Fundamentals of Deformation and Annealing

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Authors: Lucia Suarez, Roumen H. Petrov, Leo Kestens, M. Lamberigts, Yvan Houbaert
Abstract: Thin tertiary scale layers have been grown on ULC steel specimens under controlled conditions. After heating under a protective atmosphere (nitrogen), the samples have been oxidised in air for various oxidation times at 1050°C. These experiments are considered a quantitatively and qualitatively reasonable simulation of the scale formation and growth occurring before hot rolling. Immediately after controlled oxidation, some of the samples were subjected to plane strain compression, in order to simulate the finishing hot rolling process. This approach provided a better insight into the deformation behaviour of the tertiary oxide layer in the first hot rolling pass. The layers produced were examined under the SEM using the EBSD technique for texture characterisation and phase morphology determination. The texture of the deformed oxide scales, originally grown on ULC steel at 1050°C, was determined in order to achieve a better understanding of their complex deformation behaviour. This paper gives a first approach of the study of deformed oxides by EBSD. Strongly textured wustite grains with a clearly pronounced columnar structure were observed after oxidation at 1050°C. As the substrate deformation probably affects the oxide layer, orientation relationships between scale layer and substrate were observed. The detailed EBSD study reveals that the oxide layer can accommodate a significant amount of deformation. The oxide layers exhibit good adhesion to the substrate and remain homogeneous over the thickness after compression.
557
Authors: V.G. García, Jose María Cabrera, Jose Manuel Prado
Abstract: Modelling hot flow stress during grain refinement operations of fcc metals has largely included the use of an Avrami type equation to describe the decrease in stress due to Dynamic Recrystallization (DRX). However when refining large-grained copper, the processing temperatures and strain rates often produce a multi peak behaviour, which is not predictable by an Avrami equation alone. If an initial grain size, D0, is greater than the stable dynamically recrystallized grain size, Drex, which is a function of the Zener-Hollomon Parameter, Z, then the material will tend to refine. However if the current the Zener-Hollomon value, given by current temperature and strain rate conditions, is lower than a critical value, Zc, which depends on D0, then a multi peak stress behaviour is expected while refining. The latter Relative-Grain-Size model (i.e. the D0-Zc and Drex-Z relationships plotted on the same log-log graph) is a practical model that allows determination of whether a material will grain coarsen or refine and whether the dynamic recrystallization behaviour will be monotonic or with multi peaks. The present authors devised a dynamic recrystallization algorithm to measure the stress due to the diminishing initial grain volume and to measure the correction stress due to recrystallizing grains. Analysis on the hot (600°C-950°C) compression data of a 99.9% pure copper inductively lead to the use of an Avrami type equation to describe the stress contribution produced by the deformation of the remaining initial grain volume and a damped cosine equation to describe the stress contribution of the synchronized volume of new grains. This work discusses the experimental evidence and analytical findings that inductively support the mathematical description of the stress-strain curve given by a Damped Cosine Avrami Model for discontinuous DRX.
565
Authors: Ming Xin Huang, Pedro E.J. Rivera-Díaz-del-Castillo, Sybrand van der Zwaag
Abstract: Non-equilibrium thermodynamics theory is applied to the description of plastic deformation in pure FCC metals at the steady state. The saturation flow stress is predicted as a function of temperature and strain rate for Al, Cu, Ni and Ag. The implications on the cell/subgrain size and dislocation density are explored.
571
Authors: Afaf Saai, Laurent Tabourot, Christophe Déprés, Herve Louche
Abstract: In this paper, we present a fundamental model of FCC single crystal behaviour at room temperature: this model includes kinematic work hardening derived from the elementary description of the collective dislocations density evolution during cyclic loading. This kinematic work hardening is then coupled with the isotropic work hardening mechanism. Using this original model, a simulation of a tensile test on a single crystal sample is carried out in the case of an initial crystal orientation that promotes single glide even at rather large strains. The evolution of resolved shear stresses on the primary and secondary slip systems are interpreted by means of the interaction between the evolution of isotropic and kinematic work hardening variables. The evolution of the model state-variables including applied resolved shear strain, dislocation densities, and critical shear stresses are represented as functions of the evolution of crystalline orientation during plastic deformation.
