Matthias Militzer

The mechanical properties of metals and alloys are extremely sensitive to the microstructure. Different metallurgical processes, such as recrystallization, grain growth, and phase transformations, may modify the microstructure, where each process proceeds by the migration of interfaces that may be strongly affected by the presence of solutes and/or impurities due to solute drag. Quantifying the solute drag requires expensive and time-consuming experimental trials, which are further limited due to the vastly different length scales of solute segregation (few nm) and microstructural features such as grain sizes (few µm). This study presents a computational approach that integrates the microstructure evolution model, i.e., here the phase field method, with atomistic simulations, i.e., density functional theory simulations (DFT), to identify the role of solutes on microstructural processes.First, the experimental migration rates of a single well-defined grain boundary (GB) in Au during recrystallization heat treatments are rationalized using DFT calculations in combination with a continuum solute drag model. Here, an approach to determine the effective segregation energy from atomistic calculations is proposed, suggesting strong solute drag due to 2 ppm Bi impurities in the Au sample. In the microstructural scale, different grain boundaries exist with variability in GB properties, such as GB mobility and solute drag. A phase field model with a friction pressure is used to simulate solute drag on individual GBs. The simulations considering the variability in GB properties indicate that a representative GB can be defined that mimics the average grain size evolution in the presence and absence of solutes. Using the solute binding energies for five solutes in nine different grain boundaries in FCC-Fe, the anisotropic phase field simulations suggest a minor role of segregation anisotropy on austenite grain growth, and as a result, the Σ5(310)[001] GB is considered as the representative GB. A solute trend parameter is proposed to identify solutes that promote grain refinement in agreement with experimental observations. Finally, the atomistically informed approach is extended to phase transformation in binary alloys. Here, phase field simulations that explicitly considered solute segregation in nanocrystalline materials agree with the steady-state solute drag model.

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Line pipe steels are manufactured by thermomechanically controlled processing (TMCP), which combines selected alloying additions and processing conditions, to produce engineered microstructures that confer steels their properties. During pipeline construction, segments of pipe are sourced from numerous suppliers, each of whom may have different alloying strategies, and welded by varied technologies and procedures, leading to numerous different resulting microstructures in the weld heat affected zone (HAZ). Consequently, it is critical to develop a chemistry sensitive model that describes austenite grain growth and decomposition in the coarse grain heat affected zone (CGHAZ).In this work, austenite grain growth in the CGHAZ is investigated by using Laser Ultrasonics for Metallurgy (LUMet) in 37 steels of chemistries in a range relevant to X80 line pipe steels. A model which considers the effect of solutes on the mobility of the austenite grain boundary and the pinning pressure exerted by Ti-rich precipitates was developed. The amount of C and Nb in solution were found to reduce the mobility of the austenite grain boundary. The size distribution of Ti-rich precipitates was analyzed and verified by scanning transmission electron microscopy (STEM). The model was validated for laboratory thermal cycles and industrial welding trials.Austenite decomposition kinetics were measured for 16 steels with systematic variations of C, Mo, Nb, and Cr, at continuous cooling rates of 3, 5 and 10 °C/s, and their final microstructures were characterized using electron backscatter diffraction (EBSD), optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Vickers hardness. Microstructures at 30 and 50°C/s were also produced and characterized with the same techniques. The effect of solutes in decomposition kinetics was analyzed and C, Nb, and Mo were found to decrease the transformation start temperature in continuous cooling. A novel finding was the role of Ti-rich particles. It was observed that a decrease in the interparticle spacing of these precipitates leads to an increase in the transformation start temperature, which was rationalized by their ability to pin moving bainite grain boundaries. A microstructure-properties model is proposed where the hardness in the CGHAZ is a function of the high angle grain boundary density.

