The Experts below are selected from a list of 7773 Experts worldwide ranked by ideXlab platform
E. J. Mittemeijer - One of the best experts on this subject based on the ideXlab platform.
-
austenite Ferrite Transformation kinetics under uniaxial compressive stress in fe 2 96 at ni alloy
Acta Materialia, 2009Co-Authors: Ferdinand Sommer, E. J. MittemeijerAbstract:Abstract The effect of an applied constant uniaxial compressive stress on the kinetics of the austenite (γ) → Ferrite (α) massive Transformation in the substitutional Fe–2.96 at.% Ni alloy upon isochronal cooling has been studied by differential dilatometry. All imposed stress levels are below the yield stress of austenite and Ferrite in the temperature range of the Transformation. An increase in compressive stress results in a small but significant increase of the onset temperature of the γ → α Transformation and a decrease of the overall Transformation time. A phase Transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement correction, has been employed to extract the interface-migration velocity of the γ/α interface. The interface-migration velocity for the γ → α Transformation is approximately constant at fixed uniaxial compressive stress and increases with increasing applied uniaxial compressive stress. Furthermore, the value obtained for the energy corresponding with the elastic and plastic deformation associated with the accommodation of the γ/α volume misfit depends on the transformed fraction and decreases significantly as the applied uniaxial compressive stress increases. An understanding of the observed effects is obtained, recognizing the constraints imposed on the phase Transformation due to the applied stress.
-
Abnormal Austenite-Ferrite Transformation Kinetics of Ultra-Low-Nitrogen Fe-N Alloy
Metallurgical and Materials Transactions A, 2008Co-Authors: Ferdinand Sommer, E. J. MittemeijerAbstract:The kinetics of the isochronal austenite (γ) → Ferrite (α) Transformation of ultra-low-nitrogen Fe-N alloy were investigated for cooling rates in the range of 5 to 15 Kmin−1 by high-resolution dilatometry. The explored γ → α Transformation takes place in the single α-phase region, and the onset temperature of the Transformation decreases with increasing applied cooling rate. According to the variation of the Ferrite formation rate data, an abnormal Transformation mechanism, i.e., multiply peaked nature with rate maxima appearing at fixed transformed fractions, was recognized. For the later (main) part of the Transformation, a phase-Transformation model, incorporating site saturation, interface-controlled growth, and an appropriate impingement correction, has been employed to extract the migration velocity of the γ/α interface from the measured data. The thus determined interface migration velocity is approximately constant for all applied cooling rates. The first abnormal part of the Transformation was discussed in comparison to the recently observed abnormal phenomena for the γ → α Transformation in pure iron and ultra-low-carbon Fe-C alloy. On the basis of the thermodynamic and kinetic results, it is shown that autocatalytic nucleation occurs in the first part of the Transformation and that interface-controlled growth prevails for the entire Transformation.
-
abnormal austenite Ferrite Transformation behaviour of pure iron
Philosophical Magazine, 2004Co-Authors: Yongchang Liu, E. J. Mittemeijer, F SommerAbstract:The isochronal and isothermal austenite (γ) → Ferrite (α) Transformation of pure iron was measured by high-resolution dilatometry and differential thermal analysis. Both abnormal and normal Transformation kinetics were recognized for the first time in pure iron according to the variation in the Ferrite formation rate. The occurrence of the type of γ →α Transformation strongly depends on the grain size; the Transformation type changes from abnormal to normal with decreasing grain size. The abnormal Transformation process involves the occurrence of additional peaks in the Transformation rate for the first stage of the Transformation. A phase Transformation model, involving repeated nucleation (autocatalytic nucleation), interface-controlled continuous growth and incorporating correction for impingement, has been employed successfully to describe the observed kinetics of the abnormal Transformation.
