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David Rafaja - One of the best experts on this subject based on the ideXlab platform.
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Assessment of the thermodynamic dimension of the stacking fault energy
Philosophical Magazine, 2014Co-Authors: D. Geissler, Jens Freudenberger, Alexander Kauffmann, Stefan Martin, David RafajaAbstract:Especially with respect to high Mn and other Austenitic Transformation and/or TWinning Induced Plasticity (TRIP/TWIP) steels, it is a current trend to model the stacking fault energy of a stacking fault that is formed by plastic deformation with an equilibrium thermodynamic formalism as proposed by Olson and Cohen in 1976. In the present paper, this formalism is critically discussed and its ambiguity is stressed. Suggestions are made, how the stacking fault energy and its relation to the formation of hexagonal ϵ-martensite might be treated appropriately. It is further emphasized that a thermodynamic treatment of deformation-induced stacking fault phenomena always faces some ambiguity. However, an alternative thermodynamic approach to stacking faults, twinning and the formation of ϵ-martensite in Austenitic steels might rationalize the specific stacking fault arrangements encountered during deformation of TRIP/TWIP alloys.
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Stacking fault model of ∊-martensite and its DIFFaX implementation
Journal of Applied Crystallography, 2011Co-Authors: Stefan Martin, C. Ullrich, Daniel Šimek, Ulrich Martin, David RafajaAbstract:Plastic deformation of highly alloyed Austenitic Transformation-induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of ∊-martensite within the original austenite. The ∊-martensite is often described as a phase having a hexagonal close-packed crystal structure. In this contribution, an alternative structure model is presented that describes ∊-martensite embedded in the Austenitic matrix via clustering of stacking faults in austenite. The applicability of the model was tested on experimental X-ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in the DIFFaX routine; the faulted austenite and ∊-martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X-ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction.
Herbert M. Urbassek - One of the best experts on this subject based on the ideXlab platform.
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Dislocations Help Initiate the α–γ Phase Transformation in Iron—An Atomistic Study
Metals, 2019Co-Authors: Jerome Meiser, Herbert M. UrbassekAbstract:Using molecular dynamics simulation, we studied the influence of pre-existing dislocations on the Austenitic and the martensitic phase Transformations in pure iron. The simulations were performed in a thin-film geometry with (100) surfaces. We found that dislocations alleviate the Transformation by lowering the Austenitic Transformation temperature and increasing the martensitic Transformation temperature. In all cases, the new phase nucleates at the dislocations. The orientation relationships governing the nucleation process are dominated by the Burgers, Kurdjumov–Sachs, and Nishiyama–Wassermann pathways. However, upon growth and coalescence of the transformed material, the final microstructure consists of only few twinned variants separated by twin boundaries; this simple structure is dictated by the free surfaces which tend to form conserved planes under the Transformation. After Transformation, the material also contains abundant dislocations.
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Influence of the Crystal Surface on the Austenitic and Martensitic Phase Transition in Pure Iron
Crystals, 2018Co-Authors: Jerome Meiser, Herbert M. UrbassekAbstract:Using classical molecular dynamics simulations, we studied the influence that free surfaces exert on the Austenitic and martensitic phase transition in iron. For several single-indexed surfaces—such as ( 100 ) bcc and ( 110 ) bcc as well as ( 100 ) fcc and ( 110 ) fcc surfaces—appropriate pathways exist that allow for the Transformation of the surface structure. These are the Bain, Mao, Pitsch, and Kurdjumov–Sachs pathways, respectively. Tilted surfaces follow the pathway of the neighboring single-indexed plane. The Austenitic Transformation temperature follows the dependence of the specific surface energy of the native bcc phase; here, the new phase nucleates at the surface. In contrast, the martensitic Transformation temperature steadily decreases when tilting the surface from the (100) fcc to the (110) fcc orientation. This dependence is caused by the strong out-of-plane deformation that (110) fcc facets experience under the Transformation; here, the new phase also nucleates in the bulk rather than at the surface.
