| 1. |
- Mukherjee, Deepjyoti, 1991-
(författare)
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Phase field modeling of precipitation reactions in miscibility gap systems
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As a backbone to the cutting tools and rock drilling industry, cemented carbides have been used widely due to their high hardness and wear resistance. Most commercially used cemented carbides contain a hard phase made of Tungsten carbide (WC) and a binder phase which is generally ductile. Lately, a secondary hard phase is desired by replacing W in WC partially with one or more metal substitutes such as Ti, V, Ta, Cr and Zr to better its mechanical properties. Some of these carbides are observed to exhibit unusual microstructures during ageing. For example, (Ti,Zr)C present along with WC, has been observed to undergo phase separation from a supersaturated phase, called $\gamma$, to Ti-rich and Zr-rich domains leaving behind an array of precipitates morphologically manifesting as lamellae. This phase separation process has been termed as discontinuous precipitation(DP) as it resembles the classical DP reaction observed in certain binary and multi-component systems. The behaviour comes from the presence of a miscibility gap in the carbide Ref. Borgh et al. 2014 and Ma et al. 2016 due to which the usual response of the system should be to undergo spinodal decomposition(SD), however, the carbide chooses a different path which questions the governing mechanism behind its decomposition process. Several factors are believed to affect such a process and one of such factors is the strain energy, which is generated due to the difference in lattice parameters of the separating phases. When compared to a different miscibility gap system, such as Fe-Cr, where the strain energy is quite low and the response is SD. Other factors such as grain boundary diffusion, atomic mobility, and gradient energy coefficient (κ) are also believed to have an effect on the decomposition process. Therefore, a thorough investigation of the factors is required and, a powerful tool to study the spatio-temporal evolution of the microstructure such as the phase field method, should be used.According to some experiments the lamellae are generally observed to nucleate at grain boundaries and later grow with the help of grain boundary migration Ref. Borgh et al. 2014. The moving grain boundary leaves behind a series of alternate strands of Ti and Zr rich phases. The growth mechanism behind the moving boundary is believed to be assisted by diffusion of solutes along the grain boundary and generation of the elastic strain energy by it. The phenomenon is commonly known as diffusion induced grain boundary migration(DIGM) and it is believed to be a key part of DP Ref. Hillert and Purdy 1977 and Chongmo and Hillert 1981. In order to recreate DIGM and DP an energetic coupling between the mole fraction and phase field variable is required so that it accounts for the generated strain energy during the process. The main focus of this thesis will be to develop a phase field model accounting for such coupling which will predict DIGM in binary systems and use it further as a medium to model DP in (Ti,Zr)C. The model for DP could be used to predict and control its formation as its occurrence prone to increase the hardness of the carbides. Therefore, it can be used as a tool to design alloys and develop better alternatives. The alternatives could be used to prevent DP in the carbides which could be done by using different metal substitutes that will prefer SD over DP or vice versa.
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| 2. |
- Chou, Chia-Ying, 1990-
(författare)
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Integrated Computational and Experimental Study of Additively Manufactured Steels
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The design freedom Additive Manufacturing (AM) offers provides new solutions for improving functionality in industrial applications. It also offers unique opportunities when it comes to materials design.Powder Bed Fusion – Laser Beam (PBF-LB) is currently one of the most popular commercial AM techniques for metallic materials partly due to the relatively low surface roughness and the large design flexibility. However, the number of materials suitable for the PBF-LB process is still rather low and to accelerate the development of grades tailored for this AM process, dedicated computational tools for alloy design are needed. Of importance for materials design, is computational thermodynamics and kinetics coupled with CALPHAD materials descriptions since it enables calculations for multicomponent materials making it possible to predict the effect of varying composition.In this thesis, computational thermodynamic and kinetics coupled with materials characterization are applied to study the microstructure evolution during PBF-LB. Two material classes are in focus – hot-work tool steels and ferritic stainless steels. For the hot-work tool steel, the cooling rates during PBF-LB processing are high enough to induce martensite transformation and in the as-built microstructure a martensitic matrix is observed and some fraction of retained austenite. A solidification sub-structure due to micro-segregation during printing is also observed. Solidification calculations are performed to predict the micro-segregation showing agreement with experimental measurements. The segregation results are then used as input to a semi-empirical martensite start temperature model making it possible to explain the location and amount of retained austenite.In addition to compositional optimization, a computational framework for AM alloy design needs to include the possibility to tailor the AM post heat treatments. An alternative to the conventional hardening treatment is thus studied in the current work. A model for precipitation kinetics is combined with experimental characterization to explore the effect of tempering on the as-built microstructure in comparison to the tempering effect on an austenitized microstructure. The results show that the precipitation kinetics is strongly dependent on the starting structure and that direct tempering of the as-built microstructure changes the precipitation sequence compared to the conventional heat treatment route. The calculations reproduce this result suggesting that it is a thermodynamic effect stemming from different matrix compositions.The other material class, the ferritic stainless steels, is studied in terms of its response to post-heat treatments. The as-built microstructure is characterized by high dislocation density and a fine grain structure in some cases as well as a solidification sub-structure. The mechanical properties of the as-built material are in general good for these steels, however, stress relieving is most often a required post process for the as-built components which may decrease the mechanical properties. To maximize the gained benefits from the unique process condition of PBF-LB, simulations are applied to study the possibility of post heat treatment optimization.To construct a computational framework for AM materials design, multiscale modeling capabilities are needed. This work shows the value of computational thermodynamics and kinetics for understanding the materials behavior on the microscale and hence, contributes to the construction of such a framework. By understanding the physical metallurgy, and enable modeling of the AM processes, the industrialization of AM can be accelerated.
