Richard Dronskowski RWTH Aachen University
Patrice E. A. Turchi Lawrence Livermore National Laboratory
Yoshitaka Adachi National Institute for Materials Science
Dierk Raabe Max-Planck-Institut fuer Eisenforschung
O3: Poster Session: Berlin
Tuesday PM, November 30, 2010
Exhibition Hall D (Hynes)
1:00 AM -
O3.3 Transferred to O1.2
Tuesday AM, November 30, 2010
Room 204 (Hynes)
9:30 AM - **O1.1
High Manganese 3rd Generation Advanced High Strength Steel for Automotive Applications.
Bruno De Cooman 1 Show Abstract
1 Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Steel is still the material of choice for car bodies, with 99% of the passenger cars having a steel body, and 60-70% of the car weight consisting of steel or steel-based parts. Most car makers are however routinely testing multi-materials concepts, which are not limited to the obvious use of light materials for closures, e.g. the use of Al for the front lid or thermosetting resins for trunk lids. The steel research community therefore makes a sustained effort to innovate and create advanced steels and original steel-based solutions, which help achieve demanding engineering targets in terms of safety, mass containment, stability, stiffness, comfort, acoustics, corrosion, and recycling. The contribution will review the recent development of high Mn steels. These steels achieve tensile strengths >1GPa and large tensile elongations. High Mn TWinning-Induced Plasticity (TWIP) steels are highly ductile, high strength austenitic steels characterized by a high rate of work hardening resulting from the generation of deformation-nucleated twins. Their Mn content is in the range of 15-30 mass%. Alloying additions of C, Si and/or Al are needed to obtain the high strength and the large uniform elongation associated with strain-induced twinning. Depending on the alloy system, the carbon content is either low, i.e. less than 0.05 mass-%, or high, typically in the range of 0.5-1.0 mass-%. Si and Al may be added to achieve a stable fully austenitic microstructure with low stacking fault energy in the range of 15-30mJ/m2. High Mn alloys characterized by strength ductility products 40.000-60.000MPa% have reached the stage of large scale industrial testing and the industrial focus is mainly on TWIP steels with the following compositional ranges: 15-25 mass-%Mn, with 0-3%Si, 0-3% Al and 200-6000ppm C. The dominant deformation mode in TWIP steel is dislocation glide, and the deformation-induced twins gradually reduce the effective glide distance of dislocations which results in a “Dynamical Hall-Petch effect”. The presentation will discuss the basic mechanical properties of TWIP steels in detail, with a particular focus on the stacking fault energy and the twinning process. Methods to suppress dynamic strain aging and delayed fracture phenomenon will be reviewed. In addition, important technical aspects will be discussed, such as the normal anisotropy, stretch-forming and high strain rate performance of TWIP steel.
10:00 AM - O1.2
Transformation Induced Plasticity in Fe-C-Cr-(W)-V.
Uta Kuehn 1 , Julia Hufenbach 1 , Stefanie Kohlar 1 , Horst Wendrock 1 , Juergen Eckert 1 Show Abstract
1 , IFW, Dresden Germany
On the basis of the Fe84.3C4.6Cr4.3Mo4.6V2.2 high speed tool steel, manufactured under relatively high cooling rates and highly pure conditions, a further improvement of the mechanical characteristics by slight modification of the alloy composition was attempted. For this, the alloy Fe88.9Cr4.3V2.2C4.6 was generated by elimination of Mo. By applying special preparation conditions, a microstructure composed of martensite, retained austenite and a fine network of special carbides was obtained already in the as-cast state. This material exhibits extremely high compression strength of over 5000 MPa combined with large compression strain of more than 25 % due to deformation-induced martensite formation. With this alloy a new class of TRIP assisted steels was found, which shows an extreme mechanical loading capacity.
10:15 AM - O1.3
Multilayered Steel Sheets : Combination of High Stress and Formability.
Pierre Lhuissier 1 , Junya Inoue 1 , Toshihiko Koseki 1 Show Abstract
1 Department of Materials Engineering, The University of Tokyo, Tokyo Japan
The combination of two steels phases, a brittle martensitic phase and a ductile austenitic phase, in a multi time rolled sheet gives very performant solutions combining high yield stress (1.2GPa) and formability (20%). The brittle phase (SUS420) that usually presents an ultimate strain of about 6% endures uniform strains of more than 20% when integrated in the structure. The reasons of this high elongation ability are up to know not well understood. The layer thickness barely affect the macroscopic mechanical response and thus the architecture should have changed one of the constitutive material behaviour. The present contribution deals with the deformation kinetics of the brittle phase which should be responsible of this enhanced formability.The study couples material analysis, based on in situ SEM/EBSD mechanical tests, with Finite Element model. Uni-axial tensile loading is performed both within a multilayer sheet and as bulk material on thin layers of several tens of micrometers. Crystal orientation and grain shapes are the inputs of the Finite Element model. A crystal visco-plastic model with self and latent hardening between slip systems and with a Norton type flow rule is used. The hardening parameters between the various slip systems are adjusted thanks to the comparison between the strain fields obtained by digital image correlation and by the modelling. The change of active slip systems are observed and put in relation to the ability to endure large strains. The observations on the microscopic scale are then linked to the macroscopic mechanical behaviour.
10:30 AM - O1.4
The Stability of Retained Austentite in AL-alloyed Trip Steels.
Kemal Davut 1 , Stefan Zaefferer 1 Show Abstract
1 Microstructure Physics and Metal Forming Department, Max Planck Institut for Iron Research, Dusseldorf, NRW, Germany
TRIP steels offer an excellent combination of high strength and ductility. The transformation of meta-stable austenite into martensite during straining leads to strong local hardening and prevents early localization of strain. However, the formation of hard martensite islands in the soft ferrite matrix causes strain incompatibilities which also enhance micro-void and micro-crack formation. Therefore, the mechanical properties of TRIP steels, including the damage resistance depend to a significant extent on the stability of retained austenite. The aim of this study was to find out the parameters that influence the stability of retained austenite. The deformation and transformation of austenite was followed by interrupted ex-situ bending tests under observations using electron backscatter diffraction (EBSD) in an SEM. The early transforming austenite grains were larger than the others. These grains were probably less stabilized due to lower carbon saturation. Detailed local orientation measurements reveal the Schmidt factor as a second parameter influencing the austenite stability. In fact, grains with higher Schmidt factors indicating easy deformability also transform easily. This can be related to the increase of the driving force for transformation. In order to prove the relevance of the local observations concerning texture changes due to transformation and deformation of retained austenite global texture measurements were also carried out.
10:45 AM - O1.5
Modification of Mechanical Properties of Alpha-iron by Hydrogen.
Demetra Psiachos 1 , Thomas Hammerschmidt 1 , Ralf Drautz 1 Show Abstract
1 Interdisciplinary Centre for Advanced Materials Simulation, Ruhr-Universitaet Bochum, Bochum Germany
The design of modern steels with tailored mechanical properties requires an understanding of the fabrication process and a reliable prediction of failure during operation. The complex interplay of effects on different length and time scales hampers a full understanding of the contribution of the microscopic processes causing hydrogen embrittlement in steels. Different mechanisms have been proposed in the literature to explain how the presence of hydrogen triggers mechanical failure in many steels. Our aim is to use the results obtained from atomistic simulations of the materials properties of relatively simple systems, in models on the mesoscopic length scale of realistic steels undergoing mechanical loading. To that end, we have parameterised the results of our atomistic simulations for subsequent incorporation into higher length-scale schemes.We will describe the results from our calculations of the modification of the elastic properties of alpha-iron as a function of hydrogen concentration. Upon determining the dependence of the stress-strain coefficients on H concentration using first-principles electronic structure theory, we then used these results in continuum elasticity theory in order to examine the effect of H on the anisotropic elastic moduli. We find that increasing the hydrogen concentration reduces the elastic moduli. We analyze the various contributions behind the effect of hydrogen on the material properties of iron. We find that while hydrogen leads to overall softening, most of this behaviour is caused by hydrogen increasing the local volume rather than by electronic effects.Using these results for the effect of H on the elastic properties of alpha-iron, we have further derived expressions for the dependence of the solubility of H, and by extension, the local concentration of H, on strainfields typical of those found near dislocations. Our analytical expressions for the modification of H-solubility are in excellent agreement with our first-principles results. We will describe some of the applications to themesoscale which will hopefully aid in achieving greater understanding of the underlying mechanisms of H embrittlement.
11:30 AM - **O1.6
Development of Steels for High-temperature Energy Systems.
Steven Zinkle 1 , Michael Brady 1 , Yuki Yamamoto 1 , Michael Santella 1 , Phillip Maziasz 1 , David Hoelzer 1 , Jeremy Busby 1 , Lizhen Tan 1 , Gerard Ludtka 1 Show Abstract
1 Materials Science & Technology Division, Oak Ridge National Lab, Oak Ridge, Tennessee, United States
There is increasing interest to develop new steels capable of reliable operation at high temperatures for a broad range of technologies, including fossil energy ultrasupercritical steam systems and fission energy power plants. The upper use temperature for steels has steadily increased over the past 60 years due to improvements in four generations of steels, but the rate of progress has been relatively modest (2.5 degrees K per year improvement in upper use temperature). Materials science tools such as computational thermodynamics offer the potential to substantially reduce the time period to develop new high-performance steels. Examples of the potential for rapid development of high-performance steels will be given, including both traditional ingot-based steel metallurgy methods and alternative techniques such as magnetic field processing and powder metallurgy production of oxide dispersion strengthened steels. Recent work to design a series of creep-resistant steels that form a self-healing alumina surface film at high temperatures will be highlighted. Several examples of the role of first principles modeling to guide the development of new steels will also be described.
12:00 PM - O1.7
On Assessing the Effects of V(C,N) Morphologies, Blocks and Laths on Creep of Martensitic Steels.
Mallikarjun Karadge 1 , Srinivasan Swaminathan 1 , Chen Shen 2 , Vishwanath Thippeswamy 1 , Ramkumar Oruganti 1 Show Abstract
1 , General Electric - Global Research, Bangalore, Karnataka, India, 2 , General Electric - Global Research, Niskayuna, New York, United States
There has been considerable emphasis placed on developing martensitic steels as candidate materials for high-temperature structural applications. This development heavily relies on tailoring of microstructures that would be stable at high temperatures. Microstructural evolution during creep deformation of such select advanced martensitic steels having differing chemical compositions and processing histories was studied. Particular focus was placed on understanding the microstructures seen at different length scales (blocks, laths and nano-sized precipitates) and their mechanistic contribution towards creep was elucidated through modeling. Examples include (1) the influence of the nano-precipitate [V(C,N)] morphology and chemistry and (2) combined effects of block plus lath boundaries.
12:15 PM - O1.8
Copper-nanoprecipitates in Steel Studied by Atom Probe Tomography and Ab-initio Based Monte Carlo Simulations.
