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Experimental information on the growth rate of γ' and e layers formed during gaseous nitriding iron is assessed applying the most recent thermodynamic description of the Fe-N system. The atomic mobility of N in γ' was found... more
Experimental information on the growth rate of γ' and e layers formed during gaseous nitriding iron is assessed applying the most recent thermodynamic description of the Fe-N system. The atomic mobility of N in γ' was found approximately independent of composition yielding a concentration independent self-diffusivity of N in γ' obtained as D N *(γ') = 2.7 x 10 -10 exp (-88.3 kJ mol -1 /RT) m 2 s -1 for 777 K < T < 843 K where R is the gas constant and T is the absolute temperature. The atomic mobility of N in e was assumed to be independent of concentration and evaluated as M N e = 4.6 x 10 -13 exp (-80.7 kJ mol -1 /RT) mol m 2 s -1 J -1 for 848 K < T < 1300 K. The result was applied to simulate the nitriding process and reasonably good agreement with the experiments was obtained. Furthermore, the evaluated data were compared with the previous evaluations.
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ABSTRACT The thermodynamic and kinetic basis for simulation of multicomponent diffusional transformations is reviewed. The concepts underlying a new program package called DICTRA are described. The new software utilizes a new numerical... more
ABSTRACT The thermodynamic and kinetic basis for simulation of multicomponent diffusional transformations is reviewed. The concepts underlying a new program package called DICTRA are described. The new software utilizes a new numerical procedure for solving a system of coupled diffusion equations and is interfaced with the THERMO-CALC system for calculation of local equilibrium at a moving phase interface.
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It is important to understand the WC grain growth behavior in order to obtain good mechanical properties when manufacturing cemented carbides. In the present study the WC grain growth for dierent i ...
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Recently the present authors presented a Phase-field model of sintering in a multiphase system. In the present contribution the major features of the model are summarized. The model is based on diffusion of vacancies as the atomic... more
Recently the present authors presented a Phase-field model of sintering in a multiphase system. In the present contribution the major features of the model are summarized. The model is based on diffusion of vacancies as the atomic mechanism for redistribution of material. The solid material is characterized by a low vacancy content and the surroundings by a very high vacancy content and a very low content of atoms. The surface of the solid body is characterized by a continuous variation in vacancy content. The temporal development of particles during solid state sintering with effects such as wetting has been shown previously and here we discuss the effect of a highly anisotropic interfacial energy on the morphological evolution of the particles.
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Nitriding and oxidising treatments of alloys for toolingpurposes have been investigated both experimentally andtheoretically. Specimens prepared by a combination of differentprocessing steps are investigated by light-optical, scanningand... more
Nitriding and oxidising treatments of alloys for toolingpurposes have been investigated both experimentally andtheoretically. Specimens prepared by a combination of differentprocessing steps are investigated by light-optical, scanningand transmission electron microscopy and hardness measurments.The bounds of carbon and nitrogen concentration profiles duringnitriding are obtained from simulations using the DICTRA code.As a result, a new processing route to produce oxide-dispersediron-base alloys with large volume fraction of carbides issuggested. On the theoretical side the methods based on the random walktechnique have been investigated and developed further. The newmethods have been applied to study a vide range of phenomenae.g. internal oxidation, diffusion in inhomogeneous media,phase transformations and formation of porosity due to theKirkendall effect in welded joints. Probability distributionsare introduced to replace random number generators in order toincrease computational efficiency. A general method to simulatediffusional phase transformations in multicomponent systems isdeveloped and applied to ternary alloys.
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A series of bulk glassy alloys of Ti-Zr-Cu-Pd-Sn composition without toxic elements (Ni, Be, Al) were investigated that revealed excellent bulk forming ability by exhibiting large supercooled regio ...
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The special features of diffusional transformations in ternary systems, where one component is much more mobile than the others, are discussed. It is shown how the understanding of these features developed independently from two quite... more
The special features of diffusional transformations in ternary systems, where one component is much more mobile than the others, are discussed. It is shown how the understanding of these features developed independently from two quite different starting points. The features are discussed with reference to a number of calculations. The most recent development is based upon numerical solutions of the diffusion equation written with gradients of chemical potentials as driving forces. By reference to a number of calculations it is demonstrated that many of the earlier limitations have thus been removed.
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The Thermo-Calc and DICTRA software/database/programming-interface packages, through many successful applications in the fields of Computational Thermodynamics and Kinetics, have tremendously contributed to quantitative conceptual design... more
The Thermo-Calc and DICTRA software/database/programming-interface packages, through many successful applications in the fields of Computational Thermodynamics and Kinetics, have tremendously contributed to quantitative conceptual design and processing of various advanced materials. Materials scientists and engineers can efficiently apply such unique and comprehensive tools in calculating material properties, predicting material structures and simulating material processes, which are of wide-ranging industrial and academic importance.
