| 1. |
- Becker, Chandler A., et al.
(författare)
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Thermodynamic modelling of liquids : CALPHAD approaches and contributions from statistical physics
- 2014
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 251:1, s. 33-52
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Tidskriftsartikel (refereegranskat)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
(författare)
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Low Temperature Austenite Decomposition in Carbon Steels
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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. |
- Andersson, Egil, 1981-, et al.
(författare)
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Core-valence double photoionization of the CS2 molecule
- 2010
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:9, s. 94305-
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Asp Grönhagen, Klara, et al.
(författare)
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Phase-field modelling of spinodal decomposition in TiAlN including the effect of metal vacancies
- 2015
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Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 95, s. 42-45
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Tidskriftsartikel (refereegranskat)abstract
- Using a CALPHAD approach together with a Cahn-Hilliard model, we describe the microstructure evolution in cubic Ti1-xAlxN including vacancies on the metal sublattice. Our results show that vacancy content has a pronounced effect on the decomposition kinetics. Furthermore, vacancies show a strong tendency to segregate to the coherent AlN-TiN interface regions. We illustrate how vacancies anneal to grain boundaries, and finally, we compare our prediction to experimental differential scanning calorimetry data and attribute the second peak in the thermogram to vacancy depletion.
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| 5. |
- Barkar, Thomas, et al.
(författare)
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Effect of concentration dependent gradient energy coefficient on spinodal decomposition in the Fe-Cr system
- 2018
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 143, s. 446-453
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Tidskriftsartikel (refereegranskat)abstract
- The Cahn–Hilliard equation is solved with thermodynamic and kinetic input, using the Thermo-Calc and DICTRA software packages rather than simpler models e.g. regular solution. A concentration dependent expression for the gradient energy coefficient is introduced and its effect on simulated decomposition is discussed. Simulations were carried out in 2D and 3D using the FiPy software package modified for non-linear problems.
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| 6. |
- Barkar, Thomas
(författare)
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Modelling phase separation in Fe-Cr alloys : A continuum approach
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The formation of Cr-rich and Fe-rich domains upon ageing of an initially homogeneous Fe-Cr alloy at elevated temperatures (300-600 ºC) is commonly referred to as phase separation. The behaviour originates from a miscibility gap in the Fe-Cr phase diagram. The boundary of the miscibility gap is denoted the binodal, and the line where the second derivative of the molar Gibbs energy w.r.t. composition is zero, the spinodal. In the region between the binodal and spinodal lines, the phase separation is said to occur by means of nucleation and growth. Inside the spinodal line, no thermally activated nucleation event is needed, and the initially homogeneous alloy decomposes "spinodally" into Cr-rich and Fe-rich regions. This type of phase transformation can be viewed as a continuous build up of Cr-rich regions, that also are interconnected, forming a microstructure characteristic for alloys decomposed spinodally. Phase separation has been of great interest within the metallurgical community as well as industry, due to its embritteling effect. Phase separation in Cr-rich ferritic steels, and thus embrittlement, sets a practical upper service temperature of ~300 ºC for Cr-containing ferrites. It is desirable to develop understanding and modelling capability for decomposing alloy systems, since such knowledge could be used to relieve the limitation in service temperature. The current work has been focused around the development and use of computer simulations, using thermodynamic and kinetic input from databases, in order to progress towards alloy design where decomposition is minimized. Simulations in this work are based on solving the so called Cahn-Hilliard equation, where an important parameter is the gradient energy, since it influences both the morphology and rate of decomposition in the simulations. An attempt at formulating a general model for the gradient energy coefficients in multi-component systems has been made, but has yet to be properly tried against experimental data. Improvements, and insights, to the initial state used in simulations has also been achieved. The combination of above mentioned efforts is a step towards a predictive tool for decomposition of complex alloys. Such a tool could not only be an aid in future alloy design, but also be used as an aid as a diagnosis tool in life time assessment of critical components already in use and thereby difficult to assess on site by means of in-destructive testing, typically components in nuclear power facilities.
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| 7. |
- Barkar, Thomas, et al.
