Papers by Author: Ernst Kozeschnik

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Authors: Lin Qin, Alice Redermeier, Ernst Kozeschnik, Carina Karner, Christoph Dellago
Abstract: In precipitation strengthened ferritic alloys, the Fe-Cu binary system is a well-studied model system. Still, many unsettled questions remain about the early stages of bcc Cu precipitation, most of which refer to the shape and composition of the critical and post-critical nuclei. Since the critical nucleation states are hard to investigate by experimental methods, we propose a computational strategy to reconstruct precipitation pathways and identify the nucleation states making use of Monte Carlo simulations combined with Rare Event Sampling methods. The precipitation process is reproduced by Monte Carlo simulations with an energy description based on the Local Chemical Environment approach, applying efficient pair potentials, which are dependent on the chemical environment, and the Forward Flux Sampling technique. This method provides profound insight into the shape and composition of the early-stage precipitates and also the critical cluster size and shape in dependency of the temperature and supersaturation.
Authors: Piotr Macioł, Romain Bureau, Cecilia Poletti, Christof Sommitsch, Piotr Warczok, Ernst Kozeschnik
Abstract: The multiscale modelling of the behaviour of metal alloys during processing is often limited by the computing power required to run them. The Agile Multiscale Methodology was conceived to enhance the designing and controlling of complex multiscale models through an automatic run-time adaptation of its constitutive sub-models. This methodology is used to simulate the behaviour of an 6082 aluminium alloy during its thermomechanical treatment. The macroscopic deformation, the work-hardening and the state of precipitation are computed in different modules, allowing the coupling of several software solutions (DEFORMTM2D and © MatCalc) through an external storage of the relevant data.
Authors: Piotr Warczok, Yao Shan, Michael Schober, Harald Leitner, Ernst Kozeschnik
Abstract: Formation of coherent Cu precipitates in supersaturated ferrite (1.5 at.%Cu) at 500°C is simulated using the Monte Carlo method. Bond energies used in the atomistic simulation are calibrated on the mutual solubilities given on the Fe-rich and Cu-rich side of the Fe(bcc)-Cu(bcc) phase diagram. The spatial extension of the precipitate phase is defined on basis of a composition criterion of the nearest neighbor shells. Various definition conditions are examined in terms of resulting particle densities, mean radii and composition of the precipitates, as well as the composition profiles across the precipitate/matrix interface. The predictions of the simulations are compared with the experimental results from atom probe analysis as well as small angle neutron scattering.
Authors: Alice Redermeier, Ernst Kozeschnik
Abstract: In the present study, we investigate the performance of efficient pair potentials in comparison to accurate ab initio potentials as energy descriptions for Monte Carlo simulations of solid-state precipitation. As test scenario, we take the phase decomposition kinetics in binary Fe1-xCux. In a first effort, we predict thermodynamic equilibrium properties of bcc-rich Cu precipitates in an Fe-rich solution with a temperature and composition dependent Cluster Expansion. For this Cluster Expansion, combined ab inito and phonon calculations for various configurations serve as input. Alternatively, we apply the Local Chemical Environment approach, where the energy is described by computationally efficient pair potentials, which are calibrated on the first principles cluster expansion results. We observe that these fundamentally different approaches provide similar information in terms of the precipitate radius, chemical composition and interface constitution, however, the computational effort for the Local Chemical environment approach is significantly lower.
Authors: Bernhard Sonderegger, Ivan Holzer, Ernst Kozeschnik
Abstract: Interfacial energies are essential in modelling nucleation, growth and coarsening processes in solid materials; especially nucleation rates respond very sensitively to small changes of this quantity. Thus, the prediction of interfacial energies has attracted the interest of many researchers since many years. In this work, a simple concept for the calculation of energies of coherent interfaces in multicomponent systems is presented. The model advances the classical nearest-neighbor-broken-bond concept for arbitrary interface orientations and interface curvature. The obtained result is simple enough to be expressed in a single, closed equation. Consequently, it can be easily implemented in the framework of classical nucleation theory, or in complex simulation tools for precipitate evolution based on Kampmann-Wagner type models. In this paper, the theoretical background of the model is discussed, and the results are compared to experimental data. Furthermore, a size correction function for small precipitates is presented and applied to the prediction of nucleation rates. Despite the simplicity of the model, the predictions of the model are found to be in satisfactory agreement with experimental evidence.