-
abnormal austenite Ferrite Transformation behaviour in substitutional fe based alloys
Acta Materialia, 2003Co-Authors: Yongchang Liu, E. J. Mittemeijer, F SommerAbstract:Abstract The γ → α phase Transformation behaviours of Fe-Co and Fe-Mn alloys were systematically investigated by dilatometry and Differential Thermal Analysis (DTA). Two kinds of Transformation kinetics, called normal and abnormal, were recognized for the first time and classified according to the variation of the Ferrite formation rate. These Transformation characteristics were observed for both isothermally and isochronally conducted annealing experiments. A transition, from abnormal to normal Transformation kinetics, occurs for Fe-1.79at.%Co when successive heat treatment cycles are executed, which contrasts with Fe-2.26at.%Mn for which only normal Transformation kinetics occurs after each of all successive heat treatment cycles. A possible mechanism for the appearance of abnormal Transformation kinetics is given, which is based on the austenite grain size. Light microscopical analysis indicates a repeated nucleation of Ferrite in front of the migrating γ / α interface.
H. K. D. H. Bhadeshia - One of the best experts on this subject based on the ideXlab platform.
-
austenite Ferrite Transformation in enhanced niobium low carbon steel
Materials Science and Technology, 2015Co-Authors: P Yan, H. K. D. H. BhadeshiaAbstract:AbstractThe austenite to Ferrite Transformation characteristics of a commercial high strength line pipe steel containing 0·05 wt-% carbon and 0·095 wt-% niobium have been rigorously studied by continuous cooling experiments in the range between 960 and 1260°C. A significant delay in the austenite to allotriomorphic Ferrite Transformation has been demonstrated to occur under practically relevant thermal processing conditions. The effects of prior austenite grain size and soluble niobium have been carefully evaluated and isolated and it has been concluded that the amount of niobium in solution in the austenite is primarily responsible for the retardation. Alternative hypotheses to explain the mechanism whereby niobium exerts this effect on the hardenability of steel are discussed in detail. Soluble niobium reducing the austenite grain boundary energy is argued to be the most convincing explanation of the phenomenon and a reduction of grain boundary energy of 0·286 J m−2 per wt-% of soluble niobium content h...
-
Stress and the acicular Ferrite Transformation
Materials Science and Engineering: A, 1992Co-Authors: S S Babu, H. K. D. H. BhadeshiaAbstract:Abstract Acicular Ferrite is considered to be identical in Transformation mechanism to bainite, except that it nucleates intragranularly on inclusions, and hence develops into a morphologically different microstructure. The Transformation causes displacements which, on a macroscopic scale, are characteristic of an invariant-plane strain with a large shear component. It is therefore expected to be sensitive to an appropriate, externally applied stress. This is indeed found to be the case. Results are presented which demonstrate quantitatively that large changes in the development of the microstructure are induced when acicular Ferrite grows while the austenite is in uniaxial compression. This is in spite of the fact that the applied stress used was below the austenite yield strength. Profound changes in the distribution of acicular Ferrite plates are also observed. The results are interpreted in terms of the mechanism of Transformation.
-
acicular Ferrite Transformation in alloy steel weld metals
Journal of Materials Science, 1991Co-Authors: Jer-ren Yang, H. K. D. H. BhadeshiaAbstract:In this paper, the morphology of acicular Ferrite in alloy-steel weld metals has been investigated. The effect of the grain size of prior austenite on acicular Ferrite Transformation has also been studied. It is found that acicular Ferrite can form in reheated weld metals when the austenite grain size is relatively large. On the other hand, classical sheaf-like bainite will form at the same temperature if the austenite grain size is kept small. Further results strongly suggest that acicular Ferrite is in fact intragranular bainite rather than intragranular Widmanstatten Ferrite.
Nobuhiro Tsuji - One of the best experts on this subject based on the ideXlab platform.
-
dynamic Ferrite Transformation behaviors in 6ni 0 1c steel
JOM, 2014Co-Authors: Nokeun Park, Akinobu Shibata, Lijia Zhao, Nobuhiro TsujiAbstract:Phase Transformation from austenite to Ferrite is an important process to control the microstructures of steels. To obtain finer Ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static Ferrite Transformation have been carried out for austenite phase. This article reviews the dynamic Transformation (DT), in which Ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of Ferrite grains even above the Ae3 temperature (ortho-equilibrium Transformation temperature between austenite and Ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae3 was experimentally confirmed. Moreover, the change in Ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine Ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of Ferrite.
