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G Eggeler - One of the best experts on this subject based on the ideXlab platform.
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how dislocation substructures evolve during long term creep of a 12 cr Tempered martensitic ferritic steel
Scripta Materialia, 2010Co-Authors: J Pesicka, Alexander Hartmaier, A Aghajani, Ch Somsen, G EggelerAbstract:We document the evolution of dislocation densities in Tempered Martensite ferritic steels during long-term aging and creep. Scanning transmission electron microscopy in combination with a high-angle annular dark-field detector is used to study dislocations in a 12% Cr steel. During aging, the dislocation density quickly decreases by a factor 2 and then remains constant. Long-term creep results in an initial decrease by a factor 10, and after this sharp drop, the dislocation density continues to decrease.
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on the formation and growth of mo rich laves phase particles during long term creep of a 12 chromium Tempered Martensite ferritic steel
Scripta Materialia, 2009Co-Authors: A Aghajani, Frank Richter, Christoph Somsen, S G Fries, Ingo Steinbach, G EggelerAbstract:It was recently reported (Aghajani et al. [1] ) that Laves phase particles do not reach thermodynamic equilibrium as they form and grow during long-term creep (120 MPa, 823 K, 139 971 h) of a 12% chromium Tempered Martensite ferritic steel. In the present work we show that this is due to the slow diffusion of silicon in the steel matrix.
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evolution of microstructure during long term creep of a Tempered Martensite ferritic steel
2009Co-Authors: A Aghajani, G Eggeler, D RaabeAbstract:In fossil befeuerten Kraftwerken kommen Werkstoffe auf Basis von angelassenen martensitschen Chromstahlen zum Einsatz. Die vorliegende Arbeit beschaftigt sich deshalb mit der Entwicklung der Mikrostruktur eines 12 % Chromstahls im Bereich des Langzeit-Kriechens bis zu einer Bruchzeit von ungefahr 16 Jahren bei 550°C und 120 MPa. Die mikrostrukturelle Charakterisierung erfolgte hauptsachlich uber Transmissionselektronenmikroskopie (TEM) und Elektronen-back-scatter diffraction (EBSD), wobei auch Untersuchungen an Proben aus unterbrochenen Kriechversuchen durchgefuhrt wurden. Die vorliegenden Untersuchungen zeigen, dass bei Alterung die Versetzungsdichte konstant bleibt, wahrend beim Langzeit-Kriechen die Versetzungsdichte kontinuierlich abnimmt. Als Ausscheidungsteilchen wurden Cr-reiche M23C6, VX Karbide und Laves-Phase identifiziert. Der Volumenanteil der Laves-Phasen-Teilchen nimmt wahrend des Langzeit-Kriechens zu. Begrundet wird dies uber die langsame Diffusion von Silizium.
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on the effect of long term creep on the microstructure of a 12 chromium Tempered Martensite ferritic steel
Acta Materialia, 2009Co-Authors: A Aghajani, Ch Somsen, G EggelerAbstract:Abstract In the present study we investigate the evolution of the microstructure of a 12% Cr Tempered Martensite ferritic steel under conditions of long-term aging and creep (823 K, 120 MPa, t R = 139,971 h). We show how subgrains coarsen, that the close correlation between carbides and subgrain boundaries loosens during long-term creep and that the frequency of small-angle boundaries increases. All these elementary deformation processes have been discussed in short-term creep studies. The present study shows that they also govern long-term creep. However, during long-term creep, precipitation and coarsening reactions occur that are not observed during short-term creep. Three types of particles (M 23 C 6 , VX and Laves-phase) were identified after long-term creep. M 23 C 6 particles coarsen at constant volume fraction and establish their equilibrium concentration after 51,072 h; VX particles are stable; and the Laves-phase particles never reach thermodynamic equilibrium.
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on the contribution of carbides and micrograin boundaries to the creep strength of Tempered Martensite ferritic steels
Acta Materialia, 2007Co-Authors: Aleksander Kostka, Kwan Tak, Ralph Jorg Hellmig, Yuri Estrin, G EggelerAbstract:Abstract The objective of the present study is to find out how much micrograin boundary carbides contribute to the creep strength of Tempered Martensite ferritic steels. For this purpose we compare the creep behaviour and the microstructural stability of a Tempered Martensite ferritic steel (German grade: X20) with that of a strongly deformed binary Fe10Cr alloy. The binary Fe10Cr alloy was subjected to equal channel angular pressing (ECAP). The ECAP parameters are adjusted such that the Fe10Cr alloy develops an ultrafine grain microstructure which is very similar to the microstructure of X20 with one important difference: there are no carbides on micrograin boundaries. We perform creep experiments for both materials and compare the creep curves as well as the evolution of microstructures during creep. The results show that boundary particles keep ultrafine grain structures from recrystallising. They moreover slow down micrograin boundary migration and impede reactions between dislocations and subgrain boundaries. It is concluded that micrograin boundary carbides reduce creep rates by several orders of magnitude.