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| 3. |
- Hedström, Peter, et al.
(författare)
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The 475 degrees C embrittlement in Fe-20Cr and Fe-20Cr-X (X=Ni, Cu, Mn) alloys studied by mechanical testing and atom probe tomography
- 2013
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 574, s. 123-129
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Tidskriftsartikel (refereegranskat)abstract
- In the present work the 475 degrees C embrittlement in binary Fe-Cr and ternary Fe-Cr-X (X=Ni, Cu and Mn) alloys have been investigated. The mechanical properties were evaluated using microhardness and impact testing, and the structural evolution was evaluated using atom probe tomography (APT). The APT results after aging at 500 degrees C for 10 h clearly showed that both Ni and Mn accelerate the ferrite decomposition. No evident phase separation of either the Fe-20Cr or Fe-20Cr-1.5Cu samples was detected after 10 h of aging and thus no conclusions on the effect of Cu can be drawn. Cu clustering was however found in the Fe-20Cr-1.5Cu sample after 10 h aging at 500 degrees C. The mechanical property evolution was consistent with the structural evolution found from APT. Samples aged at 450 and 500 degrees C all showed increasing hardness and decreasing impact energy. The embrittlement was observed to take place mainly during the first 10 h of aging and it could primarily be attributed to phase separation, but also substitutional solute clustering and possibly carbon and nitrogen segregation may contribute in a negative way.
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| 4. |
- Karlsson, Dennis, et al.
(författare)
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Additive manufacturing of the ferritic stainless steel SS441
- 2020
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Ingår i: Additive Manufacturing. - AMSTERDAM, NETHERLANDS : Elsevier BV. - 2214-8604 .- 2214-7810. ; 36
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the ferritic stainless steel SS441 was produced with excellent mechanical properties using laser powder bed fusion (L-PBF) compared to samples produced by conventional casting and hot-rolling. In addition, thermodynamic calculations were utilized to study the phase stability at elevated temperatures and to understand the solidification behavior. The hot-rolled sample showed a grain size up to several hundred mu m with additional precipitates of TiN and Nb(C,N). In contrast, the as-built L-PBF samples displayed a grain size in the mu m range. Spherical precipitates with a size of around 50 nm could be observed and were attributed to a corundum phase from the thermodynamic calculations. The printed material shows superior mechanical properties, with more than 30 times higher impact energy compared to the hot-rolled alloy (217 +/- 5 J vs. 7 +/- 0.5 J). Furthermore, the properties are anisotropic for the L-PBF produced alloy, with the highest tensile strength vertical to the build direction. The superior mechanical properties of the L-PBF produced sample can be attributed to a smaller grain size, giving a higher strength according to the Hall-Petch relationship. The anisotropy of the material can be eliminated by heat treatments at 900 degrees C followed by water quenching, but the absolute strength decreases slightly due to formation of intermetallic phases such as Nb(C,N) and the Fe2Nb Laves phase. The results clearly illustrates that L-PBF provides a promising manufacturing mute for enhanced strength of ferritic stainless steels.
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| 5. |
- Odqvist, Joakim, et al.
(författare)
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3D analysis of phase separation in ferritic stainless steels
- 2012
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Ingår i: Proceedings of the 1st International Conference on 3D Materials Science. - Cham : John Wiley & Sons. - 9781118470398 ; , s. 221-226
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Konferensbidrag (refereegranskat)abstract
- The embrittlement of ferritic stainless steels during low temperature aging is attributed to the phase separation with Fe and Cr demixing. The small scale of the decomposed structure with only minor compositional fluctuations and short distances between the enriched and depleted regions has been a challenge for quite some time. A wide selection of experimental and modeling tools have been used to quantify these types of structures. These analyses often focus on rather late stages of decomposition where the mechanical properties are already seriously affected. The recent advance in 3D tools like phase-field and atom probe tomography have created a need for good quantitative procedures of evaluating the structure and also to link results from the continuum approach to the individual atom measurements. This work aims at addressing this need.