-
occurrence of dynamic Ferrite Transformation in low carbon steel above ae3
Scripta Materialia, 2013Co-Authors: Nokeun Park, Sunisa Khamsuk, Akinobu Shibata, Nobuhiro TsujiAbstract:Dynamic Ferrite Transformation behavior was investigated over a wide temperature range using a 6Ni–0.1C steel. Softening in flow stress due to dynamic Transformation was observed at temperatures above Ae3, the ortho-equilibrium austenite-to-Ferrite Transformation temperature. Microstructural observation revealed that Ferrite grains formed at temperatures above Ae3 showed deformation microstructures, and those grains were reversely transformed to austenite during subsequent holding at the same temperature. Therefore, we concluded that dynamic Ferrite Transformation could certainly occur even above Ae3.
-
dynamic softening of flow stress during dynamic Ferrite Transformation
Materials Science Forum, 2013Co-Authors: Nokeun Park, Sunisa Khamsuk, Akinobu Shibata, Nobuhiro TsujiAbstract:This study using a 6Ni-0.1C steel confirmed the relationship between the change in a fraction of dynamically transformed Ferrite and the dynamic softening in stress-strain curve during dynamic Transformation above ortho-equilibrium austenite-to-Ferrite Transformation temperature. Dynamic softening in stress-strain curve was well-fitted with a form of Avrami equation as a function of strain, and it corresponded with the change in fraction of Ferrite. The slope of work-hardening rate was increased due to the additional softening phenomenon, i.e. dynamic Transformation, to dynamic recovery of austenite. Dynamic softening of austenite, which has been considered as a typical evidence of dynamic recrystallization, could be interpreted as a response of dynamic Transformation to Ferrite.
-
Effect of austenite grain size on kinetics of dynamic Ferrite Transformation in low carbon steel
Scripta Materialia, 2013Co-Authors: Nokeun Park, Sunisa Khamsuk, Nobuhiro TsujiAbstract:The effect of austenite grain size on kinetics of dynamic Ferrite Transformation above Ae3 in a 6Ni–0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic Transformation was accelerated. The increase in the fraction of dynamically transformed Ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic Transformation could be evaluated by an Avrami-type formula.
-
flow stress analysis for determining the critical condition of dynamic Ferrite Transformation in 6ni 0 1c steel
Acta Materialia, 2013Co-Authors: Nokeun Park, Akinobu Shibata, Daisuke Terada, Nobuhiro TsujiAbstract:Abstract In order to clarify the occurrence of dynamic Ferrite Transformation in a 6Ni–0.1C steel, the stress–strain behavior in uniaxial compression was analyzed for a wide range of temperatures and strain rates. Significant softening of flow stress for austenite was observed at lower temperatures at a constant strain rate, which seemed to correspond with the occurrence of dynamic Transformation to Ferrite. Analysis of the maximum stress in the stress–strain curves indicated that dynamic Ferrite Transformation occurred above a certain value of the Zener–Hollomon parameter ( Z ). The critical deformation condition ( Z C ) for the occurrence of dynamic Transformation was determined. Increasing the amount of softening resulted in an increase in the fraction of Ferrite, and the maximum flow stress came close to the flow stress of Ferrite. Microstructural observations revealed that the specimens exhibiting softening consisted of Ferrite grains with typical characteristics of deformation microstructure, such as a change in crystal orientation within the Ferrite grain, inhomogeneity in Ferrite morphology and dislocation substructures inside the grains. All these characteristics confirmed the occurrence of Ferrite Transformation during deformation, i.e. dynamic Ferrite Transformation.
Nokeun Park - One of the best experts on this subject based on the ideXlab platform.
-
nature of dynamic Ferrite Transformation revealed by in situ neutron diffraction analysis during thermomechanical processing
Scripta Materialia, 2019Co-Authors: Akinobu Shibata, Nokeun Park, Yasunari Takeda, Lijia Zhao, Stefanus Harjo, Takuro Kawasaki, Wu GongAbstract:Abstract Nowadays, a new concept of process utilizing dynamic Ferrite Transformation, which can achieve ultrafine-grained structure with a mean grain size of approximately 1 μm, has been proposed. This paper reports Transformation mode of dynamic Ferrite Transformation and formation mechanism of ultrafine-grained structure revealed by our novel technique of in-situ neutron diffraction analysis during thermomechanical processing. Dynamic Ferrite Transformation occurs in a diffusional manner, whose partitioning behavior changes from para- to ortho-equilibrium with the progress of Transformation. Moreover, we propose that dynamic recrystallization of dynamically-transformed Ferrite is the main mechanism for the formation of ultrafine-grained structure.