Aleksander Kostka - One of the best experts on this subject based on the ideXlab platform.
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the nucleation of mo rich laves phase particles adjacent to m23c6 micrograin boundary carbides in 12 cr Tempered Martensite ferritic steels
Acta Materialia, 2015Co-Authors: M I Isik, Aleksander Kostka, Victoria A Yardley, Konda Gokuldoss Pradeep, Maria Jazmin Duarte, Pyuckpa Choi, Dierk RaabeAbstract:Abstract We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr Tempered Martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
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on the nucleation of laves phase particles during high temperature exposure and creep of Tempered Martensite ferritic steels
Acta Materialia, 2014Co-Authors: M I Isik, Aleksander KostkaAbstract:Abstract This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a Tempered Martensite ferritic steel with 12 wt.% Cr and 1 wt.% Mo. The material was exposed to 550 °C for time intervals between 864 and 81,984 h. For comparison, a few creep tests were carried out at 550 °C and 120 MPa (duration between 864 and 12,456 h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M 23 C 6 carbides.
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on the contribution of carbides and micrograin boundaries to the creep strength of Tempered Martensite ferritic steels
Acta Materialia, 2007Co-Authors: Aleksander Kostka, Kwan Tak, Ralph Jorg Hellmig, Yuri Estrin, G EggelerAbstract:Abstract The objective of the present study is to find out how much micrograin boundary carbides contribute to the creep strength of Tempered Martensite ferritic steels. For this purpose we compare the creep behaviour and the microstructural stability of a Tempered Martensite ferritic steel (German grade: X20) with that of a strongly deformed binary Fe10Cr alloy. The binary Fe10Cr alloy was subjected to equal channel angular pressing (ECAP). The ECAP parameters are adjusted such that the Fe10Cr alloy develops an ultrafine grain microstructure which is very similar to the microstructure of X20 with one important difference: there are no carbides on micrograin boundaries. We perform creep experiments for both materials and compare the creep curves as well as the evolution of microstructures during creep. The results show that boundary particles keep ultrafine grain structures from recrystallising. They moreover slow down micrograin boundary migration and impede reactions between dislocations and subgrain boundaries. It is concluded that micrograin boundary carbides reduce creep rates by several orders of magnitude.
A Aghajani - One of the best experts on this subject based on the ideXlab platform.
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how dislocation substructures evolve during long term creep of a 12 cr Tempered martensitic ferritic steel
Scripta Materialia, 2010Co-Authors: J Pesicka, Alexander Hartmaier, A Aghajani, Ch Somsen, G EggelerAbstract:We document the evolution of dislocation densities in Tempered Martensite ferritic steels during long-term aging and creep. Scanning transmission electron microscopy in combination with a high-angle annular dark-field detector is used to study dislocations in a 12% Cr steel. During aging, the dislocation density quickly decreases by a factor 2 and then remains constant. Long-term creep results in an initial decrease by a factor 10, and after this sharp drop, the dislocation density continues to decrease.
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on the formation and growth of mo rich laves phase particles during long term creep of a 12 chromium Tempered Martensite ferritic steel
Scripta Materialia, 2009Co-Authors: A Aghajani, Frank Richter, Christoph Somsen, S G Fries, Ingo Steinbach, G EggelerAbstract:It was recently reported (Aghajani et al. [1] ) that Laves phase particles do not reach thermodynamic equilibrium as they form and grow during long-term creep (120 MPa, 823 K, 139 971 h) of a 12% chromium Tempered Martensite ferritic steel. In the present work we show that this is due to the slow diffusion of silicon in the steel matrix.
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evolution of microstructure during long term creep of a Tempered Martensite ferritic steel
2009Co-Authors: A Aghajani, G Eggeler, D RaabeAbstract:In fossil befeuerten Kraftwerken kommen Werkstoffe auf Basis von angelassenen martensitschen Chromstahlen zum Einsatz. Die vorliegende Arbeit beschaftigt sich deshalb mit der Entwicklung der Mikrostruktur eines 12 % Chromstahls im Bereich des Langzeit-Kriechens bis zu einer Bruchzeit von ungefahr 16 Jahren bei 550°C und 120 MPa. Die mikrostrukturelle Charakterisierung erfolgte hauptsachlich uber Transmissionselektronenmikroskopie (TEM) und Elektronen-back-scatter diffraction (EBSD), wobei auch Untersuchungen an Proben aus unterbrochenen Kriechversuchen durchgefuhrt wurden. Die vorliegenden Untersuchungen zeigen, dass bei Alterung die Versetzungsdichte konstant bleibt, wahrend beim Langzeit-Kriechen die Versetzungsdichte kontinuierlich abnimmt. Als Ausscheidungsteilchen wurden Cr-reiche M23C6, VX Karbide und Laves-Phase identifiziert. Der Volumenanteil der Laves-Phasen-Teilchen nimmt wahrend des Langzeit-Kriechens zu. Begrundet wird dies uber die langsame Diffusion von Silizium.