O. Dmitrieva 1 , P. Choi 1 , N. Tillack 1 , T. Hickel 1 , D. Ponge 1 , Dierk Raabe 1 , J. Neugebauer 1 Show Abstract
1 , Max-Planck Institut, Duesseldorf Germany
We analyze the formation and coarsening of Cu nanoprecipitates in Fe-Si steels with 1 wt.% and 2 wt.% Cu, respectively, during age hardening at 400-450°C.For this purpose we use a joint experimental and theoretical approach. The atomic-scale experimental monitoring of the Cu precipitation and ripening processes is conducted using atom probe tomography (APT). For this purpose an IMAGO LEAP 3000X HR instrument is applied. The theoretical analysis is conducted by calculated the mixing and binding energies among the different atomic species. These energies are used to feed kinetic Monte Calo simulation. The work is intended to shed light on the early stages of precipitation phenomena in steels at the atomic and electronic scale.
12:30 PM - O1.9
Determination of Precipitate Stability during Thermal Processing of Ultra High Strength Steel Using TEM and Atom Probe Techniques.
Matthew Hartshorne 1 , Caroline McCormick 1 , Paul Novotny 2 , Michael Schmidt 3 , David Seidman 4 , Mitra Taheri 1 Show Abstract
1 Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 Tool and Alloy R&D, Carpenter Technology Corporation, Reading, Pennsylvania, United States, 3 Stainless Steel Alloy R&D, Carpenter Technology Corporation, Reading, Pennsylvania, United States, 4 Northwestern Center for Atom Probe Tomography, Northwestern University, Evanston, Illinois, United States
Ultra High Strength Steels are used in critical applications such as landing gear, structural members, springs and engine components. These steels traditionally have necessitated high levels of alloying elements and extremely low levels of impurities to achieve the required level of fracture toughness for advanced applications. Carpenter Technology Corporation has developed a new family of alloys, which utilizes lower levels of a more complex array of alloying elements than have been previously employed, while still retaining comparable strength and fracture toughness to much more highly alloyed steels. The mechanisms giving rise to the strength and toughness of this new alloy are not well understood, and an analysis of the segregation of the solute elements and the structure and stability of any precipitates during thermomechanical processing is desired to tailor future alloys during microstructural evolution. The behavior of precipitates and solute elements in these steels can be more thoroughly studied by utilizing a combination of in situ TEM, electron and Atom Probe tomography (APT).
Critical to the further development of future ultra high strength alloys is an understanding of precipitate nucleation and stability, and interfacial segregation. Specifically, samples containing grain and lath boundaries will be prepared utilizing a focused ion beam (FIB) and analyzed using electron tomography and APT to characterize and correlate any fine precipitates and segregation at the grain boundaries. Additionally, we will present experiments detailing the structure, chemistry, stability and preferred nucleation sites of precipitates by utilizing a combination of energy dispersive spectroscopy (EDS), selected area diffraction (SAD) and situ annealing in the TEM. The combination of these techniques allows for an in-depth analysis of the microstructure of this new alloy system, permitting us to elucidate the microstructural mechanisms which give rise to the high strength and high fracture toughness.
12:45 PM - O1.10
Comparison between Carbide and Nitride Precipitation in Steels: An Atom Probe Tomography.
Frederic Danoix 1 6 , Thierry Epicier 2 , Michel Perez 2 , Alexis Deschamps 3 , Frederic De Geuser 3 , Philippe Maugis 4 , Mohamed Goune 5 Show Abstract
1 GPM - CNRS, Université de Rouen, Saint Etienne du Rouvray France, 6 Christian Doppler Laboratory for Early Stages of Precipitation, Department of Physical Metallurgy and Materials, Leoben Austria, 2 MATEIS, INSA de Lyon, Villeurbanne France, 3 SIMAP, Grenoble INP, St Martin d’Hères France, 4 IM2NP, Université Paul Cézanne, Marseille France, 5 , ArcelorMittal Maizieres Research , Maizieres lès Metz France
The recent development of so called ‘nitrogen steels’, where nitrogen would tentatively replace carbon, offers new perspectives in stell design.The mechanical properties of carbon steels are strongly dependant on the size and distribution of carbides. In the case of nitrogen steels, iron and alloying element nitrides would play a similar role. It is therefore interesting to study the precipitation mode of nitrides, as compared to the one of carbides.It is well known that, in ferritic and martensitic steels, alloy carbides precipitate heterogeneously on dislocations. Conversely, in the case of alloy nitrides, it has recently be shown that the first stage of precipitation occurs through a similar mechanism as the one observed in aluminium alloys, i.e. the precipitation of fully coherent GP zones. It can thus be suspected a more homogeneous distribution of nitrides.In this presentation, we will focus on the particular system FeNbC and FeNbN, and experimentally show by atom probe tomography the difference in precipitation sequence in both systems, as well as the resulting distribution of carbides and nitrides. Experimental evidence of the NbN GP zone structure will also be given, and compared to ab initio calculations. These researches were conducted under ANR (french National Research Agency) financial support, contract CONTRA-PRECI n |0|6|-|B|L|A|N|-|0|2|0|5|.
Tuesday PM, November 30, 2010
Room 204 (Hynes)
2:30 PM - **O2.1
Mechanical Properties and Magnetism: Stainless Steel Alloys from First-principles Theory.
Levente Vitos 1 2 3 Show Abstract
1 Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm Sweden, 2 Physics Department, Uppsala University, Uppsala Sweden, 3 , Research Institute for Solid State Physics and Optics, Budapest Hungary
Stainless steels are among the most important engineering materials, finding their principal scope in industry, specifically in cutlery, food production, storage, architecture, medical equipment, etc. Austenitic stainless steels form the largest sub-category of stainless steels. Fully austenitic grades are composed mainly of Fe, Cr, and Ni, and have the face centered cubic structure of γ-Fe. At low temperatures, these alloys exhibit a rich variety of magnetic structures as a function of chemical composition, ranging from ferromagnetic phase to spin-glass and antiferromagnetic alignments. At ambient conditions, the austenitic steels have very low magnetic permeability and are generally regarded as non-magnetic. Because of this, they represent the primary choice for non-magnetic engineering materials.The presence of the chemical and magnetic disorder hindered any previous attempt to calculate the fundamental electronic, structural and mechanical properties of austenitic stainless steels from first-principles theories. Our ability to reach an ab initio atomistic level in this exiting field has become possible by the Exact Muffin-Tin Orbitals (EMTO) method [1-3]. This method, in combination with the coherent potential approximation , has proved an accurate tool in the ab initio description of the concentrated random alloys [1,5-11]. Using the EMTO method, we presented an insight to the electronic and magnetic structure, and micromechanical properties of austenitic stainless steel alloys [7-11]. In the present contribution, using the above approach, we will reveal the role of magnetism on the elastic properties and stacking fault energies of paramagnetic Fe-Cr-Ni alloys. L. Vitos, Computational Quantum Mechanics for Materials Engineers: The EMTO Method and Applications. Springer-Verlag London, Series: Engineering Materials and Processes (2007). O. K. Andersen, O. Jepsen, and G. Krier, in Lectures on Methods of Electronic Structure Calculations, edited by V. Kumar, O. K. Andersen, and A. Mookerjee,World Scientific Publishing Co., Singapore, pp. 63-124 (1994). L. Vitos, Phys. Rev. B 64 (2001) 014107. L. Vitos, I. A. Abrikosov, B. Johansson, Phys. Rev. Lett. 87 (2001) 156401. L. Dubrovinsky, L., et al. Nature 422 (2003) 58. L. Dubrovinsky, L., et al. Science 316 (2007) 1880. L. Vitos, P. A. Korzhavyi, B. Johansson, Nature Materials 2 (2003) 25. L. Vitos, P. A. Korzhavyi, B. Johansson, Phys. Rev. Lett. 88 (2002) 155501. L. Vitos, P. A. Korzhavyi, B. Johansson, Phys. Rev. Lett. 96 (2006) 117210. L. Vitos, J.-O. Nilsson, B. Johansson, Acta Materialia 54 (2006) 3821. L. Vitos and B. Johansson, Physical Review B 79 (2009) 024415.
3:00 PM - O2.2
Effects of Vacancy-carbon Point Defect Clusters on Diffusivity in Metastable Fe-C Alloys.
Mukul Kabir 1 , Xi Lin 2 , Sidney Yip 1 3 , Krystyn J. Van Vliet 1 Show Abstract
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Mechanical Engineering and Division of Materials Science and Engineering, Boston University, Boston, Massachusetts, United States, 3 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Diffusivity in defected crystals depends strongly on the interactions among vacancies and interstitials. Here we present a general framework to predict self-diffusivity in alloys comprising a spectrum of point defect clusters, based on an energy-landscape survey of local energy minima (formation energies governing concentrations) and saddle points (activation barriers governing mobility). Among all possible point-defect species in ferritic or body-centered cubic (bcc) iron supersaturated with carbon, only monovacancies, divacancies and the PDC containing one vacancy and two carbon atoms are found to be statistically abundant. We find that the migration barriers of these vacancy-carbon PDCs are sufficiently high compared to that of mono- and divacancies. This leads to decreased self-diffusivity in bcc Fe with increasing carbon content for any given vacancy concentration, which becomes negligible when the local interstitial carbon concentration approaches twice that of free vacancies. These results contrast with trends observed in fcc Fe, and provide a plausible explanation for the experimentally observed carbon-dependence of volume diffusion-mediated creep in ferritic (bcc) Fe-C alloys.
3:15 PM - O2.3
The Origin of Predominance of Cementite Among Iron Carbides in Steel at Elevated Temperature.
Chaitanya Krishna Ande 1 3 , Chang Ming Fang 2 1 , Marcel Sluiter 3 , Marijn van Huis 2 4 , Henny Zandbergen 2 Show Abstract
1 , Materials innovation institute (M2i), Delft, Zuid Holland, Netherlands, 3 S&C, MSE, 3mE, Delft University of Technology, Delft, Zuid Holland, Netherlands, 2 , Kavli Institute of Nanoscience, Delft University of Technology, Delft, Zuid Holland, Netherlands, 4 EMAT, University of Antwerp, Antwerp Belgium
A long-standing challenge in steel metallurgy is to understand why cementite (Fe3C) is the predominant carbide in steel. We show that the prevalent formation of cementite can be explained only by considering its stability at elevated temperature. Finite-temperature analysis shows that contributions from lattice vibration and anomalous Curie-Weis magnetic ordering, rather than from the conventional lattice mismatch with the matrix, are the origin of the predominance of cementite during steel fabrication processes with respect to η-Fe2C.
3:30 PM - O2.4
Density-functional Studies of Manganese-rich Austenite–short and Long-range Ordering, Vacancies and Mobility.