-
dynamic Ferrite Transformation behaviors in 6ni 0 1c steel
JOM, 2014Co-Authors: Nokeun Park, Akinobu Shibata, Lijia Zhao, Nobuhiro TsujiAbstract:Phase Transformation from austenite to Ferrite is an important process to control the microstructures of steels. To obtain finer Ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static Ferrite Transformation have been carried out for austenite phase. This article reviews the dynamic Transformation (DT), in which Ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of Ferrite grains even above the Ae3 temperature (ortho-equilibrium Transformation temperature between austenite and Ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae3 was experimentally confirmed. Moreover, the change in Ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine Ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of Ferrite.
-
occurrence of dynamic Ferrite Transformation in low carbon steel above ae3
Scripta Materialia, 2013Co-Authors: Nokeun Park, Sunisa Khamsuk, Akinobu Shibata, Nobuhiro TsujiAbstract:Dynamic Ferrite Transformation behavior was investigated over a wide temperature range using a 6Ni–0.1C steel. Softening in flow stress due to dynamic Transformation was observed at temperatures above Ae3, the ortho-equilibrium austenite-to-Ferrite Transformation temperature. Microstructural observation revealed that Ferrite grains formed at temperatures above Ae3 showed deformation microstructures, and those grains were reversely transformed to austenite during subsequent holding at the same temperature. Therefore, we concluded that dynamic Ferrite Transformation could certainly occur even above Ae3.
-
dynamic softening of flow stress during dynamic Ferrite Transformation
Materials Science Forum, 2013Co-Authors: Nokeun Park, Sunisa Khamsuk, Akinobu Shibata, Nobuhiro TsujiAbstract:This study using a 6Ni-0.1C steel confirmed the relationship between the change in a fraction of dynamically transformed Ferrite and the dynamic softening in stress-strain curve during dynamic Transformation above ortho-equilibrium austenite-to-Ferrite Transformation temperature. Dynamic softening in stress-strain curve was well-fitted with a form of Avrami equation as a function of strain, and it corresponded with the change in fraction of Ferrite. The slope of work-hardening rate was increased due to the additional softening phenomenon, i.e. dynamic Transformation, to dynamic recovery of austenite. Dynamic softening of austenite, which has been considered as a typical evidence of dynamic recrystallization, could be interpreted as a response of dynamic Transformation to Ferrite.
-
Effect of austenite grain size on kinetics of dynamic Ferrite Transformation in low carbon steel
Scripta Materialia, 2013Co-Authors: Nokeun Park, Sunisa Khamsuk, Nobuhiro TsujiAbstract:The effect of austenite grain size on kinetics of dynamic Ferrite Transformation above Ae3 in a 6Ni–0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic Transformation was accelerated. The increase in the fraction of dynamically transformed Ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic Transformation could be evaluated by an Avrami-type formula.
Matthias Militzer - One of the best experts on this subject based on the ideXlab platform.
-
the effect of solute nb on the austenite to Ferrite Transformation
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2015Co-Authors: Tao Jia, Matthias MilitzerAbstract:Niobium is a widely used micro-alloying element in steels that can retard the austenite-to-Ferrite Transformation primarily by solute drag when Nb remains in solution. It is critical to develop quantitative models to predict the effect of Nb on the Transformation kinetics. In the present work, dedicated continuous cooling Transformation (CCT) studies were performed for a low-carbon steel microalloyed with 0.047 wt pct Nb starting from fully recrystallized austenite states with the same grain size but different amounts of Nb in solution. The austenite-to-Ferrite Transformation kinetics is described from a fundamental perspective by assuming a mixed-mode reaction including solute drag of Nb. Using the solute drag model of Fazeli and Militzer, the intrinsic interface mobility, trans-interface diffusivity of Nb, and its binding energy to the interface have been determined from the CCT data. The interfacial parameters are critically analyzed and compared with independent measurements of diffusion and grain boundary segregation.