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Effect of homogenization temperatureon subsequent spinodal decompositionduring aging in Fe-46.5Cr alloy
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Complex martensitic microstructure evolution in steels generates enormous curiosity among the materials scientists and especially among the Phase Field (PF) modeling enthusiasts. In the present work PF Microelasticity theory proposed by... more
Complex martensitic microstructure evolution in steels generates enormous curiosity among the materials scientists and especially among the Phase Field (PF) modeling enthusiasts. In the present work PF Microelasticity theory proposed by A.G. Khachaturyan coupled with plasticity is applied for modeling the Martensitic Transformation (MT) by using Finite Element Method (FEM). PF simulations in 3D are performed by considering different cases of MT occurring in a clamped system, i.e. simulation domain with fixed boundaries, of (a) pure elastic material with dilatation (b) pure elastic material without dilatation (c) elastic perfectly plastic material with dilatation having (i) isotropic as well as (ii) anisotropic elastic properties. As input data for the simulations the thermodynamic parameters corresponding to Fe - 0.3% C alloy as well as the physical parameters corresponding to steels acquired from experimental results are considered. The results indicate that elastic strain energy, ...
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Publisher Summary This chapter discusses multi-component diffusion in compound steel. Compound materials are increasingly being used as structural materials. A corrosion-resistant steel can be combined with an inexpensive low-alloy... more
Publisher Summary This chapter discusses multi-component diffusion in compound steel. Compound materials are increasingly being used as structural materials. A corrosion-resistant steel can be combined with an inexpensive low-alloy high-strength steel and thereby lower the costs. If the different materials are chemically incompatible, the heat treatment of such a compound material would present some difficulties. There might be strong driving forces for the transfer of atoms by means of diffusion between the two component materials. The transfer might result in a drastic change in the properties close to the interface between the two materials. In some cases, it might be beneficial but in most cases it is not and the amount of transfer must be minimized. Over the last decade, a general package of computer programs for multi-component diffusional transformations has been developed. Diffusion calculations and modified equilibrium calculations can give valuable information for the heat-treating of compound materials. As input, the diffusivities of the various components and a thermodynamic description are required.
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Publisher Summary This chapter discusses the diffusion in multi-component phases. Phase transformation between condensed phases can be treated by a method that combines an explicit description of diffusion processes with thermochemical... more
Publisher Summary This chapter discusses the diffusion in multi-component phases. Phase transformation between condensed phases can be treated by a method that combines an explicit description of diffusion processes with thermochemical calculations assuming local equilibrium. In order to make the most efficient use of experimental diffusion data and to obtain reliable extrapolations, one should take into account the thermochemical properties of the system when known. One-dimensional diffusion along the z-axis in a binary system usually obeys the well-known Fick's law. The diffusion of a species depends not only on its own concentration gradient, but also on the gradients of the other species. From a practical point of view, it means that a mobile species might be redistributed in a body if there are gradients of less mobile species even if the mobile species itself is homogeneously distributed at the beginning. The effect might even be so strong that an element might diffuse to regions of much higher concentration, so-called uphill diffusion.
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The basics of thermodynamics are reviewed with special attention to phase transformations. The distinction between internal and external variables is emphasized and the general equilibrium conditions are derived from the combined first... more
The basics of thermodynamics are reviewed with special attention to phase transformations. The distinction between internal and external variables is emphasized and the general equilibrium conditions are derived from the combined first and second law. The concepts of entropy production and driving force as well as stability are discussed. The calculation of thermodynamic properties and phase equilibria is considered and the Calphad method is briefly reviewed, including modeling of substitutional and interstitial disorder. The thermodynamic bases of phase diagrams are examined, and finally, the effect of interfaces, fluctuations and thermodynamics of nucleation are reviewed.
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A data base is being developed for calculating binary, ternary and multi component phase diagrams for systems of interest in processing novel materials. Current applications cover Zirconium Fluoride based Glasses for tunable gap... more
A data base is being developed for calculating binary, ternary and multi component phase diagrams for systems of interest in processing novel materials. Current applications cover Zirconium Fluoride based Glasses for tunable gap Electro-Optical applications, Iron-Aluminum based alloys for high temperature applications and titanium-carbo-nitride compounds for hard metal coatings.