Joerg von Appen 1 , Marck Lumeij 1 , Richard Dronskowski 1 Show Abstract
1 Institute of Inorganic Chemistry, RWTH Aachen, Aachen Germany
One of central goals of the collaborative research unit “Steel ab initio” (SFB 761) is to clarify the chemical bonding situations in all the different phases which appear within manganese-rich model steels. It is obvious that atomistic structure models may help us to understand the relative phase stabilities and different deformation mechanisms, and the latter are needed to predict new promising chemical compositions.To do so, density-functional theory-based calculations were performed. We followed the pseudopotentials/plane waves strategy (VASP code) to optimize the crystal structures and calculate their total energies. For all phases, the theoretical heats of formation were computationally determined and eventually yielded a thermochemical ranking. In addition, short-ranged orbitals (TB-LMTO-ASA package) were employed to analyze the bonding situations by means of the Crystal Orbital Hamilton Population (COHP) concept. Previous contributions within this large-scale project dealt with ordered phases, in particular with the formation enthalpies of manganese carbides and cementite-type Fe3C, and with the density-functional assessment of the entire (Fe,Mn)3C phase diagram. In particular, the COHP technique allowed a thorough understanding of the structural and magnetic properties as well as chemical stabilities, in good agreement with experimental data. In the present study, we investigate the disordered antiferromagnetic fcc-type (austenite) phase Fe3MnC0.04 and compare the stabilities of various random solutions against short- and long-ranged ordered phases. We analyze the direct and indirect influence of the interstitial carbon atom and also of point defects with respect to the bonding and lattice distortion. Additionally, we calculate the activation energy for carbon diffusion and its favored path through the lattice.  J. Hafner, J. Comput. Chem. 2008, 29, 2044. O. K. Andersen, O. Jepsen, Phys. Rev. Lett. 1984, 53, 2571.  R. Dronskowski, P. E. Blöchl, J. Phys. Chem. 1993, 97, 8617. D. Djurovic, B. Hallstedt, J. von Appen, R. Dronskowski, CALPHAD, submitted. B. Hallstedt, D. Djurovic, J. von Appen, R. Dronskowski, A. Dick, F. Körmann, T. Hickel, J. Neugebauer, CALPHAD 2010, 34, 129. J. von Appen, B. Eck, R. Dronskowski, J. Comput. Chem., in press.
3:45 PM - O2.5
Carbon-vacancy Interactions in Austenite by DFT and Some of Their Consequences.
Ron Gibala 1 , Chris Wolverton 2 , Art Counts 2 Show Abstract
1 Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States
We have used density functional theory to calculate the binding energies of carbon-vacancy pairs and several (multiple-carbon)-vacancy complexes for fcc iron and fcc iron-base alloys. In order to reveal underlying trends, baseline results were obtained for C-V binding in non-magnetic fcc Fe. These binding energies range from 0.13-0.40 eV for next-nearest-neighbor and nearest-neighbor carbon-vacancy pairs, respectively, to 0.5-0.6 eV for (di-carbon)-vacancy triplets to about 0.4 eV for (tri-carbon)-vacancy defects. The magnitude of these binding energies suggests direct application to phenomena such as point-defect anelasticity, self-diffusion, high-temperature creep, strain aging, strain-age hardening, point defect strengthening, and radiation damage and other forms of environmental degradation. The effect of magnetic state on these calculations was examined by determining the binding energies of nearest-neighbor carbon-vacancy pairs in several different magnetic structures, e.g., low-spin and high-spin ferromagnetic and 1L-100, 2L-100 and SQS anti-ferromagnetic model systems. Binding energies were obtainable for only the 2L-100 and SQS AFM systems, 0.11 eV and 0.30 eV, respectively.
4:30 PM - **O2.6
Ab-initio Based Modeling of Novel High-strength Steels: From a Predictive Thermodynamic Description to Tailored Mechanical Properties.
Jorg Neugebauer 1 , Blazej Grabowski 1 , Fritz Kormann 1 , Johann von Pezold 1 , Tilmann Hickel 1 Show Abstract
1 Computational Materials Design, Max-Planck-Institut fuer Eisenforschung GmbH, Duesseldorf Germany
The combination of accurate first principles calculations with mesoscopic/macroscopic thermodynamic and/or kinetic concepts has quickly advanced in the past few years and allows now to tackle even complex engineering systems such as novel high-strength steels. Key to these studies is the highly accurate determination of free energies and surfaces. In the first part of the talk it will be shown how efficient sampling strategies together with high convergence density-functional theory calculations allow an accurate determination of all relevant temperature dependent free energy contributions such as electronic, harmonic, anharmonic, magnetic and structural excitations. Using these results to construct coarse grained models stability issues and mechanical properties of various alloys have been computed. The flexibility and the predictive power of this approach will be discussed in the second part of the talk for a few examples relevant to the design and understanding of high-strength steels: Determination of stacking fault energies (SFE) as critical parameter in mechanism maps, chemical trends in alloy composition or unraveling the mechanisms causing H embrittlement.
5:00 PM - O2.7
Quantifying the Strength of Point Defect Based Hydrogen Traps in bcc Iron.
William Counts 1 , Chris Wolverton 1 , Ron Gibala 2 Show Abstract
1 , Northwestern University, Evanston , Illinois, United States, 2 , University of Michigan, Ann Arbor, Michigan, United States
Hydrogen embrittlement of iron and steels is a classic but still unresolved problem in metallurgy. While hydrogen can freely move through the Fe lattice, its diffusion is hindered by lattice imperfections. Experimentally quantifying the binding energy of hydrogen to these defects has proven to be difficult. Fortunately, computational tools are ideally suited to study defect trapping because it is possible to isolate individual traps. Density function theory was used to quantify the binding energy of hydrogen to a number of point defects in bcc Fe. In bcc Fe, vacancies are the strongest hydrogen trap with a binding energy of 0.57 eV. Vacancies can bind multiple H atoms, but the general trend is that the binding energy decreases with the number of H atoms. Carbon is a weak H trap with a binding energy of 0.09 eV. Carbon-H defect complexes are not stable or energetically favorable when the carbon-H separation distance is less than 3.5 Å. The H binding energies of common alloying elements are significantly less than that of the vacancy and range between 0.00 - 0.20 eV. The H-substitutional solute binding energies roughly correlate with the solute atomic size. In addition, 3d transition metals can bind up to 4 atoms and the binding energy increases with the number of H atoms. The presence of a solute atom near a vacancy does not affect hydrogen binding to the vacancy (provided the solute atom is not significantly larger than Fe). The binding energy of hydrogen to a solute-vacancy defect pair varies between 0.56-0.60 eV, values that are similar to the hydrogen-vacancy binding energy.
5:15 PM - O2.8
First-principles Study of Electronic Structure and Phase Stability of Copper Sulfide in the Grain Oriented Silicon Steel.
Jianchun Cao 1 , Xiaolong Zhou 1 , Han He 1 , Yongfeng Mi 1 , Yuanyuan Peng 1 Show Abstract
1 Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093,China, Kunming China
The first principle pseudopotential plane wave method using the generalized gradient approximation (GGA) has been applied to provide comparison and contrast for the electronic structure and phase stability of Cu2S and CuS inhibitors in the Grain oriented silicon steel. The calculated equilibrium lattice parameters of Cu2S and CuS are in good agreement with the experimental values. The density of state at the Fermi energy, N(EF), is 1.65 states/eV for Cu2S and 2.52 states/eV for CuS. The formation energies are -1.2055 eV and -0.9699 eV for Cu2S and CuS respectively. Their negative formation energies indicate that Cu2S and CuS are thermodynamically stable phases. Furthermore, the Cu2S phase with the bigger negative formation energy is much more stable than CuS phase because of its lower density of state at the Fermi energy.
5:30 PM - **O2.9
Alloy Design of Steel using Thermodynamic and Kinetic Calculations.
Karin Frisk 1 Show Abstract
1 Alloy Development, Swerea KIMAB, Stockholm Sweden
To reduce lead time for development of new types of steel computational techniques are valuable, and for the early stages of alloy development, where the alloying contents are selected and adjusted, thermodynamic and kinetic calculations are important tools. Applications to microstructure predictions in high-strength low-alloy (HSLA) steels, and to the development of a new class of high-nitrogen tool steels are presented. The precipitation of fine carbides or carbonitrides in high-strength low-alloy (HSLA) steel is tailored by alloying and heat treatments for grain refinement and/or precipitation strengthening. The alloying elements are added in very small amounts, and the small size of the precipitates makes experimental determinations of the compositions of the precipitates challenging. Predictions of microstructure development through calculations are therefore useful for understanding of the effects of alloying elements. Modelling of the precipitation depends on a good description of the thermodynamic properties, and of the phase separation of the carbides/carbonitrides(1), and ab-inito calculations can give important input(2). The thermodynamic description of the precipitates strongly influences the calculated predictions of the precipitation sequences, and new calculations are presented that illustrate the differences. In recent years new types of tool steel with high nitrogen contents (up to over 4%) have been developed(3). Thermodynamic modelling has been an important aid for this development. For this type of steels, the properties are dependent on the types and phase fractions of the hard phases, normally carbides, and for this new class of steels, nitrides. Calculated predictions give a good description of the observed types of hard phases, and of the phase fractions, and the agreement is illustrated and related to the thermodynamic description of the nitrides. The exchange of carbon for nitrogen opens the possibility for new applications through compound materials(4), and is shown how predictions of the reactions in the joint in tool steel/stainless steel compounds, using diffusion calculations, indicate the critical influence of composition. 1.K. Inoue, N. Ishikawa, I. Ohnuma, H.Ohtani and K. Ishida, ISIJ Internationa, 41 (2001) p. 175-182.2.A. Markström, D. Andersson and K. Frisk, Computer Coupling of Phase Diagrams and Thermochemistry, 32 (2008) 615-623.3.O. Sandberg: Proceedings of the 8th International Tooling Conference, Aachen, 2009, pp. 357-367.O. Sandberg and A. Sandberg: Proceedings of Dansk Metallurgisk Selskab Meeting, Gothenburg, 2010.4.Greta Lindwall, Jesper Flyg, Karin Frisk and Odd Sandberg, Submitted to Metallurgical and Materials Transactions, 2010.
O3: Poster Session: Berlin
Tuesday PM, November 30, 2010
Exhibition Hall D (Hynes)
9:00 PM - O3.1
First-principles Study on the Interfacial Energy Between bcc Fe and Transition Metal Carbides.
Na-Young Park 1 2 , Woo-Sang Jung 3 , Jung-Hae Choi 1 , Pil-Ryung Cha 2 , Seung-Cheol Lee 1 Show Abstract
1 Computational Science Center, Korea Institute of Science and Technology (KIST), Seoul Korea (the Republic of), 2 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of), 3 High Temperature Energy Materials Center, Korea Institute of Science and Technology (KIST), Seoul Korea (the Republic of)
Carbides or nitrides precipitates play an important role in strengthening of metals by blocking the movement of dislocations. It has been known that tensile strength of conventional high-strength low-alloy (HSLA) ferritic steel is ~600 MPa. Recently, an HSLA steel having much higher tensile strength of ~800MPa has been reported. According to the study, nanometer-sized fine carbides whose diameter was approximately 3 nm were the main reason for the strengthening of the steels. They also reported that the highest tensile strength was obtained when the composition of Ti and Mo was approximately 1:1. However, mechanical and chemical properties of (Ti,Mo)C in a ferrite matrix has not been known yet. We employed first-principles calculations for interfacial energies between bcc Fe and group V transition metal carbides such as TiC and MoC, which are the NaCl-type structures. In the binary carbide system of MoC and TiC, the interfacial energy of MoC/Fe system was obtained lower than TiC/Fe system. Especially, the ternary carbide system with the composition of (Ti0.5Mo0.5)C/Fe showed the lowest interfacial energy when atomic bonding between Mo and Fe was formed. In the analysis of the density of states, we founded the resonant states near the Fermi level in MoC/Fe system which was not shown in TiC/Fe system. From the results, it was shown that the dependence of the interfacial energy on the type of carbide was closely related to the electronegativity of the number of valence electrons in carbides. This understanding will help one to design a new type of carbides that exhibits low interfacial energy.  Y. Funakawa, T. Shiozaki, K. Tomita, T. Yamamoto, E. Maeda, ISIJ Int. 44 (2004) 1945-1951.
