Advanced High-Strength Steels: Fundamentals of Steel Design
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Tilmann Hickel, Max-Planck-Institut fuer Eisenforschung GmbH; Wolfgang Bleck, RWTH Aachen; Amy Clarke, Colorado School of Mines ; Young-Kook Lee, Yonsei University; Matthias Militzer, The University of British Columbia

Monday 8:30 AM
February 27, 2017
Room: 17A
Location: San Diego Convention Ctr

Session Chair: Tilmann Hickel, Max-Planck-Institut für Eisenforschung; Dong Woo Suh, Pohang University of Science and Technology


8:30 AM  Invited
Ab Initio Guided Design of High Strength Steels: Where Do We Stand?: Joerg Neugebauer1; Gerard Leyson1; Xie Zhang1; Fritz Koermann1; Blazej Grabowski1; Tilmann Hickel1; 1Max-Planck-Institut fuer Eisenforschung
    Modern high strength steels exhibit a structural complexity down to the atomic scale, employ a wide variety of active deformation mechanisms such as twinning or phase transformations and require a processing that provides control down to the nanoscale. This complexity makes it often impossible to identify and describe the relevant mechanisms and structures solely on an experimental basis. Ab initio simulation techniques based on quantum mechanical approaches are ideally suited to provide the missing data on the atomistic and nanoscale but face the challenge that a brute force description of the complexity is often computationally too expensive and thus not feasible. The talk will discuss how new formulations and approaches allow to address this complexity but will also elucidate on questions where the present approaches are insufficient.

9:00 AM  
Paving the Bridge from Ab Initio to Atomistic Modeling of Advanced High-strength Steels: Christopher Barrett1; Ricolindo Carino1; Imran Aslam1; Robert Moser2; Haitham El Kadiri1; 1Mississippi State University; 2US Army Corps of Engineers - ERDC
    Accurate modeling of advanced high-strength steels requires intricate atomistic-scale understanding of effects such as twinning, phase transitions, and dislocation interactions. However, atomistic modeling of advanced steel alloys requires incorporating simultaneously the effects of many elements. Current potentials lag far behind providing the required detail and accuracy for these many-element systems. Producing many-element potentials which predict alloy properties is a complex task requiring fitting a large database of properties to experimental or first principles data. To enable this, we developed an automated computation and calibration software. The software computes first principles data and generates new potentials by calibrating potential parameters to that data. The software enables faster potential development both by automating the computation and calibration process, and by enabling users with little background experience to perform it. The efficacy of this approach is demonstrated by illustrating the development of a new FeCSiMn potential using the modified embedded atom method.

9:20 AM  
Interface Guided Design of High-strength Steels: A Dream Coming True?: Ivan Gutierrez-Urrutia1; 1National Institute for Materials Science
    The advent of novel dislocation-based strain-hardening mechanisms in austenitic high-Mn steels (TWIP, MBIP and SBIP) has revived the need of the quantitative investigation of homophase interfaces. The formation and propagation of interfaces such as twins, microbands and shear bands is determined by several microstructural parameters such as local stress state, solute content, grain size and crystallographic orientation to mention a few. Here I present a summary of the current status of the alloy design approaches based on the microstructural control of homophase interfaces in austenitic high-Mn steels. In particular, the formation and propagation mechanisms of such interfaces will be addressed. The focus will be given on the need to attain an interface guided design approach of next generation high-strength steels.

9:40 AM  
Is Twinning Important for Twinning-induced Plasticity Steels?: M.X. Huang1; 1The University of Hong Kong
    Twinning-induced plasticity (TWIP) steels are well-known for their excellent combination of high strength and exceptional ductility. The excellent mechanical properties of TWIP steels are commonly attributed to deformation twins.This work intends to evaluate the role of twinning in the mechanical properties of TWIP steels. Synchrotron X-ray diffraction experiments were carried out to measure the dislocation density that is used to evaluate the dislocation strengthening. It is found that the dislocation density in TWIP steels is very high and the corresponding dislocation strengthening accounts for ~90% of the flow stress increment after yielding. In comparison, deformation twins contribute to only 118 MPa at a true strain of 0.4, indicating that the hardening contribution of twins in terms of kinematic hardening to the flow stress should be insignificant. In other words, deformation twins only have minor effect on the flow stress, in contradiction to the current understandings in literature.