(författare)
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Phase field modeling of spinodal decomposition in Fe-Cr based alloys
- 2015
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Ingår i: PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015. - : International Conference on Solid-Solid Phase Transformations in Inorganic Materials. - 9780692437360 ; , s. 827-828
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Konferensbidrag (refereegranskat)
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| 8. |
- Bonvalet, M., et al.
(författare)
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Modelling of prismatic grain growth in cemented carbides
- 2019
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Ingår i: International journal of refractory metals & hard materials. - : ELSEVIER SCI LTD. - 0263-4368. ; 78, s. 310-319
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Tidskriftsartikel (refereegranskat)abstract
- A mean-field model dealing with prismatic grain growth during liquid phase sintering of cemented carbides with a Co-rich binder is presented. The evolution of the size of an assembly of non-spherical grains is obtained using a Kampmann-Wagner approach and by introducing a constant shape factor between the characteristic lengths of prisms. This factor is a function of interfacial energies of the two kind of facets, basal and prismatic, considered. The growth model is based on three different mechanisms, that can be rate limiting, taking place in series: 2D nucleation of a new atomic layer, mass transfer across the interface and long-range diffusion. The driving force for coarsening is distributed between the different facets. These equations are solved numerically, and the simulation results reveal that the specific abnormal grain growth phenomena experimentally observed in cemented carbides may be reproduced with this new more realistic description of the grain shape contrary to the spherical approach developed in the past. It is also shown that the initial powder size distribution, and more specifically its shape has a strong influence on the distribution of the driving force between the different rate limiting mechanisms and thus on the occurrence of abnormal grain growth. In that case, the self-similarity of the normalized grain size distribution over time is not achieved.
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| 9. |
- Bonvalet-Rolland, Manon, et al.
(författare)
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Kinetic theory of nucleation in multicomponent systems : An application of the thermodynamic extremum principle
- 2019
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Ingår i: Acta Materialia. - : Acta Materialia Inc. - 1359-6454 .- 1873-2453. ; 171, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- Nucleation kinetics in a multicomponent supersaturated solid solution is examined. Attachment rate of atoms to a nucleus of a size close to the critical one is determined combining a thermodynamic extremum principle and the Fokker-Planck equation. Two limiting cases are examined; when bulk diffusion controls the nucleation kinetics and when the process is limited by the interfacial mobility. The mixed regime is also treated. Moreover, the growth law in multicomponent alloys is derived in the general case, when both mechanisms are considered. Additionally, the attachment rate is derived, in the classical framework, from a new macroscopic growth equations and the fundamental role of the interfacial mobility is examined. These new general expressions, for the attachment rates and the growth laws, determined either applying the thermodynamic extremum principle or derived from the classical formalism are found to be consistent.
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| 10. |
- Borgenstam, Annika, et al.
(författare)
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On the Symmetry Among the Diffusional Transformation Products of Austenite
- 2011
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 42A:6, s. 1558-1574
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Tidskriftsartikel (refereegranskat)abstract
- Information on the diffusional transformation products of austenite in high-carbon steels is reviewed and supplemented with new microscopic studies. A comparison with transformation products in low-carbon steels indicates that there is a symmetry with pearlite in the middle, where ferrite and cementite are equal partners, and with acicular ferrite or cementite on each side. They both form with a surface relief, and at lower temperatures, each one is the leading phase in a eutectoid microstructure, bainite and inverse bainite, respectively. However, there is an asymmetry because at low temperatures bainite appears in high-carbon steels but inverse bainite never appears in low-carbon steels. At a constant high carbon content, there is another kind of symmetry, which is related to temperature. At intermediate temperatures the eutectoid reaction results in spherical nodules in which the cementite constituent originates from Widmanstatten plates. It turns spiky at both higher and lower temperatures with the leading phase in the spikes being cementite at higher temperatures and ferrite at lower temperatures. In the first kind of symmetry, there is an abrupt change among the three reaction products; in the second kind of symmetry, there is a gradual change. Accepting that all the eutectoid microstructures form by diffusion of carbon, one may explain the existence of both symmetries by the variation of the ratio of the supersaturations of ferrite and cementite with carbon content and with temperature.
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