Untersuchungen erste Ansätze zur Auslegung von Warmumformprozessen von gesinterten Halbzeugen aus dem Verbundwerkstoff durch quantitative Beschreibung der Entfestigungskinetik geleistet.