577
Authors: Mahesh C. Somani, L. Pentti Karjalainen
Abstract: The flow stress behaviour of ferrite under abruptly reduced strain rate has been modelled by employing the relatively simple approach of the Bergström’s model for plastic deformation of metals. This physical model considers the rate of change of dislocation density as a difference of the rate of dislocation immobilisation and remobilization, estimated as a function of a recovery parameter. The concept of average rest time of immobilized dislocations prior to remobilization has been suitably employed to estimate the transient times and the enhancement of recovery rates after the strain rate change. The transient friction stress was estimated by using equations given in the literature and some mathematical factors concerning the estimated dislocation densities and the net rates of dislocation immobilization at the high and low strain rates. The present authors have also developed a new unique regression model that is able to predict the static recrystallisation kinetics of most of the common carbon steel grades (including microalloyed steels). A linear regression equation was established to predict the activation energy of static recrystallisation for hot-worked austenite as sum effects of potent solute elements. Further, the power of grain size in the equation was found to be a strong grain-size dependent variable. The model was later validated for several special steel grades, as well as including the consideration for an upper limit for Nb and Si in retarding recrystallisation.
583
Authors: Dana Zöllner, Peter Streitenberger
Abstract: A modified Monte Carlo Potts model algorithm for single-phase normal grain growth in three dimensions in presented, which enables an extensive statistical analysis of the growth kinetics and topological properties of the microstructure within the quasi-stationary self-similar coarsening regime. From the mean-field theory an analytical grain size distribution function is derived, which is based on a quadratic approximation of the average self-similar volumetric rate of change as a function of the relative grain size as it has been determined from the simulation. The analytical size distribution function is found to be in excellent agreement with the simulation results.
589
Authors: Carlos Capdevila, Tommy De Cock, Francisca García Caballero, Carlos García de Andrés
Abstract: In this work the prominent influence of Particle Stimulated Nucleation (PSN) on the overall process of nucleation and subsequent grain growth is discussed and a global recrystallisation kinetics model is proposed. This model accounts for the effect of the most relevant industrial parameters, such as cold reduction and annealing temperature. Moreover, not only the role of the cementite content, which is function of the nominal carbon content, is included in the model, but also the morphology of the second phase particles. Experimental evidence is given to ascertain the accuracy of the theoretical predictions.
595
Authors: Sumantra Mandal, P.V. Sivaprasad, R.K. Dube, Baldev Raj
Abstract: Kinetics, mechanism and modeling of the microstructural evolution of a 15Cr-15Ni- 2.2Mo-0.3Ti modified austenitic stainless steel (alloy D9) during dynamic recrystallization (DRX) have been investigated. The kinetics of DRX has been investigated employing a modified Johnson- Mehl-Avrami-Kolmogorov (JMAK) model. The microstructural study shows that nucleation of new grains during DRX takes place on the parent grain boundary by a bulging mechanism. No significant texture component has been found to develop in the recrystallized matrix. A substantial amount of twins have been observed in the recrystallized matrix. It is proposed that twins play an important role during the nucleation and subsequent expansion of DRX in alloy D9, which in turn moderates the texture in the recrystallized matrix. An artificial neural network model has also been developed to predict the fraction of DRX and grain size, as a function of processing conditions. A good correlation between experimental findings and predicted results has been obtained.
601
Authors: J. Bednarčík, R. Nicula, Karel Saksl, M. Stir, E. Burkel
Abstract: The magnetic, mechanical or chemical properties of nanocrystalline materials strongly differ from the ones of their coarse-grained counterparts. Moreover, significant changes of the phase diagrams were already evidenced for nanostructured alloys. Thermal processing with or without applied pressure controls the microstructure development at the nanometer scale and thus essentially decides upon the final nanomaterial behaviour and properties. A common route for the synthesis of metallic nanomaterials is the devitrification of amorphous precursors obtained via non-equilibrium processing, e.g. by rapid solidification or high-energy ball-milling. Time-resolved in-situ X-ray diffraction experiments may nowadays be performed at high-brilliance synchrotron radiation sources for a variety of temperature-pressure conditions. The temperature-time evolution of the grain-size distribution and microstrain can be monitored in detail at specimen-relevant scales. Together with local information from electron microscopy and chemical analysis, in-situ X-ray experiments offer a complete set of tools for engineering of the microstructure in nanomaterials. The effect of individual processing steps can be distinguished clearly and further tuned. An example is provided, concerning the high-temperature microstructure development in Co-rich soft magnetic nanostructured alloys.
607
Authors: Bo Jakobsen, Ulrich Lienert, Jonathan Almer, Wolfgang Pantleon, Henning Friis Poulsen
Abstract: The synchrotron based X-ray diffraction method “High angular resolution 3DXRD” is briefly introduced. The technique enables the investigation of individual dislocation free regions in a dislocation structure, in-situ within the bulk. Results on the strain distribution within a single grain in a copper sample deformed in tension to 2%, and kept under load, are presented. It is found that the dislocation free regions of the dislocation structure on average are subjected to compressive strain with respect to the mean (tensile) strain in the grain. Results on the dynamics of individual dislocation free regions during straining are further reviewed, with special focus on the observation of intermittent behaviour.
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