| 1. |
- Becker, Chandler A., et al.
(author)
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Thermodynamic modelling of liquids : CALPHAD approaches and contributions from statistical physics
- 2014
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In: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 251:1, s. 33-52
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Journal article (peer-reviewed)abstract
- We describe current approaches to thermodynamic modelling of liquids for the CALPHAD method, the use of available experimental methods and results in this type of modelling, and considerations in the use of atomic-scale simulation methods to inform a CALPHAD approach. We begin with an overview of the formalism currently used in CALPHAD to describe the temperature dependence of the liquid Gibbs free energy and outline opportunities for improvement by reviewing the current physical understanding of the liquid. Brief descriptions of experimental methods for extracting high-temperature data on liquids and the preparation of undercooled liquid samples are presented. Properties of a well-determined substance, B-2 O-3, including the glass transition, are then discussed in detail to emphasize specific modelling requirements for the liquid. We then examine the two-state model proposed for CALPHAD in detail and compare results with experiment and theory, where available. We further examine the contributions of atomic-scale methods to the understanding of liquids and their potential for supplementing available data. We discuss molecular dynamics (MD) and Monte Carlo methods that employ atomic interactions from classical interatomic potentials, as well as contributions from ab initio MD. We conclude with a summary of our findings.
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| 2. |
- Stormvinter, Albin
(author)
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Low Temperature Austenite Decomposition in Carbon Steels
- 2012
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Doctoral thesis (other academic/artistic)abstract
- Martensitic steels have become very important engineering materials in modern society. Crucial parts of everyday products are made of martensitic steels, from surgical needles and razor blades to car components and large-scale excavators. Martensite, which results from a rapid diffusionless phase transformation, has a complex nature that is challenging to characterize and to classify. Moreover the possibilities for modeling of this phase transformation have been limited, since its thermodynamics and kinetics are only reasonably well understood. However, the recent development of characterization capabilities and computational techniques, such as CALPHAD, and its applicability to ferrous martensite has not been fully explored yet.In the present work, a thermodynamic method for predicting the martensite start temperature (Ms) of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X systems obtained from experiments using very rapid cooling, and Ms values for lath and plate martensite are treated separately. Comparison with the experimental Ms of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of Ms from commercial steels.A major part of the present work is dedicated to the effect of carbon content on the morphological transition from lath- to plate martensite in steels. A range of metallographic techniques were employed: (1) Optical microscopy to study the apparent morphology; (2) Transmission electron microscopy to study high-carbon plate martensite; (3) Electron backscattered diffraction to study the variant pairing tendency of martensite. The results indicate that a good understanding of the martensitic microstructure can be achieved by combining qualitative metallography with quantitative analysis, such as variant pairing analysis. This type of characterization methodology could easily be extended to any alloying system and may thus facilitate martensite characterization in general.Finally, a minor part addresses inverse bainite, which may form in high-carbon alloys. Its coupling to regular bainite is discussed on the basis of symmetry in the Fe-C phase diagram.
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| 3. |
- Strandlund, Henrik
(author)
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Simulation of diffusional processes in alloys : techniques and applications
- 2005
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Doctoral thesis (other academic/artistic)abstract
- This thesis concerns computer simulation of diffusional processes in alloys. The main focus is on the development of simulation techniques for diffusion in single-phase domains, but also diffusion controlled phase-transformations and interfacial processes are discussed. Different one-dimensional simulation techniques for studying the Kirkendall effect are developed and analyzed. Comparisons with experimentally observed marker migration show good agreement for small shifts and comparisons with observed Kirkendall porosity show reasonable agreement under the assumption that a certain supersaturation is needed before the vacancies coalesce into pores. A convenient approach in simulations of kinetics is to use thermodynamic software, e.g. Thermo-Calc, to calculate thermodynamic quantities, e.g. chemical potentials, required in the simulation. The main drawback with such an approach is that it will generate a large amount of additional computational work. To overcome this problem a method that decreases the amount of computational work has been developed. The new method is based on artificial neural networks (ANN). By training the ANN to estimate thermodynamic quantities a significant increase in computational speed was obtained. By calculating the dissipation of available driving force due to diffusion inside migrating interfaces an approach for including the effect of solute drag in computer simulations of grain growth and phase transformations has been developed. The new method is based on an effective interfacial mobility and simulations of grain growth have been performed in binary and ternary systems using experimentally assessed model parameters.
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| 4. |
- Alvares, E. D., et al.
(author)
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An assessment of Fe-Nb-B melts using the two-state liquid model
- 2020
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In: Calphad. - : Elsevier Ltd. - 0364-5916 .- 1873-2984. ; 68
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Journal article (peer-reviewed)abstract
- Amorphous alloys of the (Fe-Co-Ni)-(Cr-Mo-Nb)-B system are promising materials to supply the demands for higher wear resistance components in the petrochemical industry. Since the development of the CALPHAD method, the development of new metallic alloys has been accompanied by thermodynamic modelling and calculations. The prediction of the formation of amorphous alloys requires special care with the modeling of the liquid and or an amorphous phase. As a initial stage in the more complex system, the basic Fe-Nb-B ternary system was selected. In order to predict the stability and tendencies of transformations of these amorphous alloys, the Fe-Nb-B system was reassessed using Ågren's two-state model to describe the liquid phase. The results of the present assessment show very good agreement with the recently reported stable phase diagram. Furthermore, the use of the two-state model for the liquid is more accurate and physically consistent when evaluating transformations from supercooled liquid, as shown it the present work.