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The control of the solid-state phase transformations between the hexagonal close-packed (hcp) α and body-centered cubic (bcc) β phases of Ti-alloys is of crucial importance to achieve optimal mechanical properties. The present study explores the applicability of laser ultrasonics for metallurgy (LUMet) as an in-situ technique for the measurement of phase transformations in Ti-alloys. Through the monitoring of ultrasound velocity changes during isothermal treatments of a metastable β Ti-5553 alloy (Ti-5Al-5Mo-5V-3Cr, wt.%) it was seen that LUMet can successfully capture the rate of the β → α + β transformations. Quantitative assessment of the transformation kinetics from the ultrasonic velocity requires, however, validation with ex-situ characterization where direct measurements of phase fractions are possible. In view of the challenges associated with most ex-situ analyses, a more systematic methodology was proposed in order to semi-automate phase fraction measurements while decreasing the number of user input parameters. It was observed that ultrasonic velocity changes and transformed phase fractions are nearly linearly correlated to each other, similar to what has been reported for steels. The proposed correlation has been verified with an analytical model which accounts for the temperature dependent macroscopic elastic constants and densities of the α and β phases. LUMet measurements on near α, quasi-binary, Ti-Mo alloys during continuous heat treatments further showed the sensitivity of the transformation behavior to alloy composition. Retardation of phase transformation due to Mo additions has been confirmed with the ultrasonic velocity measurements during both continuous heating and cooling. Further, the wave velocity behaviors observed during cooling at varying rates are consistent with the formation of diffusional transformation products. Nano-scale analyses on selected specimens using atom probe tomography (APT) indicated that Mo tends to be trapped within moving α / β boundaries, which may contribute to slower phase transformations. This work demonstrates that LUMet is an effective tool to monitor phase transformations in Ti-alloys. The results pave the way for further expansion of the use of LUMet during more complex thermo-mechanical treatments in a wide range of commercial Ti-alloys.

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To engineer the mechanical response of the material, it is crucial to predict the microstructure after thermomechanical processing and relate properties to the microstructure. In this study, the relationship between the features of the deformed state after axisymmetric extrusion of aluminum at high temperatures (e.g., the subgrain size distribution and the disorientation distribution) and the final recrystallized texture was investigated. It aims to systematically change the features of the deformed state by generating different initial synthetic microstructures and using them as inputs for a phase field model to predict the recrystallized texture. The characteristics of the deformed state and the recrystallized state were analyzed for extruded 3XXX aluminum samples with two different homogenization heat treatments. The subgrain size distribution and disorientation distribution in different texture fibres of the deformed state, i.e., ||ED and ||ED, were extracted to construct a baseline condition. After verifying that 2D simulations could be used to capture the essential phenomena in 3D, the role of the microstructural features was investigated using 2D simulations. First, the subgrain size distribution and disorientation distribution were changed from baseline condition by ±50% for individual grains with ||ED or ||ED. Second, for a mixture of grains with ||ED and ||ED orientations, ||ED baseline condition was used, then subgrain size distribution and disorientation distribution were changed by ±50% for ||ED grains. Third, an input microstructure from experimental EBSD measurements was simulated. For the first scenario, a narrower distribution and a wider disorientation distribution resulted in a larger final average subgrain size which was rationalized based on the evolution of these microstructural features. For the second scenario, the tail of the subgrain size distribution and the width of disorientation distribution within ||ED was found to have a large impact on the final texture. This was explained by the probability of large grains with high angle disorientation in ||ED fibres growing preferentially to pinch-off grains with ||ED orientations. For the third scenario, only ||ED volume fraction could be predicted accurately compared to the experiment. Possible sources of error were identified and discussed.

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This study evaluates the applicability of laser ultrasonics for metallurgy (LUMet) as a non-destructive tool in measurement of grain size evolution and quantifying recrystallization in FCC polycrystalline metallic materials. A systematic investigation has been conducted to correlate the ultrasonic attenuation parameter with the effective average grain size determined by metallography in two cobalt-based superalloys and pure copper.To correlate the ultrasonic attenuation parameter with the metallographic grain size, a series of thermo-mechanical treatments were carefully designed to generate samples with different average grain sizes. Equivalent area diameter (EQAD) and area weighted grain diameter (AWGD) including twin boundaries were selected as the measures of average grain size. The developed correlation for cobalt superalloys was used to monitor the recrystallization kinetics of cold-rolled and hot deformed specimens in real time through the refinement of the mean grain size as well as the grain growth kinetics. Furthermore, it was found that when a substantial tail exists in the microstructure, the AWGD-based correlation provides a better estimation of the average grain size than the EQAD-based correlation since the former changes according to the changes in grain size distribution. Moreover, the finite element modelling of wave propagation revealed that twin boundaries have similar scattering behavior as other high-angle grain boundaries. This suggests that the LUMet technique cannot be used to extract the fraction of twins.A versatile method was then introduced to harmonize all the existing empirical equations to evaluate the grain size change in FCC metals. It was observed that the amount of grain scattering is controlled by the single crystal elastic constants which should be known apriori. The harmonized equation can be used to measure the grain size evolution in other metals without the need to develop a new calibration or at least reduces the number of experiments and labor-intensive ex-situ characterizations required for the design of a new calibration.