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ABSTRACT Caballero et al. recently presented new evidence for different growth mechanisms of Widmanstätten and bainitic ferrite. Their argument was based on Zener’s hypothesis of diffusionless growth of bainitic ferrite. It is now... more
ABSTRACT Caballero et al. recently presented new evidence for different growth mechanisms of Widmanstätten and bainitic ferrite. Their argument was based on Zener’s hypothesis of diffusionless growth of bainitic ferrite. It is now demonstrated that Bhadeshia’s model, based on Zener’s hypothesis, predicts that some of the new measurements, claimed to fall above Bs and to be due to Widmanstätten ferrite, actually fall within the predicted temperature range of bainite, indicating that they cannot be used as new support for Bhadeshia’s model.
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ABSTRACT The properties of cemented carbides strongly depend on the WC grain size and it is thus crucial to control coarsening of WC during processing. The aim of this work was to study the effect of sintering at different carbon... more
ABSTRACT The properties of cemented carbides strongly depend on the WC grain size and it is thus crucial to control coarsening of WC during processing. The aim of this work was to study the effect of sintering at different carbon activities on the final microstructure, as well as the coarsening behavior of the WC grains, including the size distribution and the shape of WC grains. These aspects were investigated for five WC–Co alloys sintered at 1410 °C for 1 h at different carbon activities in the liquid, in the range from the graphite equilibrium (carbon activity of 1) to the eta (M6C) phase equilibrium (carbon activity of 0.33). The grain size distribution was experimentally evaluated for the different alloys using EBSD (electron backscatter diffraction). In addition, the shape of the WC grains was evaluated for the different alloys. It was found that the average WC grain size increased and the grain size distribution became slightly wider with increasing carbon activity. Comparing the two three-phase (WC–Co–eta and WC–Co–graphite) alloys a shape change of the WC grains was observed with larger grains having more planar surfaces and more triangular shape for the WC–Co–graphite alloy. It was indicated that in alloys with a relatively low volume fraction of the binder phase the WC grain shape is significantly affected by impingements. Moreover, after 1 h of sintering the WC grains are at a non-equilibrium state with regards to grain morphology.
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... (18). The species is initially distributed betweeen z min and z max as given by the distribution function x k0 (z). In Fig. 4 the division of species into these functions have been indicated; in the interior of phases overlapping... more
... (18). The species is initially distributed betweeen z min and z max as given by the distribution function x k0 (z). In Fig. 4 the division of species into these functions have been indicated; in the interior of phases overlapping triangular distribution functions are used. ...
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ABSTRACT The Ni–Pt system is assessed using the CALPHAD method. The four fcc-based phases, i.e. disordered solid solution phase, Ni3Pt–L12, NiPt–L10 and NiPt3–L12, are described by a four-sublattice model. The calculated thermodynamic... more
ABSTRACT The Ni–Pt system is assessed using the CALPHAD method. The four fcc-based phases, i.e. disordered solid solution phase, Ni3Pt–L12, NiPt–L10 and NiPt3–L12, are described by a four-sublattice model. The calculated thermodynamic properties and order/disorder phase transformations are in good agreement with the experimental data. In order to facilitate the assessment, first-principles pseudopotential calculations are also performed to calculate the enthalpy of formation at 0 K, and comparison with the assessed values is discussed. By combining the assessments of Al–Ni and Al–Pt, the Al–Ni–Pt ternary system is assessed within a narrow temperature range, focusing on the fcc-based phases and their phase equilibria with B2 phase.
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A simplified model for the growth of particles in a multicomponent system has been developed. The model takes into account of the effects of cross diffusion and finite interface mobility. It can be applied in a straightforward way to... more
A simplified model for the growth of particles in a multicomponent system has been developed. The model takes into account of the effects of cross diffusion and finite interface mobility. It can be applied in a straightforward way to treat precipitation under the conditions of paraequilibrium (PARA) or non-partitioning local equilibrium (NPLE).
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A steady-state model for austenite-to-pearlite transformation in multicomponent steel is presented, including Fe, C, and eight more elements. The model considers not only classic ingredients (formation of ferrite–cementite interface,... more
A steady-state model for austenite-to-pearlite transformation in multicomponent steel is presented, including Fe, C, and eight more elements. The model considers not only classic ingredients (formation of ferrite–cementite interface, volume diffusion, boundary diffusion, and optimization of lamellar spacing) but also finite austenite–pearlite interfacial mobility that resolves some previous difficulties. A non-Arrhenius behavior of interfacial mobility is revealed from growth rate and lamellar spacing data. A smooth and physical transition between orthopearlite and parapearlite is realized by optimizing the partitioning of substitutional alloying elements between ferrite and cementite to maximize growth rate or dissipation rate while keeping carbon at equilibrium. Solute drag effect is included, which accounts for the bay in growth rate curves. Grain boundary nucleation rate is modeled as a function of chemical composition, driving force, and temperature, with consideration of grain...