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Ferrite-to-Austenite and Austenite-to-Martensite Phase Transformations in the Vicinity of a Cementite Particle: A Molecular Dynamics Approach
MDPI AG, 2018Co-Authors: Jerome Meiser, Herbert M. UrbassekAbstract:We used classical molecular dynamics simulation to study the ferrite–austenite phase Transformation of iron in the vicinity of a phase boundary to cementite. When heating a ferrite–cementite bicrystal, we found that the Austenitic Transformation starts to nucleate at the phase boundary. Due to the variants nucleated, an extended poly-crystalline microstructure is established in the transformed phase. When cooling a high-temperature austenite–cementite bicrystal, the martensitic Transformation is induced; the new phase again nucleates at the phase boundary obeying the Kurdjumov–Sachs orientation relations, resulting in a twinned microstructure
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The α↔γ Transformation of an Fe1−xCrx alloy: A molecular-dynamics approach
International Journal of Modern Physics C, 2016Co-Authors: Emilia Sak-saracino, Herbert M. UrbassekAbstract:Using molecular dynamics (MD) simulation, we study the temperature-induced α↔γ phase Transformation of an Fe0.9Cr0.1 alloy. We find that the Austenitic transition temperature is increased with respect to that of an Fe0.9Ni0.1 alloy containing the same concentration of impurity atoms. During the Austenitic Transformation, heterogeneous nucleation of close-packed (cp) nuclei leads to a polycrystalline structure. The microstructure formed closely resembles that found in pure Fe and in FeNi alloys.
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Effect of uni- and biaxial strain on phase Transformations in Fe thin films
International Journal of Computational Materials Science and Engineering, 2016Co-Authors: Emilia Sak-saracino, Herbert M. UrbassekAbstract:Using molecular-dynamics simulation, we study the phase Transformations in Fe thin films induced by uni- and biaxial strain. Both the Austenitic Transformation of a body-centered cubic (bcc) film at the equilibrium temperature of the face-centered cubic (fcc)–bcc Transformation and the martensitic Transformation of an undercooled fcc film are studied. We demonstrate that different strain states (uni- or biaxial) induce different nucleation kinetics of the new phase and hence different microstructures evolve. For the case of the Austenitic Transformation, the direction of the applied strain selects the orientation of the nucleated grains of the new phase; the application of biaxial strain leads to a symmetric twinned structure. For the martensitic Transformation, the influence of the strain state is even more pronounced, in that it can either inhibit the Transformation, induce the homogeneous nucleation of a fine-dispersed array of the new phase resulting in a single-crystalline final state, or lead to the more conventional mechanism of heterogeneous nucleation of grains at the free surfaces, which grow and result in a poly-crystalline microstructure of the transformed material.
Dierk Raabe - One of the best experts on this subject based on the ideXlab platform.
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new insights into the austenitization process of low alloyed hypereutectoid steels nucleation analysis of strain induced austenite formation
Acta Materialia, 2014Co-Authors: Han Zhang, Konda Gokuldoss Pradeep, Suvendu Mandal, Dirk Ponge, Dierk RaabeAbstract:Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at 973 K (60 K below Ae1, the equilibrium austenite–pearlite Transformation temperature). Scanning electron microscopy, electron backscatter diffraction and atom probe tomography results reveal the mechanism of Austenitic Transformation below Ae1. The initial fine-grained microstructure, in combination with the warm deformation process, determines the occurrence of strain-induced austenite formation below Ae1. The initial fine-grained microstructure essentially contains a higher dislocation density to facilitate the formation of Cottrell atmospheres and a larger area fraction of ferrite/carbide interfaces which serve as austenite nucleation sites. The warm deformation promotes the Ostwald ripening process and the increase in dislocation density, and hence promotes the accumulation of local high carbon concentrations in the form of Cottrell atmospheres to reach a sufficiently high thermodynamic driving force for austenite nucleation. The critical carbon concentration required for the nucleation of austenite was calculated using classical nucleation theory, which correlated well with the experimental observations.