10:00 AM  
New Law to Describe Plastic Anisotropy in BCC Metals: Lucile Dezerald1; David Rodney2; Emmanuel Clouet3; Lisa Ventelon3; François Willaime3; 1Université de Lorraine; 2Université Lyon 1; 3CEA Saclay
    Body-Centered Cubic (BCC) metals are known to display an atypical plasticity at low temperature, namely, a marked dependence of the elastic limit on crystal orientation in clear violation of the Schmid law. These properties are controlled by ½<111> screw dislocations that display strong core effects at the atomic scale. Here, we show using DFT calculations that these dislocations glide in {110} planes only on average. The path they follow systematically deviates towards the twinning region, and the amplitude of this deviation can directly be linked to the dislocation Peierls potential, which is metal dependent. We propose a new law to account for these atomic-scale deviations that enables predicting Schmid law deviations in good agreement with both experimental and DFT measurements of Peierls stress variations with crystal orientation. This work paves the way towards a better understanding of BCC metals’ plasticity and can be used as input for larger scale models.

10:20 AM Break

10:40 AM  
Deformation-Induced Martensite: A Thermodynamic Study: Gh. Ali Nemaollahi1; Soundes Djaziri1; Yujiao Li1; Blazej Grabowski1; Christoph Kirchlechner1; Aleksander Kostka2; Shoji Goto1; Dierk Raabe1; Gerhard Dehm1; Jörg Neugebauer1; 1Max-Planck Institut für Eisenforschung; 2Lehrstuhl Werkstoffdesign Institut für Werkstoffe Fakultät für Maschinenbau Ruhr-Universität Bochum
    Cold-drawn pearlitic steel wires revealing ultra-high tensile strengths of up to 7 GPa are the world’s strongest steels. Experimental observations reveal that cementite gradually decomposes during wire drawing. The C atoms resulting from the cementite decomposition are mechanically alloyed into the ferrite phase and accommodated in trapping sites around defects, such as dislocations. Surprisingly, there is also a high oversaturation of the bulk ferrite phase and experiments indicate a transformation to a tetragonally distorted system. We have therefore developed an ab initio informed model that accurately takes into account the interaction of C with the wire-drawn strained host matrix. The effect of the applied strain is captured by introducing a renormalized C formation energy and computing it by density functional theory. Applying the model we demonstrate that the experimentally observed tetragonal distortion is due to a mechanically driven phase transformation from ferrite (bcc) to martensite (bct).

11:00 AM  
The Development and Application of a Thermodynamic Database for Low-density Steels: Reza Naraghi1; 1Thermo-Calc Software AB
    Development and production of “low-density steels” is of great interest in steel industry since it can save both material and energy for applications in automotive, cryogenic, electrical steels, etc. In order to design such a steel it is necessary to predict the phase evolution during various stages of the production. The calculation of phase diagrams (CALPHAD) method allows different phenomena to be calculated mathematically. These calculations require an internally consistent and reliable thermodynamic database. Fe-Mn-Al-C system is the core system of low density steels, with higher Mn and Al content than conventional steels. In order to provide an efficient tool for alloy/process design, the thermodynamic description of the quaternary Fe-Mn-Al-C and its subsystem in Thermo-Calc Steels/Fe-alloy database (TCFE) are being revised in the present work. The phase diagram information reported in the literature and available thermodynamic assessments are utilized in order to model Gibbs energies of the relevant phases.

11:20 AM  
Data Science Approaches for Predicting Fatigue Strength of Steels: Ankit Agrawal1; Alok Choudhary1; 1Northwestern University
    The application of data science approaches in materials science opens up new avenues for accelerated discovery and design, as also recognized by MGI. In this talk, I will describe our ongoing work employing state-of-the-art data analytics on experimental steel data from NIMS to decipher PSPP linkages in steels. Fatigue strength is one of the most important properties of steel. High cost and time for fatigue testing, and potentially disastrous consequences of fatigue failures motivates this work. Our data-driven predictive models have a high cross-validated accuracy of >98%, and have been deployed in a user-friendly online web-tool, which can make very fast predictions of fatigue strength for a given steel composition and processing parameters. Such a tool is expected to be a very useful resource for the materials science researchers and practitioners to assist in their search for new and improved quality steels. The tool is available at http://info.eecs.northwestern.edu/SteelFatigueStrengthPredictor

11:40 AM  
Three Dimensional Atom Probe and First-principles Studies on Spinodal Decomposition of Cr in a High Strength Maraging Stainless Steel: Wei Wang1; 1Institute of Metal Research
    The effect of Co addition on spinodal decomposition of Cr in a high strength maraging stainless steel was investigated by three dimensional atom probe. The concentration profile of Cr and analysis by maximum likelihood method indicated an increase of spinodal decomposition amplitude of Cr in the Co-alloyed maraging stainless steel. The first-principles calculations showed that the increased Fe-Fe ferromagnetic interaction caused by Co addition might facilitate the formation of Cr-rich clusters in bcc Fe.