Authors: Sabine Zamberger, Ernst Kozeschnik
Abstract: In the present work, the precipitation behavior of a V-microalloyed, quenched and tempered steel with 0.3wt % C is investigated experimentally and by computer simulation. The specimens are analyzed by means of transmission electron microscopy using selected area diffraction (SAD) and energy dispersive x-ray spectroscopy (EDX). The analysis is done on electropolished foils and on extraction replica. The numerical simulation is performed with the thermokinetic software package MatCalc, where the precipitation kinetics is examined for the experimentally applied thermo-mechanical cycles. Good agreement between experiment and simulation is obtained and the experimentally observed precipitate microstructure can be well explained on the basis of these simulations.
Authors: Ernst Kozeschnik, Bernhard Sonderegger, Ivan Holzer, Joachim Rajek, Horst Cerjak
Abstract: Precipitates are the key ingredient for the strength of heat treatable alloys. To optimize the mechanical properties of alloys it is important to know the response of precipitates to thermomechanical treatments. In the past, application of computer models to describe the evolution of precipitates in the course of these processes has proven difficult due to the complexity of the problem. In this work, a new model based on a mean-field representation of precipitates in a multicomponent matrix is applied to heat treatments of steels. Example simulations are presented for a 9- 12% Cr ferritic/martensitic heat resistant steel for power plant application and a complex tool steel with both carbides and intermetallic phases using the software MatCalc. The predictions of the model are verified on experimental results and the potential application to industrial heat treatment simulation is discussed.
Authors: Markus Rath, Ernst Kozeschnik
Abstract: In this work, we utilize recent software for precipitation kinetics simulation and couple it with models for grain growth. Basis of our studies is the thermo-kinetic software MatCalc, which has been designed for simulation of the evolution of precipitates in general multi-component multi-phase alloy systems. Grain growth approaches of different complexity are incorporated into the precipitation kinetics framework, investigated with respect to their coupling behavior with precipitation and precipitate dissolution and, finally, benchmarked on typical grain growth problems of industrial practice. The example presented in this work comprises austenite grain growth studies accompanying TiN and NbC precipitation/dissolution during austenitization of a Ti/Nb microalloyed steel. It is demonstrated that the present approach represents a versatile tool for analysis of simultaneous grain growth and precipitation in industrially important alloy systems.
Authors: Erwin Povoden-Karadeniz, Ernst Kozeschnik
Abstract: We present successful examples of CALPHAD thermodynamics-based precipitation simulations for three important alloy groups: Single-crystal Ni-base superalloy, austenitic stainless steel and hardenable Al-alloy. Underlying physical models for special features, such as, energies of diffuse interfaces between coherent precipitates and matrix, precipitation of incoherent particles at grain boundaries, evolution of excess vacancies during quenching and continuous aging and their role for metastable precipitate nucleation, are discussed.
Authors: Horst Cerjak, Gerhard Dimmler, Ivan Holzer, Ernst Kozeschnik, Peter Mayr, Cornelia Pein, Bernhard Sonderegger
Abstract: The research activities on ferritic / martensitic 9-12% Cr steels at the Institute of Materials Science, Welding and Forming (IWS) are represented by a network of interacting projects focusing on mechanical properties of base and weld metal, microstructural characterisation of creep and damage kinetics, weldability, microstructure analysis in the course of creep, modelling of precipitation and coarsening kinetics, simulation of complex heat treatments and the deformation behaviour under creep loading. The individual projects are briefly described and the conceptual approach towards a quantitative description of the creep behaviour of 9-12% Cr steels is outlined.
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