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| 5. |
- Andersson, Egil, 1981-, et al.
(author)
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Core-valence double photoionization of the CS2 molecule
- 2010
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:9, s. 94305-
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Journal article (peer-reviewed)abstract
- Double photoionization spectra of the CS2 molecule have been recorded using the TOF-PEPECO technique in combination with synchrotron radiation at the photon energies h nu=220, 230, 240, 243, and 362.7 eV. The spectra were recorded in the S 2p and C 1s inner-shell ionization regions and reflect dicationic states formed out of one inner-shell vacancy and one vacancy in the valence region. MCSCF calculations were performed to model the energies of the dicationic states. The spectra associated with a S 2p vacancy are well structured and have been interpreted in some detail by comparison to conventional S 2p and valence photoelectron spectra. The lowest inner-shell-valence dicationic state is observed at the vertical double ionization energy 188.45 eV and is associated with a (2p(3/2))(-1)(2 pi(g))(-1) double vacancy. The spectrum connected to the C 1s vacancy shows a distinct line at 310.8 eV, accompanied by additional broad features at higher double ionization energies. This line is associated with a (C 1s)(-1)(2 pi(g))(-1) double vacancy. (C-) 2010 American Institute of Physics. [doi: 10.1063/1.3469812]
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| 6. |
- Andersson, Magnus, et al.
(author)
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Effect of Prestraining and Training on the γ→ε Transformation in Fe-Mn-Si Alloys
- 1995
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In: Journal de Physique IV. - : EDP Sciences. - 1155-4339 .- 1764-7177. ; 5:C8, s. 457-462
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Journal article (peer-reviewed)abstract
- The effect of training and predeformation on the martensitic transformation is investigated. It is concluded that MS as a function of predeformation strain must have a maximum at around 3% strain. Prestrains less than 5% will enhance the martensitic transformation whereas larger prestrains depress it due to strain hardening of the γ phase.
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| 7. |
- Andersson, Magnus, et al.
(author)
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Effects of recrystallization and grain size on the martensitic transformation in Fe-31% Mn-5% Si alloy
- 1996
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In: Materials transactions, JIM. - : Japan Institute of Metals. - 0916-1821 .- 2432-471X. ; 37:7, s. 1363-1370
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Journal article (peer-reviewed)abstract
- The effects of recrystallization and grain size have been studied in an Fe-31%Mn-5%Si shape memory alloy. The amount of martensite formed in samples which have been hot rolled at 1373 K and subsequently homogenized for 24 h at 1273 K followed by a final 30 min anneal at 1323 K is twice as large as in samples which have been hot rolled and homogenized in the same way as the former followed by a compression and a subsequent recrystallization at 1323 K for 30 min. The only difference between the samples is the plastic deformation induced by the compression. We thus conclude that some effect of the compressive deformation remains despite the recrystallization and suppresses the subsequent martensite formation. However, we have not yet been able to identify this effect. This matter will be subject to further investigations. No significant change is found in the M(S) and A(S) temperatures for the gamma <----> epsilon transformations determined by DSC. When the grain size varies between 40 and 160 mu m in completely recrystallized samples the variation in transformation temperatures is small, less than 10 K.
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| 8. |
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| 9. |
- Andersson, Magnus, et al.
(author)
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The Influence of External Stresses on the Transformation Behaviour in a Fe-Mn-Si Alloy
- 1997
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In: Journal de Physique IV. - : EDP Sciences. - 1155-4339 .- 1764-7177. ; 7:C5, s. 411-416
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Journal article (peer-reviewed)abstract
- The influence of external stresses on the transformation behaviour has been studied for a polycrystalline Fe-3 IMn-SSi alloy exhibiting the y-+& martensitic transformation. Samples have been loaded at temperatures above the M, temperature, cooled with a constant external load to a temperature below the M, temperature and subsequently heated without load to a temperature above the A, temperature of the alloy. Stress, strain and temperature have been continuously measured and transformation temperatures and strains have been determined from the change in strain during the transformation. It has been found that the M, temperature during cooling with load is not influenced by 0.3-2 % plastic pre-deformation of the austenite whereas there is a substantial increase in the transformation strain, i.e. variants with a favourable orientation are enhanced by pre-strain of austenite.
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| 10. |
- Andersson, Magnus, et al.
(author)
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Unified thermodynamic analysis of the stress-assisted γ→ε martensitic transformation in Fe–Mn–Si alloys
- 1998
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In: Acta Materialia. - 1359-6454 .- 1873-2453. ; 46:11, s. 3883-3891
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Journal article (peer-reviewed)abstract
- A unified thermodynamic analysis has been developed which allows the combined complex effects of applied stresses and alloy composition on the martensite start temperature, Ms(σ), to be explained and predicted. In this unified analysis, a thermodynamic analysis of the effect of applied stress on the relative stability of the high and low temperature phase is combined with existing thermodynamic descriptions of the alloy system. The calculated results are compared with experimental results for the γ→ martensitic transformation in Fe–Mn–Si shape memory alloys. For single crystals, the comparison shows that the influence of applied stresses can be directly predicted with a reasonable accuracy for a wide composition range. By integrating a polycrystallinity parameter in the analysis it is shown that the calculations are also valid for polycrystalline alloys. Application of this analysis to experimental values of Ms(σ) can also generate useful data for future optimizations of the thermodynamic description of several alloy systems.
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