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Microstructure evolution during material processing is determined by a number of factors, such as the kinetics of grain boundary migration in the presence of impurities, which can take form of solid solution, second-phase precipitates or clusters. The dynamic interaction between grain boundaries and clusters has not been explored. In this work, a variety of simulation tools are utilized to approach this problem from an atomistic perspective. Atomistic simulations are first implemented to explore the parameter space of the solute drag problem, i.e. grain boundary migration in a binary ideal solid solution system, via a kinetic Monte Carlo framework. Depending on their diffusivity, solute atoms are capable of modifying the structure of a migrating boundary, leading to a diffusion-dependent drag pressure. A phenomenological model adapted from the Cahn model is proposed to explain the simulation results. The interaction between clusters and a migrating grain boundary is studied next using molecular dynamics simulations. The iron helium (Fe-He) system is chosen as the object of the study. A preliminary step towards such a study is to investigate the grain boundary migration in pure bcc Fe. An emphasis is placed upon demonstrating the correlation between the migration of curved and planar boundaries. Evidence that verifies such a correlation is established, based on the analyses on the shapes, the kinetics and the migration mechanism of both types of boundaries. Next, the formation of He clusters in the bulk and grain boundaries of Fe is examined. The cluster formation at the boundary occurs at a lower rate relative to that in the bulk. This is attributed to the boundary being a slow diffusion channel for interstitial He atoms. The overall effect of clusters on the boundary migration is twofold. Clusters reduce the boundary mobility via segregation; the magnitude of their effect can be rationalized using the Cahn model in the zero velocity limit. Clusters also act as pinning sources, delaying or even completely halting the boundary migration. A phenomenological model adapted from the Zener pinning model is used to discuss the role of clusters on grain boundary migration.

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Advanced line pipe steels are microalloyed with Nb to promote the formation of complex microstructures leading to the required mechanical properties. In addition to its role during thermo-mechanical processing Nb in solution affects significantly the austenite decomposition kinetics and the resulting microstructure. A systematic study has been carried out to quantify the influence of Nb on the austenite decomposition in a commercial X80 line pipe steel containing 0.06C- 0.034Nb- 0.24Mo- 0.012Ti- 0.0005N (in wt. %) for a variety of austenite grain sizes and cooling rates that are relevant for the heat affected zone. To quantify the influence of Nb on transformation kinetics, two distinct amounts of Nb in solution were obtained through carefully designed reheat treatments prior to continuous cooling transformation tests conducted with a Gleeble 3500. The amount of Nb in solution was quantified based on ageing experiments. To investigate the combined influence of Nb and Mo on austenite decomposition two laboratory cast low-carbon steels containing 0.06 wt. % Nb and 0.045 wt. % Nb and 0.145 wt. % Mo, respectively, were compared with the X80 steel. The obtained transformation products include irregular ferrite, upper and lower bainite and martensite/ austenite.Electron backscatter diffraction (EBSD) was used to distinguish upper and lower bainite based on their orientation relationship with the prior austenite and to quantify microstructural features which are relevant for the tensile properties. Based on the quantitative measures obtained from the EBSD analysis structure-property relationships were developed to predict the yield strength, uniform elongation and ultimate tensile strength of the studied X80 line pipe steel. An effective grain size was defined including martensite/ austenite to consider grain refinement and the kernel average misorientation was used to quantify dislocation strengthening. A phenomenological model was applied and modified to capture the austenite decomposition of the X80 steel considering the effect of prior austenite grain size, amount of Nb in solution and cooling rate. The amount of martensite/ austenite depends on the surrounding matrix microstructure and is predicted as a function of the transformation start temperature with an empirical fit.