9:00 PM - O3.10
Detection of Corrosion in Steel-reinforced Concrete by Ferromagnetic Resonance.
David Plusquellic 1 , William Egelhoff 1 , Frank Gayle 1 , James Baker-Jarvis 2 , Mark Stiles 1 , Robert McMichael 1 , Keith Gilmore 1 , Edward Garboczi 1 , Virgil Provenzano 1 , Jack Surek 2 , Shin Chou 1 Show Abstract
1 , NIST, Gaithersburg, Maryland, United States, 2 , NIST, Boulder, Colorado, United States
In this study, high resolution THz and microwave-based absorption measurements are carried out as a non-destructive means to probe the corrosion process deeply buried within complex steel-reinforced structural material, such as concrete, which has been shown to transmit far IR radiation. By taking advantage of the inherently different ferromagnetic resonance properties between the different phases of iron oxides and their precursors, the absorption measurements could be developed into a platform to perform detailed optical characterization on deeply buried steel bar undergoing corrosion, aging, and structural fatigue. The results can also provide insights into steel chemistry occurring within complex structural material that could not be readily understood using conventional surface science methods.
9:00 PM - O3.2
Effect of Impact Velocity on the Solid Particle Erosion Behavior of Fe-12Cr-5Mn-0.4C Alloy for Overlay Material.
Tae ho Kim 1 , Ki Nam Kim 1 , Jun-Ki Kim 2 , Seon Jin Kim 1 Show Abstract
1 Division of Advanced Materials Science and Engineering, Hanyang university, Seoul Korea (the Republic of), 2 Welding&Joining R&D Department, Korea Institute of Industrial Technology, Incheon Korea (the Republic of)
Solid particle erosion (SPE) implies the removal of material from component surfaces due to successive impact of hard particles, and has been recognized as a serious problem for many years in various engineering applications such as hydrotransport lines, power plants, and gas turbine blades. In particular, energy lines used in oil-sand, deep-sea mining and petroleum extraction industries are exposed to the serious erosion environment. Recently, present authors confirmed that a strain-induced martensitic transformation (SIMT) improved abrasive, adhesive, and cavitation erosion resistance in Fe-based alloys. Thus, we have tried to apply SIMT to improve SPE resistance of Fe-based alloys. In this study, the SPE behavior of Fe-12Cr-5Mn-0.4C alloy was investigated in comparison to Inconel 625 at impact velocities ranging from 5 to 35 m/s at angles of 30, 90 degrees. With an impact velocity of 35 m/s, the SPE resistance of the Fe-12Cr-5Mn-0.4C alloy was worse than that of Inconel 625, but at 5m/s, it was found that the Fe-12Cr-5Mn-0.4C alloy had better SPE resistance than Inconel 625 at all angles. This result was most likely attributed to SIMT which occurred on the eroded surface of the Fe-12Cr-5Mn-0.4C alloy. Strain- induced martensite had a negative effect on the SPE resistance due to its brittle fracture at an impact velocity of 35m/s, but this brittle fracture of strain-induced martensite did not occur and absorption of impact energy improved the SPE resistance of the Fe-12Cr-5Mn-0.4C alloy during SIMT at the impact velocity of 5m/s.
9:00 PM - O3.4
Influence of Deformation Rate on Overload Performance of Spot Welded Metastable Austenitic Stainless Steel.
Wei Liu 1 , Li Qiang 1 , Chuanjing Sun 1 Show Abstract
1 Mechanical Engineering, Beijing Jiaotong University, Beijing China
ABSTRACT: Spot welded specimens were prepared using high strength (R0.2>700MPa) cold-rolled 301LN sheets. Overload tests with slow 1mm/min and fast 60mm/min displacement rates were carried out under tensile-shear loading condition to investigate ultimate strengths and fracture energy absorptions of spot welds at normal and collided failure. All spot welded specimens were interfacial fracture. Although the deformation behaviors of specimens with two rates are different, the ultimate strengths and fracture energy absorptions are same. The deformation behaviors of spot welds can be divided into two stages; loading force with fast displacement rate increased more along with displacement in the first stage due to the higher yield strength and work hardening caused by more active dislocations; force with slow rate increased more in the second stage due to more alpha martensite formed along with displacement induced by failure deformation. Hardness under the fractures was higher than HV400, which are more than two times of their original. Keywords: cold-rolled 301LN, spot welds, displacement rate, overload performance
9:00 PM - O3.5
Micromechanical Characterization of Multilayered Steel Composites.
Koichi Hirashita 1 , Mitsuhiro Matsuda 1 , Masaaki Otsu 1 , Kazuki Takashima 1 Show Abstract
1 Materials Science and Engineering, Kumamoto University, Kumamoto Japan
Multilayered steel composites composed of high-strength steel and high-ductility steel exhibit features of both its components, and potential materials for reducing the weight and improving the safety of transportation systems. The properties of these multilayered materials depend on the mechanical properties of each of its constituent layers and their respective interfaces; hence, the measurement of these mechanical properties is essential for realizing high-performance multilayered steel composites. It is, however, rather difficult to measure the mechanical properties of each layer using a conventional testing method as the thickness of each layer is several tens of microns. Hence, we have developed a testing machine and a measurement method that enables the measurement of mechanical properties of micro-sized materials; by applying this testing method to multilayered steel composites, the mechanical properties of each layer and its interface can be measured directly. In this investigation, the mechanical properties and deformation behavior of each constituent layer of multilayered steel composites have been examined. Three-layered integrated steels consisting of SUS420 and SPCC (cold-reduced carbon steel sheets) were fabricated by a cold-rolling process. The thickness of the three-layered steel is 0.66 mm. Different heat-treatment processes were used to prepare three types of specimens (as-rolled, 550 °C-2 min heat-treated, and 550 °C-500 min heat-treated) and the effect of heat-treatment on their mechanical properties was investigated. Thin foils with a thickness of 20 µm were prepared from SUS420 and SPCC layers by mechanical polishing, and micro-sized tensile specimens having a parallel part with dimensions of ~50×20×20 µm3 were fabricated from the foils by FIB (focused ion beam) machining. Micro-tensile tests were performed by means of micro-tensile testing equipment that we have developed. In the case of the as-rolled specimens, the average tensile strengths in SUS420 and SPCC layers were 1063 and 606 MPa, respectively, while in the case of the specimen heat-treated for 500 min, they were 680 and 451 MPa, respectively. The tensile strength decreased with the increase in the heat-treatment time. The tensile strength of the specimens was also calculated by using the rule of mixture. For the as-rolled specimens and the 550 °C-2 min heat-treated specimens, the calculated value was consistent with the measured value; however, for the 550 °C-500 min heat-treated specimens, the calculated value was lower than the measured value. This result suggests that the necking of this layered structure was effectively obstructed by the outer ductile layer. Microtensile testing at the interface was also performed to verify this effect. The micromechanical characterization technique used in this study is useful not only for investigating the deformation behavior but also for enhancing the mechanical properties of multilayered steel composites.
9:00 PM - O3.6
Superior Mechanical Properties of FeCrMoWVC.
Julia Hufenbach 1 , Uta Kuehn 1 , Stefanie Kohlar 1 , Horst Wendrock 1 , Juergen Eckert 1 Show Abstract
1 Institute for Complex Materials, Leibniz Institute for Solid State and Materials Research Dresden, Dresden Germany
This work presents results on microstructure and mechanical properties of the steel composition Fe86Cr4.3Mo2.4W1.5V1.1C4.7 (at. %) subjected to preparation conditions typically used for manufacturing of bulk metallic glasses. Thermodynamical aspects and kinetic limitations on the specific solidification process of phase formation, particularly those, which are strongly dominated by diffusion controlled mechanisms, promote the formation of nonequilibrium phases, such as martensite and complex carbide structures already in the as-cast state. This combination of high strength phases yields a material with highly desirable properties, such as an engineering compression strength of more than 4500 MPa combined with large compression strain of over 20 %.
9:00 PM - O3.7
A Molecular Dynamics Study of Growth and Melting of Semi-spherical bcc-iron Nanoparticles on a Substrate Contacting Undercooled Liquid Iron.
Yasushi Shibuta 1 , Yusuke Watanabe 1 2 , Toshio Suzuki 1 Show Abstract
1 Department of Materials Engineering, The University of Tokyo, Tokyo Japan, 2 , JFE Steel Corporation, Kurashiki Japan
The growth and melting of semi-spherical bcc-iron nanoparticles on the substrate contacting undercooled liquid iron have been investigated as functions of particle size and temperature by a classical molecular dynamics simulation. As in the case of the freestanding nanoparticles in the undercooled melt , there is a critical radius that demarcates the growth or melting of a semi-spherical nanoparticle on the substrate. The critical temperature seems higher (i.e., the lower undercooling temperature) for the case of the stronger interaction between the particle and substrate. That is, the lower contact angle due to the strong interaction resulted in the lower undercooling temperature.Moreover, it is interesting to see that the contact angle of the semi-spherical nanoparticle remains almost constant during solidification and melting. Thus the idea of balances of the surface tensions of solid-liquid, liquid-substrate and solid-substrate interface at the triple point in the Young’s equation seems to be valid in the range of the interaction energy used in this study.In addition, it was confirmed that depression of the critical temperature was proportional to the inverse of the curvature radius of the solid-liquid interface, which is the Gibbs-Thomson effect.  Y. Shibuta, Y. Watanabe, T. Suzuki, Chem. Phys. Lett., 475 (2009) 264.
9:00 PM - O3.9
Correlation of Deformation Microstructures in Steels Created by Single-pass Machining with Conventional SPD Microstructures.
Srinivasan Swaminathan 1 2 , Srinivasan Chandrasekar 2 , Richa Verma 2 Show Abstract
1 , GE Global Research, Bangalore India, 2 School of Industrial Engineering, Purdue University, West Lafayette, Indiana, United States
The classical work by Embury and Fisher demonstrated the creation ofhigh-strength, ultrafine-grained (UFG) pearlitic microstructures in steelsusing multiple passes of large deformation. It is shown here thatsingle-pass, severe plastic deformation (SPD) of a eutectoid steel usingchip formation by machining results in microstructures similar to thosecreated by the multi-pass SPD.The correlations are established for strainsof up to 6, similar to those realized in the classical work. The possibilityof varying deformation parameters such as strain rate over a range isanother characteristic of the machining-SPD. Implications for manufacture ofbulk steel samples with UFG microstructures are briefly discussed.
Richard Dronskowski RWTH Aachen University
Patrice E. A. Turchi Lawrence Livermore National Laboratory
Yoshitaka Adachi National Institute for Materials Science
Dierk Raabe Max-Planck-Institut fuer Eisenforschung
Wednesday AM, December 01, 2010
Room 204 (Hynes)
9:30 AM - **O4.1
Ab-initio Modeling of Structural and Magnetic Phases of Transition Metal Alloys and Nanoclusters.
