Advanced High-Strength Steels: Planar Defects and Interfaces
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

Tuesday 8:30 AM
February 28, 2017
Room: 17A
Location: San Diego Convention Ctr

Session Chair: Tadashi Furuhara, Tohoku University; Chad Sinclair, University of British Columbia

8:30 AM  Invited
Parameter-free Finite-temperature Computations of Stacking Fault Energies for Magnetic Materials: Fritz Körmann1; Ivan Bleskov2; Björn Alling2; Blazej Grabowski2; Biswanath Dutta2; Tilmann Hickel2; Jörg Neugebauer2; 1Delft University of Technology and Max-Planck-Institut für Eisenforschung; 2Max-Planck-Institut für Eisenforschung
    Within the last years computationally guided design strategies based on finite-temperature ab initio calculations have become a key pillar in materials design. Ab initio Gibbs energy computations have been successfully applied in the past to tackle various materials science problems related to steels. The stacking fault energy is a key quantity linking such atomistic predictions to microstructure, twin formidability and hence to durability and ductility of such materials. Theoretical challenges for predicting finite-temperature SFEs from first principles arise in particular due to the difficulties in modeling anharmonic contributions and magnetism, and their interactions with other degrees of freedom. This talk gives a brief overview on the developed methods, which allow us to include magnetic excitations and lattice vibrations on the same footing and discusses the predictive power achievable by these new approaches. Examples include SFE predictions of elemental Al, Cu, Ni, as well as FeMn steels.

8:50 AM  
Analysis of the Aging Behavior and Orientation Relationships with Respect to β-Mn Phase in Austenite-based Low-density Steel: Keunho Lee1; Seong-Jun Park2; Jun-Yun Kang2; Siwook Park1; Anthony Rollett3; Sukbin Lee4; Kyu Hwan Oh1; Heung Nam Han1; 1Seoul National University; 2Korea Institute of Materials Science; 3Carnegie Mellon University; 4Ulsan National Institute of Science and Technology (UNIST)
    Recently, many researches in steel industry have been focused on reducing the density of steel while maintaining high strength for automotive applications, leading to development of various low-density steels. Among them, austenite-based low-density steels based on the Fe-Al-Mn-C system show superior mechanical properties and weight reduction rates. After aging steps, micro/nano-scale precipitates are formed in such steel system. Especially among them, β-Mn phase causes harmful effect on mechanical properties, inducing brittle fracture. However, there is still a paucity of research on precipitation characteristics of the β-Mn. In this study, the aging behavior and orientation relationships in high Mn low-density steel are investigated. The microstructure evolutions and mechanical response during aging treatment are characterized by Vickers hardness measurement combined with microstructural observations. The misorientation-angle distribution, Rodrigues–Frank vector space, and orientation relationship stereogram (ORS) from EBSD measurements are applied for analyzing the orientation relationships for austenite/β-Mn and β-Mn/D03 interphase boundaries, respectively.

9:10 AM  
Relationship between Impact Toughness, Prior Austenite Grain Boundaries and Microstructural Morphology in Medium Mn Steel: Jeongho Han1; Alisson Kwiatkowski da Silva1; Dirk Ponge1; Dierk Raabe1; Sang-Min Lee2; Young-Kook Lee2; Sang-In Lee3; Byoungchul Hwang3; 1Max-Planck-Institut für Eisenforschung; 2Yonsei University; 3Seoul National University of Science and Technology
    Here we investigate the relation between microstructural morphology and impact toughness of intercritically Fe-7Mn-0.1C steel. The hot-rolled and annealed (HRA) specimen exhibited dual-phase lath-shaped microstructure due to absence of recrystallization of martensite matrix before the reverse transformation. The cold-rolled and annealed (CRA) specimen showed a dual-phase globular-shaped microstructure, because recrystallization and reversion were actively occurred during annealing. HRA specimen showed a poor low temperature toughness due to rapid intergranular crack propagation along the coarse prior austenite grain boundaries. In contrast, CRA specimen exhibited an improved low temperature toughness, because intergranular crack propagated along the fine ferrite and martensite grain boundaries.

9:30 AM  
Experimental Determination of Magnitude of Shear of Stacking Faults, Twins and Alpha’-martensite in TRIP/TWIP Steels: Anja Weidner1; Horst Biermann1; 1TU Bergakademie Freiberg
    Modern CrMnNi TRIP/TWIP steels exhibit deformation-induced martensitic phase transformation or mechanical twinning depending on the austenite stability and the stacking fault energy. Although the magnitude of shear for epsilon-martensite and twins are known from theoretical considerations, up to now the experimental determination of the individual local shear strain values associated with different mechanisms like alpha’-nuclei or mechanical twins is still missing. An excellent method for the experimental determination of individual strain levels is the performance of quasi in situ deformation experiments in a scanning electron microscope complemented by the application of high-resolution digital image correlation. The results of image correlation show significant difference in the formation of local strain fields depending on the primary deformation mechanism. Moreover, the magnitude of shear for individual constituents of the microstrucutre like deformation bands, alpha’-martensite or mechanical twins were calculated. Furthermore, the magnitude of shear of alpha’-martensite depends significantly on their crystallographic orientation.