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Interaction between recrystallization and phase Transformation during intercritical annealing in a cold-rolled dual-phase steel: A cellular automaton model
Acta Materialia, 2013Co-Authors: Chengwu Zheng, Dierk RaabeAbstract:The concurrent ferrite recrystallization and Austenitic Transformation during intercritical annealing of cold-rolled DP steels is investigated by cellular automaton (CA) modeling. The simulations provide insight into the microstructural phenomena that result from the interaction of primary recrystallization and phase Transformation. We find that the interaction between ferrite recrystallization and austenite formation affects not only the Transformation kinetics but also the morphology and spatial distribution of the austenite. From this we can interpret experimental data of the observed temperature-dependent hardness and its dependence on the two metallurgical processes. The influence of the initial heating rate on subsequent isothermal Transformation kinetics and the microstructure evolution is also obtained by the model.
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atomic scale effects of alloying partitioning solute drag and austempering on the mechanical properties of high carbon bainitic Austenitic trip steels
Acta Materialia, 2012Co-Authors: Jaebok Seol, Puckpa Choi, Yungrok Im, Dierk Raabe, Changyung ParkAbstract:Abstract Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic–Austenitic Transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite Transformation.
E. Garcia-sanchez - One of the best experts on this subject based on the ideXlab platform.
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Thermal stability and phase Transformations of a FV535 steel
Journal of Thermal Analysis and Calorimetry, 2016Co-Authors: L. Guerra-fuentes, R. Deaquino Lara, M. A. L. Hernandez-rodriguez, A. Salinas-rodriguez, E. Garcia-sanchezAbstract:Thermal stability and phase Transformation temperatures in Firth-Vickers 535 (12Cr–Mo–VNbWCo) steel have been studied by differential scanning calorimetry and dilatometry. The ( M _s), ( Ac _1) and ( Ac _3) temperatures of this steel were measured using different heating and cooling rates; the results showed good agreement between both techniques, and in these studies was found that Austenitic Transformation is strongly dependent on the heating rate during continuous heating. The α – γ Transformation enthalpy for this steel was about 17 J g^−1. Microstructure was analyzed by optical and scanning electron microscope, and in the results can be observed that this steel has a martensitic structure with Mo- and Nb-rich carbides.
Stefan Martin - One of the best experts on this subject based on the ideXlab platform.
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Assessment of the thermodynamic dimension of the stacking fault energy
Philosophical Magazine, 2014Co-Authors: D. Geissler, Jens Freudenberger, Alexander Kauffmann, Stefan Martin, David RafajaAbstract:Especially with respect to high Mn and other Austenitic Transformation and/or TWinning Induced Plasticity (TRIP/TWIP) steels, it is a current trend to model the stacking fault energy of a stacking fault that is formed by plastic deformation with an equilibrium thermodynamic formalism as proposed by Olson and Cohen in 1976. In the present paper, this formalism is critically discussed and its ambiguity is stressed. Suggestions are made, how the stacking fault energy and its relation to the formation of hexagonal ϵ-martensite might be treated appropriately. It is further emphasized that a thermodynamic treatment of deformation-induced stacking fault phenomena always faces some ambiguity. However, an alternative thermodynamic approach to stacking faults, twinning and the formation of ϵ-martensite in Austenitic steels might rationalize the specific stacking fault arrangements encountered during deformation of TRIP/TWIP alloys.
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Stacking fault model of ∊-martensite and its DIFFaX implementation
Journal of Applied Crystallography, 2011Co-Authors: Stefan Martin, C. Ullrich, Daniel Šimek, Ulrich Martin, David RafajaAbstract:Plastic deformation of highly alloyed Austenitic Transformation-induced plasticity (TRIP) steels with low stacking fault energy leads typically to the formation of ∊-martensite within the original austenite. The ∊-martensite is often described as a phase having a hexagonal close-packed crystal structure. In this contribution, an alternative structure model is presented that describes ∊-martensite embedded in the Austenitic matrix via clustering of stacking faults in austenite. The applicability of the model was tested on experimental X-ray diffraction data measured on a CrMnNi TRIP steel after 15% compression. The model of clustered stacking faults was implemented in the DIFFaX routine; the faulted austenite and ∊-martensite were represented by different stacking fault arrangements. The probabilities of the respective stacking fault arrangements were obtained from fitting the simulated X-ray diffraction patterns to the experimental data. The reliability of the model was proven by scanning and transmission electron microscopy. For visualization of the clusters of stacking faults, the scanning electron microscopy employed electron channelling contrast imaging and electron backscatter diffraction.