Peter Entel 1 , Heike Herper 1 , Sanjubala Sahoo 1 , Denis Comtesse 1 , Fred Hucht 1 Show Abstract
1 Faculty of Physics, University of Duisburg-Essen, Duisburg Germany
New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co, Ni and Z = C, Si with emphasis on the Fe-Mn steels. The structural and magnetic optimization is performed by using different methods. In particular, the finite temperature magnetic properties such as free energy and entropy differences are simulated by a Heisenberg-like model using the magnetic exchange interactions from first-principles calculations as input. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and atiferromagnetic trends influence the nucleation, morphologies and growth of the Fe-Mn nanoparticles.
10:00 AM - O4.2
Ab Initio Interfacial Austenite/Martensite Energies for Accurate Stacking Fault Energy Calculations in High-Mn Steels.
Alexey Dick 1 , Tilmann Hickel 1 , Joerg Neugebauer 1 Show Abstract
1 Computational Materials Design, Max-Planck-Institute for Iron and Steel Research, Düsseldorf Germany
The dominating plasticity mechanism in high-Mn steels (twinning- or transformation-induced) correlates with the value of the intrinsic stacking fault energy (SFE) . The most commonly used approach to theoretically evaluate SFE relies on analysis of the energy balance between austenite and martensite bulk phases, as well as of the interface energy between austenite and martensite phases. The latter is typically used as a fit parameter since no direct measurements of it are available, which results in uncertainty of the SFE calculation [2,3]. We have performed, therefore, a first-principles study on the interface austenite/martensite energy of FeMn-alloy which is a prototype structure for realistic high-Mn-steels. The interface austenite/martensite energy is obtained from comparison of the ab initio SFE and the ab initio free energy difference between bulk alloys in austenite and martensite phase. The relevant atomic configurations have been identified by the cluster-expansion method and the concept of special quasirandom structures. The resulting structures and energies have been used as an input for the axial interaction model and for explicit calculations of the SFE. The temperature dependencies have been included using concepts of equilibrium thermodynamics. The calculated ab initio interfacial austenite/martensite energies are compared to previously used thermodynamic assessments and show a remarkable dependence on the magnetic ordering and Mn content. G. Frommeyer, U. Brüx, and P. Neumann, ISIJ Int. 43, 438 (2003). A. Saeed-Akbari, J. Imlau, U. Prahl, and W. Bleck, Met. and Mat. Trans A. 40, 3076 (2009). J. Nakano and P. J. Jacques, Calphad 34, 167 (2010).
10:15 AM - O4.3
Effect of Segregation on Grain Boundary Cohesion in Aluminum Σ11(113) Grain Boundary from Ab-initio Study.
Tokuteru Uesugi 1 , Kenji Higashi 1 Show Abstract
1 Department of Materials Science, Osaka Prefecture University, Sakai Japan
The fracture mechanism due to the segregation of harmful impurities to grain boundary has been a well established mechanism. In aluminum alloys, segregation of impurities, such as alkali metals, at grain boundaries are sometimes discounted as a cause of grain boundary embrittlement. In this work, we investigate the energies of segregation of various solutes including Na and Ca at symmetric tilt Σ11(113) grain boundary in aluminum from the first principles calculations. As energy of segregation of Na and Ca is negative, these alkali elements tend to segregate at the grain boundary. Furthermore, on basis of the Rice and Wang model, we study the effect of the segregation of these alkali metals on the grain boundary embrittlement of aluminum. Our first principles calculations of energies of segregation at grain boundary and free surface show that these alkali metals behave as embrittler. The decreasing charge density at the gain boundary also demonstrates that the Na and Ca atoms form weaker metallic bonds with neighboring Al atoms in the grain boundary region.
10:30 AM - **O4.4
Modeling Magnetism-driven Phase Transformations in Fe-Cr Alloys and Ferritic-martensitic Steels.
Duc Nguyen-Manh 1 , Mikhail Lavrentiev 1 , Sergei Dudarev 1 Show Abstract
1 Theory and Modeling, Culham Centre for Fusion Energy, Abingdon United Kingdom
Developing predictive models for Fe-Cr -based ferritic/martensitic steels, which are candidate structural materials for fusion and advanced fission power generation, requires formulating controlled approximations and efficient algorithms for large-scale atomistic modeling. Understanding the stability and dynamics of Fe-Cr alloy system at high temperature under irradiation is a formidable challenge for computational materials science because magnetic fluctuations involving the constituent elements of the alloy result in complex point-defect behavior and phase transformations occurring in the high temperature limit[1-3]. Here we describe two complementary approaches based on first-principles spin-polarized density functional theory (DFT) calculations which address the frustrated magnetic behavior in this system. The first model  involves a many-body Anderson Hamiltonian for localized 3d-electrons. The model allows treating spin-fluctuation effects and at the same time gives the mean-field Stoner solution, representing a starting point for the development of magnetic inter-atomic potentials for atomistic simulations of Fe-Cr alloys. Using this model, we prove that the unusual Bain transformation pathway and the negative enthalpy-of-mixing anomaly characterizing the alloy in the low chromium concentration limit are entirely due to presence of the on-site magnetic exchange Stoner terms. The second, Magnetic Cluster Expansion (MCE), model makes it possible to investigate and accurately describe structural phase transformations in magnetic Fe-Cr alloys at elevated temperatures. Using equilibrium Monte Carlo simulations performed using this model we find that the predicted Curie, Néel, and the structural phase-transition temperatures are all in good agreement with experimental observations. Also, we find that it is the thermal excitation of magnetic and phonon degrees of freedom that stabilizes the fcc (γ) phase. The model describes the occurrence of the γ-loop in the phase diagram of Fe-Cr alloys for a realistic interval of temperatures and Cr concentrations. Finally, the effect of magnetic environmentally dependence for the point-defect migration energies is discussed in presence of carbon impurities to modeling the kinetics of phase transformations in ferritic/martensitic steels.This work was funded partly by the United Kingdom Engineering and Physical Sciences Research Council under grant EP/G003955 and the European Communities under the contract of Association between EURATOM and CCFE.  D. Nguyen-Manh, A.P. Horsfield, S.L. Dudarev, Phys. Rev. B 73, 020101 (2006). D. Nguyen-Manh, M.Yu. Lavrentiev, S.L. Dudarev, J. Computer-Aided Mater. Des., 14, 159 (2007). D. Nguyen-Manh, M.Yu. Lavrentiev, S.L. Dudarev, C.R. Phys. 9, 379 (2008). D. Nguyen-Manh, S.L. Dudarev, Phys. Rev. B 80, 104440 (2009). M. Yu. Lavrentiev, D. Nguyen-Manh, S.L. Dudarev, Phys. Rev. B 81, 184202 (2010).
11:30 AM - **O4.5
Interplay Between Magnetic and Chemical Disorder in Theoretical Simulations of Phase Transformations in Steels.
Igor Abrikosov 1 , Alena Ponomareva 2 , Marcus Ekholm 1 , Andrei Ruban 3 Show Abstract
1 Department of Physics, Chemistry and Biology (IFM), Linkoping University, Linkoping Sweden, 2 Department of Theoretical Physics and Quantum Technology , National University of Science and Technology “MISiS”, Moscow Russian Federation, 3 Department of Materials Science and Engineering, Applied Materials Physics, Royal Institute of Technology, Stockholm Sweden
At present, there are several state-of-the-art methods for parameter-free simulations of phase equilibria in alloys . However, in case of magnetic alloys the majority of simulations are still carried out either for non-magnetic or for fully magnetically ordered states. We show that the effective chemical interactions are sensitive to the details of the magnetic state of the system, because the interactions between magnetic atoms depend on the relative orientation of their local magnetic moments through the corresponding exchange interactions. We investigate thermodynamic properties and phase transitions in Fe-Cr, Fe-Si, and Fe-Ni alloys and demonstrate the importance of the proper treatment of spatial arrangements of local magnetic moments. We reconsider a classical example of Fe-Cr system, where mixing enthalpies differ qualitatively between the magnetically ordered and magnetically disordered states . We demonstrate that in the ferromagnetic Fe-Cr alloys magnetic exchange interactions almost completely determine the chemical interactions leading to the anomalous stability of low-Cr steels. We show that effective chemical interactions in Fe-Si alloy can be tuned by its global magnetic state, which opens exciting possibilities for materials synthesis. We predict theoretically that at pressure of 20 GPa and at high temperatures the magnetic disorder favors a formation of cubic Fe2Si phase with B2 structure, which is not present in the alloy phase diagram. The prediction is confirmed experimentally . Taking Fe25Ni75 permalloy as an example, we show from ab-initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme in which the magnetic reference state used for the calculations of chemical interactions, consists of partially disordered local magnetic moments with the degree of disorder determined from Heisenberg Monte-Carlo simulations for the corresponding alloy composition and temperature. We demonstrate that it allows us to reproduce the chemical order-disorder transition temperature for the permalloy in good agreement with experimental data. A. V. Ruban and I. A. Abrikosov, Rep. Prog. Phys 71, 046501 (2008).  P. Olsson, I. A. Abrikosov, L. Vitos, and J. Wallenius, J. Nucl. Mater. 321, 84 (2003). A. V. Ponomareva, A. V. Ruban, N. Dubrovinskaia, L. Dubrovinsky, and I. A. Abrikosov, Appl. Phys. Lett. 94, 181912 (2009).
12:00 PM - O4.6
Thermodynamic Data of Fe-based Alloys Derived from First Principles.
Tilmann Hickel 1 , Fritz Koermann 1 , Alexey Dick 1 , Joerg Neugebauer 1 Show Abstract
1 Computational Materials Design, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
The iron-carbon phase diagram plays an important role in the thermodynamic evaluation of steels. Besides the ferrite and the austenite, in particular cementite (Fe3C) is a decisive ingredient. Since the experimental data for the heat capacity of cementite are not completely conclusive, the thermodynamics of this phase in CALPHAD evaluations was traditionally often based on a temperature independent heat capacity. Making use of the continuous progress in the field of ab initio modeling, we recently develop new routes to determine heat capacities without experimental input. These approaches are based on density functional theory in combination with statistical methods. For Fe-based alloys the incorporation of magnetic excitations turned out to be a particular challenge in this context.In this presentation we discuss our recently developed concepts to capture all the relevant physical mechanisms contributing to the heat capacity such as vibronic, electronic, and magnetic effects in iron and cementite. A particular focus will be on the magnetic entropy of these materials. To capture these contributions an effective model Hamiltonian is constructed and numerically solved using Quantum Monte Carlo simulations. For the example of ferrite, for which accurate experimental data are available, we demonstrate the excellent performance of our theoretical approach below as well as above the Curie temperature. The subsequent application to the heat capacity of cementite can explain the experimental data and allows an evaluation of their temperature dependence. Consequently, the application of first principles concepts to Fe-based materials can provide a valuable contribution to a systematic CALPHAD reassessment of their phase diagrams.
12:15 PM - O4.7
First-principles Prediction of Partitioning of Alloying Elements Between Cementite and Ferrite.