9:50 AM  
Effect of Interfacial Mn Partitioning on Carbon Partitioning and Interface Migration during Quenching and Partitioning: Zongbiao Dai1; Jianguo He1; Zhigang Yang1; Chi Zhang1; Hao Chen1; 1Tsinghua University
    Fundamentals of carbon partitioning and austenite-martensite interface migration during the Quenching and Partitioning (Q&P) process is essential for tailoring the microstructure of Q&P steels. A physical model, in which the interfacial partitioning of substitutional alloying element is taken into account, has been proposed to investigate the effect of Mn on the kinetics of austenite-martensite interface migration and carbon partitioning during the Q&P process in Fe-C-Mn-Si alloys. It is predicted that interfacial partitioning of Mn plays a significant role in carbon partitioning and interface migration. Furthermore, the physical Q&P model has also been integrated with a 2D phase field model (PFM) to simulate the microstructure evolution during the Q&P process. The PFM simulations show carbon concentration in individual austenite and interface migration are strongly dependent on austenite and martensite morphologies. The effect of size distribution of martensite and austenite on the Q&P process has also been discussed.

10:10 AM Break

10:30 AM  
Molecular Dynamics Simulations of the Interaction of Helium Clusters with Grain Boundaries and Dislocations bcc Iron: Tegar Wicaksono1; Yu Yue2; Matthias Militzer1; 1The University of British Columbia; 2Tsinghua University
    Solute atoms and precipitates significantly affect dislocation glide and grain boundary migration in alloys. Models for solute solution strengthening and solute drag as well as precipitation strengthening and particle pinning are well established. The intermediate case of solute clusters has, however, not yet been fully explored despite its technologically importance, e.g. embrittlement of irradiated steel reactors due to helium (He) clusters. In this work, the interplay between He segregation, He cluster formation and dislocation glide as well as curvature-driven grain boundary migration in bcc iron (Fe) has been investigated using molecular dynamics simulations. The restraining force on grain boundary migration increases with cluster size via a pinning mechanism. Similarly, the critical resolved shear stress for dislocation glide increases with the size of the He clusters. Phenomenological models for cluster pinning and strengthening are proposed based on these simulations results.

10:50 AM  
Interface Dominated Process in Modern Steels: Goune Mohamed1; Fréderic Danoix2; Xavier Sauvage2; Didier Huin3; Lionel Germain4; 1ICMCB-Bordeaux1; 2GPM - Université de Rouen; 3ArcelorMittal; 4LEM3-Université de Lorraine
    The development of modern steels depends on the increased understanding of the interactions between alloying elements such as carbon (C) and manganese (Mn) and the α-ferrite/γ-austenite transformation interface. Their analysis is often difficult for the reason that they occur at the nanoscale and in the vicinity of an interface of few atomic plans thick. However, the experimental and theoretical techniques had become much more powerful over the last decade, that is seems fruitful to return to this topic of interest with fresh eyes. In the present work, from a combined selective approach by 3D Atom Probe Tomography, TEM and EBSD, we propose to analyse the behaviour of both C and Mn at the vicinity of the migrating α-ferrite/γ-austenite as a function of its physical nature. Some effects of co-segregation and co-precipitation of both C and Mn at the α/γ interface are evidenced and discussed.

11:10 AM  
New Insights in the Atomic Interface Structure of Kappa Carbides in High-Mn Steels: Christian Liebscher1; Marta Lipinska-Chwalek2; Menji Yao1; Michael Herbig1; Baptiste Gault1; Joachim Mayer2; Dierk Raabe1; Christina Scheu1; 1Max-Planck-Max-Planck-Institut fuer Eisenforschung GmbH; 2Ernst Ruska-Centrum and RWTH Aachen
    High-Mn steels show excellent strength, which is attributed to a regularly arranged network of nano-sized (Fe,Mn)3AlC kappa carbides embedded in an austenitic matrix. In the present work the kappa/austenite interface structure was studied at the atomic scale by aberration-corrected scanning transmission electron microscopy (STEM). The results, obtained on samples from a Fe-29.8Mn-7.7Al-1.3C (wt%) alloy annealed at 600°C for 24h, show that the interface between the kappa carbides and the austenitic matrix is fully coherent and devoid of misfit dislocations. However, the STEM images indicate that the matrix between two closely spaced carbides is heavily strained. Image analysis reveals chemical fluctuations at the interface in the order of few atomic distances. The intensities in the Z-contrast images reveal that the carbide is ordered and a part of the Mn atoms are located at Al positions forming anti-sites as predicted by theory [1]. [1] Yao et al. Acta Materialia 106 (2016)

11:30 AM  
An Interface Controlled Transformation Model Predicting the Kinetics for Isothermal Bainite Formation in Medium Mn Steels: Hussein Farahani1; Wei Xu2; Sybrand van der Zwaag1; 1Delft University of Technology; 2Northeastern University, China
    Following the previous TU Delft work on the effect of partitioning of alloying elements on the interface velocity for the austenite/ferrite and austenite/bainite transformations, a refined kinetic model for isothermal bainite transformation is presented. In the new model, the rate of heterogeneous nucleation of bainitic plates is calculated explicitly considering the effect of thermal treatment and alloy composition. The Gibbs Energy Balance (GEB) approach is used to incorporate the effect of interstitial (in particular C) and substituional (in particular Mn) alloying elements on the velocity of migrating austenite/ferrite interface. The model is coupled with numerical calculation of the distributions of alloying elements along the parent austenite phase at each time step. The new model allows prediction of the effect of alloying element partitioning on the morphology and the aspect ratio of bainitic plates. The results are compared to previous simulations and experiments and new metallurgical insights are presented.