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During welding, the heat affected zone (HAZ) of X80 linepipe steel is subjected to very steep spatial variations in temperature and concentration of Nb bearing particles which results in a strongly graded microstructure. Therefore, models on the length scale of the microstructure, i.e. the so-called mesoscale, are useful to simulate microstructure evolution in the HAZ. Among mesoscale models, phase field modelling (PFM) is selected because it is based on diffusional time steps and it is a robust tool to capture complex morphologies, e.g. bainitic ferrite. A PFM is developed for austenite grain growth in 2D and 3D that is applicable to rapid heat-treatment cycles taking the pinning/dissolution effects of Nb bearing particles into account by using an effective mobility concept. In addition, a PFM is developed for the austenite decomposition to predict the simultaneous formation of polygonal ferrite and bainite. PFM is coupled with a carbon diffusion model and an effective interface mobility is introduced to implicitly account for the solute drag effect of Nb. For simplicity, the formation of carbide-free bainite is considered and a suitable anisotropy approach is proposed for the austenite-bainite interface mobility. The model is first applied to a TRIP steel in which ferrite and bainite form separately, and bainite can be considered carbide-free bainite. Then the model is applied to simulate the microstructural evolution in the HAZ of the X80 linepipe steel accounting for the thermal and microstructural gradients and validated with microstructure observations made in a weld trial.

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Intercritical annealing is used widely in the steel industry to produce advanced high strength steels for automotive applications, e.g. dual-phase steels. A phase-field model is develop to describe microstructure evolution during intercritical annealing of low-carbon steels. The phase-field model consists of individual sub-models for ferrite recrystallization, austenite formation and austenite to ferrite transformation. In particular, a Gibbs-energy dissipation model is coupled to the phase-field model to describe the effects of solutes on migration of austenite/ferrite interfaces. The model is applied to a low-carbon steel with a cold-rolled pearlite/ferrite microstructure suitable for industrial production of dual-phase steels (DP600 grade). The sub-model parameters, e.g. nucleation parameters and interface mobilities, are tuned using experimental data. The interaction of concurrent ferrite recrystallization and austenite formation is investigated using the developed model. The simulation results reveal that ferrite recrystallization can be inhibited by the pinning effect of austenite particles and concurrent ferrite recrystallization can lead to intragranular distribution of austenite in the final microstructure. The transition of austenite morphology from a network structure to a banded structure with increasing heating rates is replicated by the phase-field model. The model is validated using a simulated industrial intercritical-annealing cycle. Moreover, the developed phase-field model is used to describe cyclic phase transformations in the intercritical region for a plain-carbon steel and a manganese-alloyed low-carbon steel. The consideration of Gibbs-energy dissipation in the phase-field model rationalizes the existence of stagnant stages during cyclic phase transformations in the manganese-alloyed low-carbon steel. In summary, the developed model provides a single tool that is able to describe various physical phenomena occurring in an entire intercritical-annealing cycle. Phase-field modeling can be a promising approach for developing process models for advanced steels in the future.

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The kinetics of the recrystallization and austenite-ferrite (fcc-bcc) phase transformation in steels are markedly affected by substitutional alloying elements. Nevertheless, the detailed mechanisms of their interaction with the grain boundaries and interfaces are not fully understood. Using density functional theory, we determine the segregation energies of commonly used alloying elements (e.g. Nb, Mo, Mn, Si, Cr, Ni) in the Σ5 (013) tilt grain boundary in bcc and fcc Fe, and the bcc-fcc interfaces. We find a strong interaction between large solutes (e.g. Nb, Mo and Ti) and grain boundaries or interfaces that is consistent with experimental observations of the effects of these alloying elements on delaying recrystallization and the austenite-to-ferrite transformation in low-carbon steels. In addition, we compute the solute-solute interactions as a function of solute pair distance in the grain boundaries and interfaces, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk.Besides the prediction of solute segregation, the self- and solute-diffusion in Fe-based system are also investigated within a framework combining density functional theory calculations and kinetic Monte Carlo simulations. Good agreement between our calculations and the measurements for self- and solute diffusion in bulk Fe is achieved. For the first time, the effective activation energies and diffusion coefficients for various solutes in the α-Fe Σ5 (013) grain boundary are determined. The results demonstrate that grain boundary diffusion is significantly faster than for lattice diffusion, confirming grain boundaries are fast diffusion paths. By contrast, the effective activation energy of self-diffusion in a bcc-fcc Fe interface is close to the value of fcc bulk self-diffusion, and the investigated bcc-fcc interface provides a moderate "fast diffusion" path.