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As a step in the further development of models and databases to support design of new steels, i.e., the “steel genome”, the growth of bainitic ferrite plates is accounted for by a thermodynamic and kinetic approach. The thermodynamic... more
As a step in the further development of models and databases to support design of new steels, i.e., the “steel genome”, the growth of bainitic ferrite plates is accounted for by a thermodynamic and kinetic approach. The thermodynamic aspects are represented by CALPHAD databases and a Gibbs energy barrier for growth B m. Experimental information on ferrite-plate growth rates for a number of Fe-C alloys, some of high-purity, are analyzed in terms of a modified Zener–Hillert model and the barrier as well as some kinetic parameters are evaluated. It is found that the barrier varies in a smooth way with carbon content and lengthening rate. In order to improve the agreement with the experimental information it was necessary to adjust the diffusion coefficient of carbon in austenite at low temperatures. It is concluded that the representation of the experimental data is satisfactory.
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The microstructure evolution of a martensitic Stainless steel subjected to hot compression is simulated with a physically based model. The model is based on coupled sets of evolution equations for dislocations, vacancies,... more
The microstructure evolution of a martensitic Stainless steel subjected to hot compression is simulated with a physically based model. The model is based on coupled sets of evolution equations for dislocations, vacancies, recrystallization, and grain growth. The advantage of this model is that with only a few experiments, the material-dependent parameters of the model can be calibrated and used for a new alloy in any deformation condition. The experimental data of this work are obtained from a series of hot compression, and subsequent stress relaxation tests performed in a Gleeble thermo-mechanical simulator. These tests are carried out at various temperatures ranging from 900 to 1200 °C, strains up to 0.7, and strain rates of 0.01, 1, and 10 s−1. The grain growth, flow stress, and stress relaxations are simulated by finding reasonable values for model parameters. The flow stress data obtained at the strain rate of 10 s−1 were used to calibrate the model parameters and the predictio...
Research Interests: Materials Engineering, Mechanical Engineering, Materials Science, Microstructure, Modeling, and 11 moreMetallurgy, Strain Rate, Stress relaxation, Grain Growth, Recrystallization, Dislocation Density, Dynamic Recrystallization, Martensite, Martensitic Stainless Steel, Water Softening, and Flow Stress
As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of... more
As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of ferrite and austenite, leading to both high strength and large elongation. The growth of austenite during intercritical annealing is crucial for the final properties, primarily because it determines the fraction, composition, and phase stability of austenite. In the present work, the growth of austenite from as-quenched lath martensite in medium Mn steels has been simulated using the DICTRA software package. Cementite is added into the simulations based on experimental observations. Two types of systems (cells) are used, representing, respectively, (1) austenite and cementite forming apart from each other, and (2) austenite forming on the cementite/martensite interface. An interfacial dissipation energy has also been added to take into account a fi...
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ABSTRACTDepth sensing nano-indentation investigations have been performed to determine the radial dependence of the hardness through the cross section of a Fe-based bulk glassy rod. We have found the hardness of the material decreases... more
ABSTRACTDepth sensing nano-indentation investigations have been performed to determine the radial dependence of the hardness through the cross section of a Fe-based bulk glassy rod. We have found the hardness of the material decreases along radius from the centre to the outermost surface. This phenomenon is attributed to the ‘cooling rate induced surface softening’. Furthermore, a significant change (~15 %) in elastic modulus is noticed along the radius as well.
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ABSTRACTWe have succeeded in producing bulk metallic glass by partial substitution of Fe with Ni in Fe-B-Nb alloys which could otherwise be only melt spun into amorphous ribbons. Substitution by Ni in the Fe72-xB24Nb4Nix alloys with (x... more
ABSTRACTWe have succeeded in producing bulk metallic glass by partial substitution of Fe with Ni in Fe-B-Nb alloys which could otherwise be only melt spun into amorphous ribbons. Substitution by Ni in the Fe72-xB24Nb4Nix alloys with (x ~2, 4, 6, 8, 10, 12 and 14) improves the glass forming ability of the materials and as a result rods of same compositions can be fabricated. Magnetically the BMG alloys remain soft with coercitivities below 500 mOe- However, the electrical resistivity of the system decreases significantly by as much as a factor of two with the increase of Ni concentration, and becomes more metallic like with a positive temperature coefficient.