Chaitanya Krishna Ande 1 2 , Marcel Sluiter 2 Show Abstract
1 , Materials innovation institute (M2i), Delft, Zuid Holland, Netherlands, 2 S&C, MSE, 3mE, Delft University of Technology, Delft, Zuid Holland, Netherlands
At long tempering times in steels when both cementite (Fe3C) and ferrite (bcc-Fe-rich solid solution) phases are present, alloying elements tend to segregate to either of the two phases. V, Cr, Mn, Mo and W are found to partition to the cementite phase while the elements Al, Si, P, Co, Ni and Cu partition to ferrite. We use first-principles calculations to show that partitioning of alloying elements and cementite (de)stabilization by alloying in mixtures of bcc-Fe and cementite are intimately related through the introduction of a partitioning enthalpy. The formation enthalpy of alloying element substituted cementite is shown not to be a proper gauge for addressing these questions. Magnetism is also shown to play an important role in the partitioning of the alloying elements.
12:30 PM - O4.8
Elastic Properties Screening of Fe-Mn Alloy Thin Film Spreads Using Nanoindentation.
Thomas Gebhardt 1 , Denis Music 1 , Jochen Schneider 1 , Marcus Ekholm 2 , Igor Abrikosov 2 , Levente Vitos 3 , Joerg von Appen 4 , Richard Dronskowski 4 Show Abstract
1 Materials Chemistry, RWTH Aachen University, Aachen Germany, 2 Department of Physics, Chemistry and Biology, Linkoping University, Linkoping Sweden, 3 Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm Sweden, 4 Solid-State and Quantum Chemistry, RWTH Aachen University, Aachen Germany
Fe-Mn alloys spark interest of material scientist of various disciplines due to their fascinating properties and wide range of applications. These properties are mainly related to the face centered cubic (fcc) → hexagonal close packed (hcp) martensitic transformation as well as the magnetism and magnetic transformations present in these alloys. Among these applications high-Mn steels feature high strength and exceptional plasticity due to twin formation (TWIP effect) or via multiple martensitic transformations (TRIP effect) under mechanical load, providing therefore great potential for structural components in automotive engineering. The microstructural parameter, which governs the appearance of the deformation mechanism, is the stacking fault energy of the austenite, which is strongly dependant on the chemical composition. However, a systematic experimental study of the dependence of the elastic properties of Fe-Mn alloys on the chemical composition has not been done.It is our goal to contribute towards understanding the correlation between structure, chemical composition, magnetic configuration and elastic properties. We use the so-called combinatorial thin film approach to investigate the influence of the Mn content and additions of the alloying elements Al and Si on the elastic properties of fcc Fe-Mn alloys. Ab initio calculations serve to probe both the impact of magnetic effects and the influence of the chemical composition on the elastic properties.Fe-Mn thin films with a graded chemical composition were synthesized by combinatorial magnetron sputtering. The elastic properties of these thin films were probed by nanoindentation. The magnetic state description with disordered local moments yields the best agreement with the experimental results, whereas with nonmagnetic and antiferromagnetic configurations the bulk modulus is overestimated. This may be understood based on the differences in the chemical bonding and the magnetovolume effect. These results show that the combinatorial thin film approach is an appropriate tool to systematically investigate the elastic properties of Fe-Mn alloys as a function of the chemical composition and to validate theoretical models.
Wednesday PM, December 01, 2010
Room 204 (Hynes)
2:30 PM - **O5.1
Structures of Complex-oxide Nanoparticles in Fe-Cr MA/ODS Ferritic Steels with Alloying Additions of Al and Ti.
Luke Hsiung 1 Show Abstract
1 Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, California, United States
Crystal and interfacial structures of oxide nanoparticles in Fe-16Cr-4.5Al-0.3Ti-2W-0.37Y2O3 (K3) and Fe-20Cr-4.5Al-0.34Ti-0.5Y2O3 (MA956) ODS ferritic steels produced by mechanical alloying (MA) and hot extrusion at 1150 °C have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. The formation of Y-Al-O complex-oxide nanoparticles in both ODS steels implies the occurrence of decomposition of Y2O3 in association with internal oxidation of Al during mechanical alloying. While the complex-oxide nanoparticles observed in both ODS steels are mainly Y4Al2O9 (YAM) with a monoclinic structure, the complex-oxide of YAlO3 (YAP) with an orthorhombic structure was occasionally observed in MA956-ODS steel. These results indicate that Y4Al2O9 is the most stable among three Y-Al-O complex-oxide compounds (Y4Al2O9, YAlO3, and Y3Al5O12) when the ODS steels are consolidated at 1150°C, and Ti plays an insignificant role in forming complex-oxide nanoparticles at the presence of Al in these two ODS steels. The observations of oxide nanoparticles with core/shell or partially crystallized structures in both ODS steels support a three-stage (fragmentation, amorphization, and crystallization) mechanism proposed for the formation of nanoparticles in ODS steels. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
3:00 PM - O5.2
Precipitation Strengthening of a Nano-cluster-strengthened Ferritic Steel.
Zhongwu Zhang 1 2 , Chain-tsuan Liu 2 3 , Sheng Guo 3 , Jialin Cheng 1 , Guang Chen 1 , Takeshi Fujita 4 , Mingwei Chen 4 , Yip-Wah Chung 5 , Semyon Vaynman 5 , Morris E Fine 5 , Bryan A Chin 2 Show Abstract
1 Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing China, 2 Materials Research & Education Center, Auburn University, Auburn, Alabama, United States, 3 Department of Mechanical Engineering, the Hong Kong Polytechnic University, Hong Kong China, 4 Institute for Materials Research, and World Premier International Research Center for Atoms, Molecules and Materials, Tohoku University, Sendai Japan, 5 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Precipitation strengthening of steels by nanoscale precipitates is of great importance because the carbon content of high strength steels can be designed to be very low giving improved ductility and weldability. The strength of Cu-rich nanocluster strengthened ferritic steels is derived mainly from nanocluster precipitation. This gives the ability to control the alloy’s strength and ductility through the control of the precipitation process of nanocluster. In this study, the effect of composition and processing routes on the nanocluster precipitation and finally on the mechanical properties of Cu-rich nano-cluster-strengthened ferritic steels were investigated. Cu-rich nanocluster precipitation was observed by using STEM and atom probe tomography. The microhardness results show a typical dependence on Cu-rich precipitate aging time. Tensile tests showed that the thermomechanical treatment can significantly improve the yield strength and ductility of the ferritic steel compared to that without thermomechanical treatment. There exist three mechanisms influencing the ductility of the nano-cluster-strengthened ferritic steel. They are nano-cluster-precipitation strengthening, hydrogen embrittlement including moisture-induced hydrogen embrittlement, and grain boundary fracture. Our study reveals that boron strengthens the grain boundary and suppresses the intergranular fracture. Hydrogen-induced embrittlement can be eliminated by heat treatment in vacuum.This research was supported mainly by internal funding from Auburn University and Hong Kong polytechnic University, together with the Science and Technology Development Foundation of Nanjing University of Science and Technology of China (XKF09055), the National Natural Sciences Foundation of China (No 50871054), the Research Fund for the Doctoral Program of Higher Education of China (20093219110035), and the US National Science Foundation (CMMI-0826535).
3:15 PM - O5.3
Use of Model High Purity Alloys to Study the Tempering Behaviour of Low-alloyed Medium Carbon Steels.
Anna Fraczkiewicz 1 , Muriel Hantcherli 1 Show Abstract
1 Centre SMS, UMR CNRS 5146, Ecole des Mines de St-Etienne, St-Etienne France
The purpose of this work is to optimise the strength of tempered martensitic medium carbon steel, containing low-level of Cr, V and Si/Al. A series of high purity "model" ternary alloys (Fe-C–X, X=Cr, V, Si, Al) was prepared (EMSE) by the cold crucible method. The initial structures of quenched alloys were optimised through appropriate austenitizing conditions. Analysis of alloys behaviour on tempering was performed through carbide precipitation characterisation (X-ray diffraction, SEM and TEM), completed by hardness and dilatometric measurements. The behaviour of "model" alloys was compared to that of the industrial material.Vanadium addition is responsible for two effects: (i) vanadium carbides lock the grain growth during the austenitizing; (ii) dissolved vanadium promotes the precipitation of complex carbides containing also some chromium; a slight secondary hardening peak near 550°C is observed in their presence. Silicon addition has an interesting effect, as it stabilises the martensitic matrix and increases the temperature of cementite precipitation. Therefore, a high value of hardness is conserved up to high tempering temperatures.Unfortunately, because of some noxious properties, the industrial process in concern excludes the presence of Si. That’s why two new model alloys have been prepared and tested, in which the silicon addition (i) was absent, or (ii) replaced by Al.Aluminium has been shown to be "neuter" vs tempering behaviour. Moreover, the interesting effect of secondary hardening (550°C) has been observed in both Si-free steels, showing that alloying with only Cr and V can be enough to get the expected behaviour of the studied steel.
3:30 PM - O5.4
The Effect of Nano-precipitates on Strength in a Micro-alloyed Ferritic Steel.
Hesamaldin Askari 1 , Yongfeng Shen 3 , C m Wang 3 , Xin Sun 2 , Hussein Zbib 1 Show Abstract
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States, 3 Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang China, 2 , Pacific Northwest National Laboratory, Richland, Washington, United States
A high strength ferritic steel with finely dispersive precipitates was investigated to reveal the fundamental strengthening mechanisms. Using energy dispersive X-ray spectroscopy (EDXS) and transmission electron microscope (TEM), fine carbides with an average diameter of 10 nm were observed in the ferrite matrix of the 0.08%Ti steel, and some cubic M23C6 precipitates were also observed at the grain boundaries and the interior of grains. The increase in precipitation strengthening by the carbides was approximately 450 ~ 630 MPa, depending on the average size of nanoscale precipitates. This value is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. The effect of the particle parameters, size and density and strength, have been investigated through dislocation dynamics simulations and the relationship for resolved shear stress for single crystal has been determined as a function of these parameters. The DD simulations as well as experimental analysis showed that the finely dispersive precipitates can strengthen the steel by pinning the dislocations up to a critical shear stress and retarding the recovery as well as annihilation of dislocations.
3:45 PM - O5.5
Evolution of Interfacial Toughness of Dissimilar Metal Bonding.