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Accelerated cooling on the run-out table of a hot rolling mill is a key technology to tailor microstructure and properties of advanced steels. Thus, it is crucial to develop accurate heat transfer models in order to predict the temperature history of the steel plates on run-out tables.The present study describes a strategy to develop a mechanistic cooling model to simulate the temperature of the plate cooled by top water nozzles on a run-out table. Systematic experiments have been carried out on a pilot scale run-out table facility using two types of top nozzles: planar (curtain) and circular (axisymmetric) nozzles. Experimental results for cooling of stationary plates showed that the heat transfer rate depends strongly on the distance from the jet especially in the temperature range where the transition boiling regime occurs. Based on experimental results, a boiling curve model has been proposed that takes into account boiling heat transfer mechanisms and maps local boiling curves for cooling of stationary steel plates. The effects of water flow rate and water temperature on the heat extraction from the plate have been included in the model.Then, systematic experimental heat transfer studies were conducted to investigate the effect of plate speed on the heat transfer rate. It was found that the plate motion influences the heat transfer rate in the film boiling and transition boiling regimes; however, it does not have an effect on the heat flux in the nucleate boiling regime. Moreover, for the circular nozzle system, it was found that the nucleate boiling heat flux does not change with lateral distance. However, heat flux in the film boiling and transition boiling regimes decreases with increasing distance from the longitudinal centerline of the plate. In the next step, a cooling model was proposed by accounting for the boiling curves of single nozzle cooling for moving plates. Transient heat conduction within the plate was analyzed and surface heat flux and temperature histories were predicted. The validity of the cooling model was examined with multiple nozzles experimental data from the literature. Very good agreement with experimental results has been obtained.

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A model was developed to describe the microstructure evolution during intercritical annealing of a low-carbon steel suitable for industrial production of dual-phase steels (DP600 grade) on a hot-dip galvanizing line. The microstructure evolution model consists of individual submodels for recrystallization, austenite formation in a fully recrystallized material and austenite decomposition after partial austenization. These submodels were developed using the Johnson-Mehl-Avrami-Kolmogorov approach and the additivity principle. The model parameters were obtained based on the results of systematic experiments addressing the effects of initial microstructures and processing conditions on the microstructure evolution in the course of intercritical annealing. The initial microstructures included 50 pct cold-rolled ferrite-pearlite, ferrite-bainite-pearlite and martensite. If heating to an intercritical temperature was sufficiently slow, recrystallization was completed before austenite formation, otherwise austenite formed in a partially recrystallized microstructure. The recrystallization-austenite formation interaction accelerated austenization in all three starting microstructures by providing additional nucleation sites and enhancing growth rates; this complex process could not be accounted for with the current modelling approach. A variety of austenite morphologies was produced by using different initial microstructures and/or by means of the interaction of recrystallization and austenite formation. Following the complete intercritical annealing cycle, the final microstructure was composed of ferrite, bainite and martensite; the latter two components inherited the distribution and morphology of those for intercritical austenite. The microstructure evolution model was validated using simulated industrial thermal paths for intercritical annealing. Laser ultrasonics was employed for in-situ monitoring of phase transformations to facilitate the validation of the microstructure evolution model.

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Second phase particles and solute atoms have been used as an important constituent in the design of materials and processes due to their ability to restrain the motion of grain boundaries. The drag effect occurs on a scale comparable to the particle diameter and interface thickness. However, to simulate grain growth with numerical efficiency one requires a model that captures the drag pressure on the interfaces but does not resolve the particles or solute segregation spike. In this work, a multi-scale modelling scheme is proposed to simulate grain growth with particle pinning and solute drag. The interaction of a grain boundary with an ensemble of particles is simulated to obtain the pinning pressure. A phase field model is then developed that incorporates the drag pressure in the meso-scale and simulates grain growth. The accuracy of the model is confirmed in comparison with analytical expressions. The application of the model is then presented for grain growth in two- and three-dimensional systems under the influence of particle pinning. Measuring the curvature of the grain boundary network reveals that in the completely pinned structure, the average driving pressure is not equal to but lower than the pinning pressure. The results of the nano-scale simulations for pinning pressure is combined with the results from the meso-scale to produce a limiting grain size that coincides with the experiments. This curvature analysis provides a kinetic model that describes the evolution of the structure more accurately than that of the mean field theories.The proposed phase field formulation is also applied to simulate grain growth in the presence of solute drag. The grain growth kinetics follows a phenomenological relationship that is described using a power law, with a time exponent in the range of 0.35 to 0.50. The deviation from ideal grain growth, associated with a time exponent lower than 0.50, and its correlation with the solute drag parameters is investigated.