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| 6. |
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| 7. |
- Pettersson, N., et al.
(författare)
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Nanostructure evolution and mechanical property changes during aging of a super duplex stainless steel at 300°C
- 2015
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 647, s. 241-248
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Tidskriftsartikel (refereegranskat)abstract
- The nanostructure evolution and the corresponding changes in mechanical properties of a super duplex stainless steel 2507 (UNS S32750) during aging at 300. °C up to 12,000. h have been investigated. Microstructural studies using transmission electron microscopy and atom probe tomography show that subtle Cr concentration fluctuations develop during aging. The amplitude of the concentration fluctuations is proportional to the hardness of the ferrite phase, and it is also proportional to the decrease in room temperature impact toughness during aging. The fracture behaviour of the alloy changes gradually from ductile to cleavage fracture, upon aging. The cracks were found to propagate through the ferrite phase, partly along deformation twin interfaces, and delamination between the austenite and ferrite phases was observed.
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| 8. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Experimental and theoretical investigation of precipitate coarsening rate in Z-phase strengthened steels
- 2018
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Ingår i: Materialia. - : Elsevier B.V.. - 2589-1529. ; 4, s. 247-254
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Tidskriftsartikel (refereegranskat)abstract
- Two Z-phase strengthened 12% Cr steels were investigated: they are similar in composition, however one steel contains Nb and the other contains Ta. Z-phase precipitates (CrMN, M = Nb or Ta) provide precipitation hardening for creep resistance at 650 °C in these steels. Experimental data based on the transmission electron microscopy investigation of the size evolution of Z-phase precipitates during isothermal ageing at 650 °C show that the Ta-based Z-phase benefits from a five times smaller coarsening constant compared to the Nb-based Z-phase. Theoretical calculations show that this is attributed to the smaller diffusivity of Ta compared to Nb in the steel matrix. Besides, comparing the size of the Ta-based Z-phase precipitates in the gauge and head portion of a crept specimen, it is shown that Z-phase coarsens faster under stress.
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| 9. |
- Xu, Xin, et al.
(författare)
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Effect of cooling rate after solution treatment on subsequent phase separation during aging of Fe-Cr alloys: A small-angle neutron scattering study
- 2017
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 134, s. 221-229
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Tidskriftsartikel (refereegranskat)abstract
- The effect of cooling rate after solution treatment on the initial structure of concentrated binary Fe-Cr alloys and the effect of the initial structure on phase separation during subsequent aging has been investigated. The nano-scale compositional fluctuations in the bulk of the alloys are studied using small angle neutron scattering and the results are compared with simulations using the Cahn-Hilliard-Cook (CHC) model. The alloys investigated represent different mechanisms of phase separation and at higher Cr content, when spinodal decomposition (SD) is favored, the initial Cr compositional fluctuations due to slow cooling after solution treatment reduce the kinetics of phase decomposition, whereas, at lower Cr composition when nucleation and growth is favored, the kinetics of phase decomposition is more rapid. Regardless of the nominal Cr composition of the alloy, the phase decomposition after extended aging up to 300 h at 748 K is always larger for the more non-random initial structure. The CHC modeling of the cooling process and subsequent initial aging (below 10 h) is in reasonable qualitative agreement with the experimental results for the Fe-40 wt.% Cr alloy decomposing via SD. However, the modeling approach must be refined for accurate quantitative modeling of the full SD process, including coarsening.
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| 10. |
- Zhou, Jing, et al.
(författare)
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Concurrent phase separation and clustering in the ferrite phase during low temperature stress aging of duplex stainless steel weldments
- 2012
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 60:16, s. 5818-5827
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Tidskriftsartikel (refereegranskat)abstract
- The concurrent phase separation and clustering of alloying elements in the ferrite phase of duplex stainless steel weldments after stress aging at 325 degrees C have been investigated by atom probe tomography analysis. Both phase separation, into Fe-rich and Cr-rich ferrite, and solute clustering were observed. Phase separation in the heat-affected zone (HAZ) is most pronounced in the high alloyed SAF 2507, followed by SAF 2205 and SAF 2304. Moreover Cu clustering was observed in the HAZ of SAF 2507. However, decomposition in the weld bead (25.10.4L) was more pronounced than in the HAZs, with both phase separation and clustering of Ni-Mn-Si-Cu. The observed differences in the decomposition behaviors in the HAZ and weld bead can be attributed to the high Ni content and the characteristic microstructure of the weld bead with high internal strains. In addition, an applied tensile stress during aging of weldments has been found to further promote the kinetics of phase separation and clustering.
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