-
Superledge model for interphase precipitation during austenite-to-Ferrite Transformation
Metallurgical and Materials Transactions A, 2014Co-Authors: Meng-yang Chen, Mohamed Gouné, Matthias Militzer, Yves Bréchet, Jer-ren YangAbstract:A model for interphase precipitation has been developed based on the ledge mechanism of austenite-to-Ferrite Transformation. Carbide precipitation is considered on the migrating Ferrite/austenite interface as an interaction of Transformation and precipitation kinetics. The derived equations describe sheet spacing and particle spacing of interphase-precipitated carbides as well as the overall interface velocity which are related to the nucleation rates of carbides and Ferrite ledges, respectively. The microstructure characteristics of interphase precipitation are predicted as a function of Transformation temperature and steel composition and replicate trends observed experimentally.
-
phase field modeling of microstructure evolution in steels
Current Opinion in Solid State & Materials Science, 2011Co-Authors: Matthias MilitzerAbstract:This article provides an overview on the application of phase field models to describe microstructure evolution in steels. The focus will be on phase field modeling of the austenite–Ferrite Transformation as this has emerged as a particularly active area of research in the past few years. Phase field models are powerful tools to deal with the complex morphologies, e.g. Widmanstatten Ferrite, that may result from these Transformations. Even though much progress has been attained there is still significant work to be done in applying these models to processing of advanced steels with complex multi-phase microstructures. In particular, the phase field approach promises to have significant impact on modeling of bainite formation and the microstructure evolution in the heat affected zone of welds.
-
grain refinement in dual phase steels
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2009Co-Authors: Sujoy S Hazra, K Mukherjee, Matthias MilitzerAbstract:Deformation-induced Ferrite Transformation (DIFT) was applied in laboratory tests to produce fine-grained dual-phase (DP) steels. Four different chemistries were investigated, starting from a conventional DP 600 chemistry of 0.06 wt pct C-1.9 wt pct Mn-0.16 wt pct Mo and subsequently varying Nb and Mo additions. For all investigated steels, ultrafine Ferrite (UFF) with a grain size of 1 to 2 μm can be obtained when a sufficient amount of deformation (e.g., a true strain of 0.6 or above in axisymmetric compression) is applied to an austenite microstructure with a grain size in the range of 10 to 20 μm at 25 °C to 50 °C above the austenite-to-Ferrite Transformation start temperature (Ar 3) characteristic for the given cooling condition. Rapid post-deformation cooling at rates of approximately 100 °C/s yields the desired UFF-martensite microstructure. Electron backscattered diffraction (EBSD) mapping reveals a high percentage (approximately 40 pct) of low-angle boundaries in these microstructures, except for the steel that is just microalloyed with Nb. The steel with the plain-carbon-base chemistry was subjected to hot torsion simulations of a hot strip rolling processing schedules that incorporate a DIFT pass after a conventional seven-stand finish mill schedule. Executing the DIFT pass at 650 °C to 675 °C produced an UFF microstructure, illustrating the potential for the design of novel thermomechanical processing paths to produce hot-rolled ultrafine DP steels.
-
three dimensional phase field modelling of the austenite to Ferrite Transformation
Acta Materialia, 2006Co-Authors: Matthias Militzer, Jilt Sietsma, M G Mecozzi, S Van Der ZwaagAbstract:The phase field method has been used to simulate the austenite-to-Ferrite phase Transformation in steel, for the first time in three-dimensional (3D) space. The effect of the exact morphology of the initial 3D austenitic microstructure is shown to be small, but the nucleus density and the temperature range in which nucleation takes place, both derived from experimental observations, are of primary importance for the kinetics. When 3D simulations are compared to two-dimensional (2D) simulations, it becomes evident that 2D simulations predict faster Transformation rates. In the often-used practice of considering the interface mobility as an adjustable model parameter, systematically too low values will be obtained if 2D simulations are compared with experiments. Moreover, unrealistic features can be observed in the simulated 2D microstructures, whereas the 3D microstructures show a realistic representation of the actual microstructure. The overall conclusion is therefore that phase field modelling is distinctly more powerful when applied in 3D space.