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Copper interconnects in advanced integrated circuits are manufactured by processes that include electrodeposition, chemical mechanical polishing and annealing. The as-deposited copper is nano-crystalline and undergoes a microstructure evolution at room temperature (self-annealing) or during an annealing step. During this process, significant changes in resistivity and grain size are observed. In this work, the microstructure evolution in 0.5-3 μm-thick electrodeposited copper thin films was studied. Resistivity measurements were used to quantify the role of deposition conditions on the microstructure evolution rate. In-situ electron backscatter diffraction (EBSD) was employed to observe self-annealing at the film surface. The resistivity-microstructure correlation during self-annealing was examined. A phenomenological model using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach was developed to describe recrystallization during isothermal and continuous annealing treatments. The microstructure evolution in copper-silver alloys and films produced by variable deposition rates was investigated. Phase-field model was applied to simulate self-annealing and the effect of deposition current density.The results show that the drop in resistivity during self-annealing is accompanied by significant changes of the microstructure at the film surface. Different criteria were developed to assess self-annealing rate from EBSD maps including grain size, image quality and local orientation spread. Adopting a grain size threshold, it was found that there is a reasonable correlation between resistivity and microstructure during self-annealing. The recrystallization in copper thin films appears to be thermally activated with an activation energy of 0.89-0.93 eV. Adopting the principle of additivity, it was found that the recrystallization rate during continuous annealing can be described by the JMAK model using the isothermal resistivity profiles. A method was proposed to accelerate recrystallization based on a capping layer deposition. No recrystallization was observed when silver was co-deposited with copper in the absence of chloride (even when annealed at 100 °C for 5 hours). Phase-field model was able to describe self-annealing and the effect of deposition current density. The results in this thesis are of significance to the microelectronic industry where recrystallization is a crucial step in the fabrication of copper interconnects for the high performance integrated circuits.

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The present work deals with grain refinement and austenite formation in a plain C-Mn steel with 0.17C-0.74Mn (wt pct). To improve the limited work hardening capability of ultrafine grained ferritic steels, new approaches were adopted to develop bimodal ferrite grain size distributions and ultrafine grained dual phase microstructures. The first approach is based on deformation and annealing of a ferrite-martensite microstructure. Ultrafine grained dual phase steels were obtained through rapid heating of very fine ferrite-carbide aggregates into the intercritical annealing region where partial austenite formation takes place. Hence, austenite formation was systematically investigated using a combination of microstructure characterization and detailed dilatometry analysis. The effect of initial structure and heating rate on austenite formation was examined. The resulting microstructure characteristics and mechanical properties of dual phase steels were also investigated. A multi-phase field modelling approach was adopted to simulate austenite formation from a variety of initial structures including ferrite-spheroidized carbide aggregates, fully pearlitic and ferrite-pearlite structures.The results show that a bimodal distribution of ferrite grains negates the Lüdering effect, yet the improvement of work hardening rate remains marginal compared to fine grained ferrite structures. Very fine grained initial structure and rapid heat treatment cycle are essential parameters to achieve ultrafine grained dual phase steels with improved mechanical properties in the steel employed in this study. For austenite formation, dilatation data can be used to distinguish different stages of microstructure evolution upon heating into the single austenite phase region including ferrite recrystallization, pearlite to austenite and ferrite to austenite transformation. Heating rate has a pronounced effect on the size and morphology of austenite grains in the intercritical annealing region. It is shown that phase field modelling is capable of predicting microstructural changes during austenite formation. It is well suited to capture complex interaction between microstructure processes such as spheroidization, carbide dissolution and coarsening during austenite formation especially in fine grained structures where the length scale is comparable with carbon diffusion distance.