Shoichi Nambu 1 , Junya Inoue 1 , Toshihiko Koseki 1 Show Abstract
1 , The University of Tokyo, Tokyo Japan
Multilayered steels have been proposed by our group to achieve better strength-ductility combination, and the interfacial toughness at steel/steel interface is an important parameter to enhance the ductility and improve the reliability of multilayered steels. The control of interfacial toughness is a key to obtain better properties of multilayered steels. Although the bonding strength at metal/metal interface can be varied by post-heat treatment, the development has not been fully understood due to the changes in microstructures and mechanical properties of constituent materials. In this study, development of interfacial toughness at steel/steel, steel/Ni and steel/Cu interfaces during heat treatment was investigated in order to clarify the mechanism of interface formation and control the interfacial toughness at metal/metal interface.Austenitic stainless steel SS304 with 0.6mm thickness, interstitial free (IF) steel with 0.15mm thickness, pure Ni with 0.15mm thickness and pure Cu with 0.15mm thickness were prepared. Three kinds of interfaces of IF steel/SS304, Ni/SS304 and Cu/SS304 were bonded by surface activation bonding method. Interfacial toughness was evaluated by 180 degree peel test after a heat treatment. Changes in microstructures at interface and fracture profiles of peel test were also investigated by SEM and TEM.The results of peel test demonstrated that the interfacial toughness increases with time during heat treatment and the development of interfacial toughness was clearly divided into two stages. In the first stage, interfacial toughness increases rapidly, while it increases slowly in the second stage, and the increasing rate of interfacial toughness was varied by heat treatment temperature. Furthermore, it was clarified that the activation energy of increasing rate of interfacial toughness and microstructure at interface changed at each stage.
4:30 PM - **O5.6
Atomistically Informed Continuum Models for Deformation and Failure of Multiphase Steels.
Alexander Hartmaier 1 , Aenne Koester 1 , Anxin Ma 1 Show Abstract
1 Interdisciplinary Centre of Advanced Materials Simulation (ICAMS), Ruhr-University Bochum, Bochum, NRW, Germany
Micromechanical modeling of deformation and failure of multiphase materials requires the mechanical properties of all individual phases and their interfaces to be known. If furthermore the microstructural evolution during deformation is to be described, phase transformations, grain growth, and recrystallization need to be modeled as well. In this work, an approach is described to develop representative volume elements (RVE) of multiphase steels containing multiple grains of ferrite, austenite, and martensite. In the work presented here, only plastic deformation shall be considered, while models for microstructural changes and crack initiation are only touched in the sense of an outlook. Plasticity in the ferrite and austenite grains of the RVE is described on the basis of crystal plasticity models. While such models have been demonstrated to yield reliable results for the plastic deformation of face centered cubic (fcc) metals, it is known that body centered cubic phases (bcc), like ferrite, exhibit a pronounced non-Schmid behavior and a large difference in the mobilities of edge and screw dislocations. Furthermore, the mobility of screw dislocations depends on orthogonal components of the local shear stress that do not contribute to the Peach-Koehler force on these dislocations. This dependence is assessed with atomistic simulations with semi-empirical potentials. Together with experimental and numerical results from the literature this yields a new atomistically informed constitutive relation for the deformation of ferrite grains. Hence, it is demonstrated that atomistic methods support the constitutive modeling by providing quantities and relations that are difficult, if not impossible, to measure directly with experimental methods.
5:00 PM - O5.7
Local Residual Stress Measurement in Slightly Deformed TWIP Steels.
Tom Jaepel 1 , Stefan Zaefferer 1 , Myrjam Winning 1 , Dierk Raabe 1 Show Abstract
1 Dept. Microstructurephysics and Metal Forming , Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
High manganese steels are characterized by a high work hardening and work hardening rate. Depending on the stacking fault energy – which itself depends on composition and temperature - the deformation mechanisms of these materials exhibit planar glide, mechanical twinning (twinning induced plasticity TWIP) or formation of martensite (transformation induced plasticity TRIP). The high strength and work hardening cause the formation of high localized residual stresses, mainly because of the restriction of dislocations to planar glide on only a few lattice planes and dislocation pile up. These high residual stresses are of great importance for technological applications, because they can decrease the fracture strength significantly (e.g. hydrogen embrittlement). Additionally the analysis of local residual stresses can help to clarify the influence on the work hardening behavior and is therefore of general interest for the development of these materials. In this work the Kikuchi-pattern cross-correlation method according to Wilkinson et al. was used to determine small lattice distortions and rotations. They are determined by measuring small shift of diffraction poles in Kikuchi-patterns from neighboring spatial positions. An iterative cross-correlation algorithm calculates a unique solution for eight independent components of the deviatoric displacement gradient tensor. The method was applied to determine the evolution of local elastic and plastic strain in and ex-situ bending experiment. To this end a recrystallized TWIP steel sample was placed in a self-designed SEM sample holder for three point bending. A comparison of the stress and strain state before and after twin formation allows to draw first conclusions on the formation mechanisms of twins in these steels.
5:15 PM - O5.8
Dislocation Density-based Constitutive Model for TWIP Steel.
David Steinmetz 1 , Franz Roters 1 Show Abstract
1 Department of Microstructure Physics and Metal Forming, Max Planck Institute for Iron Research, Dusseldorf Germany
In this work we present a dislocation density-based constitutive model for the hardening behavior in twinning induced plasticity (TWIP) steels. There have been numerous attempts to model the hardening behavior of low stacking fault energy (SFE) fcc metals that are either phenomenological, based on crystal plasticity finite element (CPFE) modeling, or are physically-based but lack certain decisive features, such as being able to model multiple deformation routes. In contrast, our model is a physically-based strain rate and temperature-sensitive model based on electron channeling contrast imaging (ECCI) microstructural investigations of twins and dislocation structures in TWIP steels.One distinct advantage of the physically-based model is that the fitting parameters are known within an order of magnitude. This allows more complex microstructural information to be included in the model without losing the ability to predict a reasonable initial value for all of the parameters. Dislocation cell sizes, grain sizes, and twin volume fraction evolution are included and combined with a new formulation for the critical twinning stress.A Matlab optimization routine was used for parameter optimization. Various strain rates, temperatures, and deformation paths were considered in the parameter optimization process in order to obtain a universal parameter set for the used Fe-23Mn-0.6C TWIP steel.
5:30 PM - O5.9
Microstructure Evolution in TWIP Steels During Deformation with Strain Path Changes.
Nahid Elhami 1 , Stefan Zaefferer 1 , Ingo Thomas 2 , Harald Hofmann 2 Show Abstract
1 Microstructure Physics and Metal Forming, Max-Planck-Institute for Iron Research, Dusseldorf Germany, 2 Research and Development , ThyssenKrupp Steel Europe AG , Duisburg Germany
Steels with twinning induced plasticity (TWIP) are known to exhibit a well balanced work hardening rate which leads to high ductility and strength of deformed material. The well balanced work hardening is based on the continuous formation of twins even at high strain and the interaction of twins with each other and with dislocations.It has been found that the twin structure develops very differently depending on the deformation mode. At the same time it is known that certain deformation modes are more prone to hydrogen-induced embrittlement than others. The present study therefore aims at understanding the evolution of the defect structure during deformation processes that are particularly critical for hydrogen embrittlement, namely cup drawing. We separated the cup drawing process into 3 independent deformation processes, namely plane strain compression, bending and unbending. The microstructures are compared to those obtained after similar strain in a simple tensile test and to the microstructure of each foregoing step. After each step the microstructure was investigated using ECCI (electron channeling contrast imaging) and EBSD-based orientation microscopy in the SEM. The ECCI technique allows observation of the finest twin and dislocation structures while EBSD yields information on the distribution of texture and matrix grain size. While TEM observations are not very helpful at high strain, the ECCI technique appears to be an appropriate alternative, since the information is obtained only from a thin surface layer. In TEM, in contrast, the relatively large sample thickness (~100 nm) leads to overlap and therefore invisibility of many of the important deformation features. An important change of deformation mechanisms proceeds in the bending process: before bending the dominating process is twinning and stacking fault (SF) creation is only a side process; in contrast, during bending, twinning becomes the less dominant process while all areas in between the twins show massive stacking fault formation.
5:45 PM - O5.10
Microstructural Modeling of Failure Modes in Martenistic Steel Alloys.
P. Shantraj 1 , T. Hatem 1 , M. Zikry 1 Show Abstract
1 Mechanical engineering, North Carolina State University, Raleigh, North Carolina, United States
The major objective of this research is to develop a unified physically-based representation of how the microstructure in martenistic steels, and how it affects the initiation and evolution of failure modes at different physical scales that occur due to a myriad of factors, such as texture, grain size and shape, grain heterogeneous microstructures, and grain boundary misorientations and distributions. The microstructurally-based formulation for inelastic deformation is based on coupling a multiple-slip crystal plasticity formulation that accounts for variant distributions, orientations, and morphologies. This dislocation density based multiple-slip crystal plasticity formulation is then coupled to specialized finite-element methods to predict the scale-dependent microstructural behavior, the evolving heterogeneous microstructure, and failure phenomena, such as crack propagation, shear-strain localization, and void coalescence. This methodology is then used to predict and understand intergranular and transgranular failure and behavior for different failure scenarios.
Richard Dronskowski RWTH Aachen University
Patrice E. A. Turchi Lawrence Livermore National Laboratory
Yoshitaka Adachi National Institute for Materials Science
Dierk Raabe Max-Planck-Institut fuer Eisenforschung
Thursday AM, December 02, 2010
Room 204 (Hynes)
9:00 AM - **O6.1
Integrated Computational Materials Design: Flying Quantum Steel.
Greg Olson 1 Show Abstract
1 MSE, Northwestern University, Evanston, Illinois, United States
The numerical implementation of established materials science principles in the form of purposeful engineering tools has brought a new level of integration of the science and engineering of materials. Parametric materials design integrating materials science, applied mechanics and quantum physics within a systems engineering framework has brought a first generation of designer "cyberalloys" that have now entered successful commercial applications, and a new enterprise of commercial materials design services has steadily grown over the past decade. The success of materials design established a basis for the DARPA-AIM initiative which broadened computational materials engineering to address acceleration of the full materials development and qualification cycle. A new level of science-based modeling accuracy has now been achieved under the ONR/DARPA "D3D" Digital Structure consortium using a suite of advanced 3D tomographic characterization tools to calibrate and validate a set of high fidelity explicit 3D microstructural simulation tools spanning the hierarchy of microstructural scales. Surface thermodynamic databases predicted directly from DFT quantum mechanical calculations employing the FLAPW method have generated novel “Quantum Steels” completely eliminating intergranular stress corrosion cracking at the highest strength levels, recently achieving accelerated flight qualification for aircraft landing gear through application of the integrated computational design + AIM methodology.
9:30 AM - **O6.2
Chemistry and Mechanics of Steel Erosion from First Principles Simulations.
Emily Carter 1 Show Abstract
1 Mechanical and Aerospace Engineering and Applied and Computational Mathematics, Princeton University, Princeton, New Jersey, United States
Experimentally characterizing and theoretically modeling the complex chemistry and mechanics involved in steel corrosion-erosion is highly nontrivial. Existing engineering models should benefit from first principles quantum mechanical predictions of chemical energetics and kinetics and mechanical properties such as tensile and shear strength, since such calculations are non-empirical and therefore should be predictive. By necessity, one must resort to approximate quantum mechanics techniques; here we employ periodic Kohn-Sham density functional theory (DFT) and an ab initio version of DFT+U theory we developed for cases where standard DFT fails (e.g., for first row transition metal oxides). We apply these techniques to explore a subset of chemical degradation reactions involved in steel erosion. We focus on the known chemical contaminants, CO, O2, and H2S, which are thought to lead to carburization, oxidation, sulfidation, and hydrogen embrittlement of steel. We first examine the adsorption and dissociation of CO and H2S on iron surfaces (as a model for steel). We then evaluate the incorporation of carbon and hydrogen into steel. Then we consider whether any sort of pretreatment, such as alloying the surface of steel, might inhibit these damaging surface reactions. We then characterize the mechanical response of iron and chromium oxides. Lastly, we suggest a possible multilayer coating design for steel that could protect simultaneously against thermal shock and deleterious chemistry.
10:00 AM - O6.3
Helium Retention in Oxide Dispersion Strengthened Steels from First-principles.
Paul Erhart 1 , Jaime Marian 1 , Mike Fluss 1 Show Abstract
1 Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, California, United States
Oxide dispersion strengthened (ODS) steels have been proposed for structural components in next generation nuclear reactors in part thanks to their larger tolerance to He induced swelling than their oxide-free counterparts. Here, we explore this behavior by means of quantum-mechanical calculations. To this end, we have studied the properties of He-related point defects in several characteristic oxides within density functional theory using both conventional and hybrid exchange-correlation functionals. We find that interstitial He has very low formation energies compared to the surrounding iron matrix. Helium does not bind to intrinsic vacancies although depending on the processing conditions of the oxide the occupation of intrinsic vacancies by He can make a substantial contribution. We discuss the effect of the metal-oxide interface on the thermodynamic properties of the oxide particles and their ability to store helium. Our results indicate that there is a strong thermodynamic driving force for storing He inside the oxide particles that should thus serve as volumetric sinks. It therefore appears that the observed dependence of He retention on particle-matrix interface area has kinetic origins.
10:15 AM - **O6.4
CALPHAD Approach of Fe-Mn Based Alloys—phase Equilibria and Phase Transformations.
Kiyohito Ishida 1 Show Abstract
1 Materials Science, Tohoku University, Sendai Japan
Fe-Mn based alloys have received much attention as TWIP or TRIP type high strength steels with low density. We have developed a thermodynamic database on the Fe-Mn based alloys including the elements of C, Al, Si and Cr by the CALPHAD approach. In this paper, the phase equilibria in some Fe-Mn based alloys and the design of alloys with high strength and low density using the database will be shown. Furthermore, the unique martensitic transformation from the bcc parent ferrite phase to the fcc or fct austenite phase is shown in the Fe-Mn-Al and Fe-Mn-Ga systems, this transformation being due to the large temperature dependence of the partial molar free energy change of Mn between ferrite and austenite by the magnetic effect. The characteristic magnetic properties of both systems are also shown.
10:45 AM - O6.5
A Study of the Elastic Properties of Fe-Mn-C by Ab-initio Calculations and Nanoindentation.
Stephanie Reeh 1 , Denis Music 1 , Thomas Gebhardt 1 , Jochen M. Schneider 1 Show Abstract
1 Materials Chemistry, RWTH Aachen University, Aachen Germany
For a tailored design of new steels a calculation based development is indispensable. In this work ab initio calculations as theoretical method for determining the elastic properties for high manganese steels are chosen. Steels with manganese contents of 15 to 30 wt.-% exhibit a high ductility and an extraordinary strengthening behavior during plastic deformation. As major deformation mechanisms the TRIP and TWIP effects occur dependent on the stacking fault energy (SFE). The SFE is basically influenced by the chemical composition. The intention of this work is to contribute towards understanding the correlation between chemical composition, structure and elastic properties of Fe-Mn steels.The elastic properties of the Fe-Mn-C-system have not been described systematically before neither through ab initio calculations nor experimentally. The theoretical data will be compared with experimental results. Therefore Fe-Mn-C thin films are synthesized by combinatorial magnetron sputtering in an UHV chamber. These films exhibit a graded chemical composition hence, the characterization of the mechanical properties is carried out along the chemical composition gradient. The elastic properties of these films are probed by nanoindentation.The thin film data is compared with characterization of bulk samples of Fe-Mn-C through nanoindentation for providing a comparison between these different types of samples.
11:30 AM - **O6.6
Ab-initio Calculations for the Understanding of Phase Transitions in Steel.
Philippe Maugis 1 , David Tingaud 2 Show Abstract
1 IM2NP, Aix-Marseille University, Marseille France, 2 LPMTM, University Paris 13, Villetaneuse France
Ab initio calculations, in conjunction with atomic-scale characterisation, are nowadays used in the steel-making industry not only for the understanding of mechanisms of phase transformation, but also as a guide to product development and process improvement. I will present two examples of the use of the Density Functional Theory (DFT), coupled to higher-scale modelling techniques and characterisation techniques, and applied to phase transformations in laboratory carbon steel material.Example 1. The development of high aluminium-alloyed steels suffers from the lack of data on the stability of the various phases involved (solid solutions, intermetallics and carbides). Ab initio calculations have provided thermodynamic data on the following topics:(i) Stability of the non-stoichiometric ternary Fe3AlCx carbide, via point defect calculations and statistics of independent defects.(ii) Stability of various stoichiometric binary compounds as a function of temperature (phonon calculations).(iii) Formation energies of various disordered phases via the Special Quasi-random Structures (SQS) technique.These data were used for the construction of the iron-rich part of the Fe-Al-C phase diagram in the framework of the CALPHAD formalism. Example 2. As confirmed by 3D Atom Probe, ionic images and High Resolution TEM, niobium precipitation in the ferrite of Fe-Nb-C-N microalloyed carbon steel starts with the formation of Guinier-Preston zones (GPZ). To understand the mechanisms and define the influence of carbon and nitrogen, various types of DFT calculations have been performed:(i) Stability of clusters of point defects in ferrite (antisites, interstitials, vacancies)(ii) Structural optimisation of GPZ’s of various compositions(iii) Ab initio calculation of the elastic response of niobium carbides and nitrides to coherency stains. From these results, the stability of nitrogen-rich GP zones appears to stem from a structural adaptation to coherency strains in the form of a prismatic plane of the hexagonal WC-type.
12:00 PM - O6.7
Thermodynamics of High Manganese TRIP/TWIP Steels.
Dejan Djurovic 1 , Bengt Hallstedt 1 Show Abstract
1 Materials Chemistry, RWTH Aachen, Aachen Germany
The high manganese TRIP (''transformation induced plasticity'') and TWIP (''twinning induced plasticity'') steels exhibit excellent tensile strength-ductility property combination. Since the ternary Fe-Mn-C system forms the basis for these steels, an accurate thermodynamic description for the system is essential as a foundation for further modeling-based approaches. With such a description the calculation of phase equilibria and different thermodynamic properties is possible.The objective of this work is to build a complete thermodynamic description of the phases existing in the ternary Fe–Mn–C system by means of the CALPHAD method. The present evaluation of the Fe-Mn-C system is based on previous evaluations of the Fe-Mn system by Huang , the Fe-C system by Gustafson  and the Mn–C system by the present authors . In the Fe-Mn system, the description of the hcp phase was slightly modified to better reproduce data on the fcc/hcp martensitic transformation and in the Fe-C the description of Fe3C was modified by taking in account experimental data on heat capacity. Based on current ab initio data the stability of the metastable carbides Fe5C2 and Fe7C3 were changed considerably compared to previous evaluations .The relevant literature information is critically assessed and the inconsistencies are revealed. A self-consistent set of Gibbs energy functions describing the phases in the Fe-Mn-C system is obtained by least-squares fit to the selected experimental data. The backward compatibility of the refined parameters with the preferred datasets is demonstrated by calculation of variousphase diagrams and thermodynamic properties, which are compared with literature data and the results of previous assessments . W. Huang, Calphad, 13 (1989) 243-52 P. Gustafson, Scand. J. Metall., 14 (1985) 259-67 D. Djurovic, B. Hallstedt, J. von Appen and R. Dronskowski, Calphad W. Huang, Metall. Trans. A, 21A (1990) 2115-23
12:15 PM - O6.8
Solubility Products for Precipitate Phases in Steels from First-principles Calculations.
Tetyana Klymko 1 2 , Chaitanya Ande 1 2 , Marcel Sluiter 2 Show Abstract
1 , Materials Innovation Institute, Delft Netherlands, 2 Department of Materials Science and Engineering, Delft University of Technology, Delft Netherlands
Microstructure and mechanical properties of steel such as high temperature strength, ductility and creep resistance are to a great extent defined by various microalloying elements and precipitate phases. Alloying elements that remain in solid solution are responsible for the solute drag effect and solid solution strengthening while the phases that precipitate out of the solid solution give rise to precipitate strengthening. Hence, it is important to know the solubility limits of the alloying elements with respect to various precipitate phases. This is determined by the solubility products of the precipitate phases in the solid solution both for the austenitic and ferritic phases of steel. Much experimental and theoretical research has been performed on this subject. Among theoretical studies, first principles, or ab initio, calculations are of growing interest. Most of the time they are used for computing formation enthalpies, diffusion or elastic properties, however are not used much yet for finite temperature solubility products calculations involving multiple alloying elements simultaneously. The work presented gives an insight into using formation enthalpies determined from ab initio calculations for computing solubility products in steels. The role of enthalpy and entropy contributions to the solubility product is discussed. As an illustration of the method, we present solubility products for observed stoichiometric precipitate phases in ferrite. The results of first-principles calculations are compared with experimental data where available.
12:30 PM - **O6.9
CVM-based First-principles Calculations for Fe-based Alloys.
Tetsuo Mohri 1 Show Abstract
1 Division of Materials Science and Engineering, Hokkaido University, Sapporo Japan
Cluster Variation Method (CVM) has been regarded as one of the most reliable theoretical tools to incorporate wide range of atomic correlations into a free energy formula. The accuracy of the calculated thermodynamic quantities including a phase diagram is, therefore, significantly improved as compared with Bragg-Williams (equivalently regular solution model)-based calculations. By combining CVM with electronic structure total energy calculations, one can perform first-principles calculations of alloy phase equilibria. The author attempted such CVM-based first-principles calculations for a wide range of alloy systems including noble metal alloys, transition-noble alloys, III-V semiconductor alloys and Fe-based alloy systems. It is, however, pointed out that the conventional CVM is not able to include local lattice distortion effects and that the resulting order-disorder transition temperatures are overestimated. In order to circumvent such inconveniences, Continuous Displacement Cluster Variation Method (CDCVM) has been developed. In the CDCVM, additional points, termed quasi lattice points, are introduced around each Bravais lattice point and an atom is allowed to be displaced to one of quasi lattice points. Then, the system is able to find a lower energy state by distributing atomic pair distance which was uniquely fixed to a single value in the conventional CVM calculations. In the preliminary calculations for a two-dimensional square lattice with Lennard-Jones potentials, it is confirmed that the order-disorder phase boundary considerably shifts downward as compared with the results of conventional CVM. CVM can be extended to two kinds of kinetics calculations. One is Path Probability Method (PPM) which is the natural extension of the CVM to time domain and is quite powerful to investigate atomistic kinetic phenomena. The other one is the combination with Phase Field Method (PFM) by employing CVM free energy as a homogeneous free energy density term in the PFM. The author’s group applied this procedure to study time evolution process of Anti Phase Boundary in Fe-Pd and Fe-Pt systems. The validity of the method, however, is subject to the careful examination in the light of coarse graining procedure. Nevertheless, CVM has a potential applicability for the systematic study covering atomistic to microstructural scales.