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In this work, a potential production route of ultra fine grained dual phase (DP) steels was studied. Deformation induced ferrite transformation (DIFT) was applied in laboratory tests employing a Gleeble 3500 thermo-mechanical simulator to produce fine grained dual phase steels in two chemistries: a conventional DP 600 chemistry with 0.06 wt% C-1.9 wt% Mn-0.16 wt% Mo and the C-Mn base chemistry of 0.06 wt% C-1.8 wt% Mn with no Mo addition. This thermo-mechanical treatment consisted of cooling the steel from the austenitization temperature at a rate of 40°C/s to a deformation temperature, which was 25 to 50°C above the austenite to ferrite transformation start temperature (Ar3) specific for the given austenitization and cooling conditions. Then the steel was immediately deformed to a true strain of up to 0.7 followed by rapid quenching. The effects of prior austenite grain size, amount of strain and deformation temperature on DIFT microstructures were studied to identify the most suitable thermo-mechanical path to obtain an ultra fine grained dual phase structure. Microstructures were characterized by scanning electron microscopy (SEM) including electron back scatter diffraction (EBSD) mapping. For the investigated steels the highest amount of deformation with a true strain of 0.6 or above resulted in optimized microstructures consisting of 70-80% polygonal ferrite with a mean grain size of 1-2 μm. Simulation of DIFT hot rolling schedules were conducted with hot torsion tests to investigate the viability of the proposed approach. A two-dimensional phase field model was developed to describe the austenite to ferrite transformation during DIFT. Several nucleation schemes were examined in terms of time and position of forming ferrite nuclei in the austenite domain to replicate the experimentally observed ferrite grain size spread. The austenite-ferrite interface mobility was used as the adjustable parameter to match the experimentally observed ferrite fraction.

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A comprehensive study on the microstructural evolution of a new generation Nb-Momicroalloyed model complex-phase (CP) steel under hot strip rolling conditions hasbeen conducted. The experimental investigation includes the austenite conditioningduring reheating, work hardening and static softening of austenite during hot deformation,austenite decomposition to multiphase structure during run out table cooling operationand finally precipitation strengthening during coiling at downcoiler.The flow stress and static softening behaviour of austenite is modeled by thephysically based approaches of Kocks-Mecking and Zurob et al., respectively, whereasempirical approaches are employed to model recrystallized austenite grain size and graingrowth after recrystallization. The start of ferrite formation is described by the earlygrowth of comer nucleated ferrite. A limiting carbon concentration concept is postulatedabove which ferrite formation ceases. A semi-empirical approach based on the Johnson-Mehl-Avrami-Koknogorov (JMAK) theory adopting additivity is employed to describeferrite as well as bainite growth with individual parameters for each reaction. The presentferrite model includes the formation of the transformation stasis regime, where a criticaldriving pressure approach is adopted to describe the stasis initiation. Present researchconcludes that the same driving pressure approach is applicable to describe bainite startand the transition from stasis to bainite start occurs at 620°C. The effect of carbonenrichment in the remaining austenite after ferrite formation is included to describebainite growth. Martensite + retained austenite volume fraction is calculated empiricallyas a function of carbon enrichment resulting from the ferrite formation. The isothermalaging kinetics is modeled by a modified Shercliff-Ashby approach, which is thenextended for coil cooling path to predict the optimum coiling temperature range (580-610°C) to maximize the precipitation strengthening of microalloying elements. Finallythe hardness of the material is expressed as a function of the volume fractions of varioustransformation products and the precipitation strength contribution. The overall modelprediction is validated successfully by torsion simulation of the entire hot rolling andcontrolled cooling schedule. Current research suggests that fine multiphase structure ispossible to achieve in the present steel through proper austenite conditioning and adoptingcomplex cooling strategies.

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The Direct-Chill (DC) casting process is used in the non-ferrous metals industry toproduce ingots, blooms and cylindrical billets. During DC casting, primary cooling in the mouldis followed by secondary cooling, in which the cast product surface is directly cooled by waterjets. The formation of defects during the direct-chill casting process can be reduced bycontrolling the heat extraction in the secondary cooling zone during the start-up phase. Thecontrol and optimization of this process requires an accurate knowledge of the boundaryconditions and their relationship with casting parameters.This research project studied the effect of different parameters on the heat transfer in thesecondary cooling zone of the direct-chill casting process. This process was simulated byquenching instrumented samples of industrial DC-cast aluminum AA5 182 and magnesiumAZ3 1 with water jets and recording the thermal history within the sample using sub-surfacethermocouples. An inverse heat conduction algorithm specifically developed for this researchproject converted this thermal history into surface heat fluxes and surface temperatures. Therelationship between heat flux and surface temperature was expressed by a boiling curve.Cooling experiments showed the influence of the cooling water flow rate oncharacteristic features of the boiling curve. The effect of thermophysical properties, initialsample temperature and water temperature on high temperature boiling regimes was alsoquantified. The influence of other parameters such as the water jet velocity and the surfaceroughness was determined in a qualitative fashion.Results from the quench tests were used as boundary conditions in a finite elementmodel for the direct-chill casting of AZ3 1 billets. Simulations of the process start-up phaseshowed the critical role played by stable film boiling and water film ejection in determining thethermal history within the billet.

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