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on the effect of long term creep on the microstructure of a 12 chromium Tempered Martensite ferritic steel
Acta Materialia, 2009Co-Authors: A Aghajani, Ch Somsen, G EggelerAbstract:Abstract In the present study we investigate the evolution of the microstructure of a 12% Cr Tempered Martensite ferritic steel under conditions of long-term aging and creep (823 K, 120 MPa, t R = 139,971 h). We show how subgrains coarsen, that the close correlation between carbides and subgrain boundaries loosens during long-term creep and that the frequency of small-angle boundaries increases. All these elementary deformation processes have been discussed in short-term creep studies. The present study shows that they also govern long-term creep. However, during long-term creep, precipitation and coarsening reactions occur that are not observed during short-term creep. Three types of particles (M 23 C 6 , VX and Laves-phase) were identified after long-term creep. M 23 C 6 particles coarsen at constant volume fraction and establish their equilibrium concentration after 51,072 h; VX particles are stable; and the Laves-phase particles never reach thermodynamic equilibrium.
M I Isik - One of the best experts on this subject based on the ideXlab platform.
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the nucleation of mo rich laves phase particles adjacent to m23c6 micrograin boundary carbides in 12 cr Tempered Martensite ferritic steels
Acta Materialia, 2015Co-Authors: M I Isik, Aleksander Kostka, Victoria A Yardley, Konda Gokuldoss Pradeep, Maria Jazmin Duarte, Pyuckpa Choi, Dierk RaabeAbstract:Abstract We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr Tempered Martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
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on the nucleation of laves phase particles during high temperature exposure and creep of Tempered Martensite ferritic steels
Acta Materialia, 2014Co-Authors: M I Isik, Aleksander KostkaAbstract:Abstract This paper reports on the formation of an Mo-rich Laves phase during high-temperature exposure and creep of a Tempered Martensite ferritic steel with 12 wt.% Cr and 1 wt.% Mo. The material was exposed to 550 °C for time intervals between 864 and 81,984 h. For comparison, a few creep tests were carried out at 550 °C and 120 MPa (duration between 864 and 12,456 h). All tests were interrupted after specific time periods and microstructures were investigated using transmission electron microscopy and atom probe tomography. Laves phase formation occurs during both heat treatment and creep. Creep stress and strain have no significant effect on the early stages of Laves phase formation. In the present work we show that prior to Laves phase nucleation Si and Mo segregate to micrograin boundaries, where subsequently Laves phase particles appear next to M 23 C 6 carbides.
J Pesicka - One of the best experts on this subject based on the ideXlab platform.
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how dislocation substructures evolve during long term creep of a 12 cr Tempered martensitic ferritic steel
Scripta Materialia, 2010Co-Authors: J Pesicka, Alexander Hartmaier, A Aghajani, Ch Somsen, G EggelerAbstract:We document the evolution of dislocation densities in Tempered Martensite ferritic steels during long-term aging and creep. Scanning transmission electron microscopy in combination with a high-angle annular dark-field detector is used to study dislocations in a 12% Cr steel. During aging, the dislocation density quickly decreases by a factor 2 and then remains constant. Long-term creep results in an initial decrease by a factor 10, and after this sharp drop, the dislocation density continues to decrease.
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on the relation between carbide density and grain boundary character in Tempered Martensite ferritic steels
Steel Research International, 2005Co-Authors: Andre Dronhofer, J Pesicka, G EggelerAbstract:In the present study we investigate the microstructure of Tempered Martensite ferritic steels. It is well known that inside former austenite grains and inside packets of former Martensite laths ultrafine micro grains (average size near: 1 pm) govern the strength of this material class. Micro grain boundaries are decorated by carbides (average size after creep near: 0.05 μm). However, in transmission electron micrographs it is commonly found that there are micro grain boundaries with a high carbide density while there are others where no carbides can be detected. In the present study we make an attempt to decide whether the crystallographic character of micro grain boundaries can be related to the number density of carbides at the boundaries. Kikuchi line diffraction patterns were used to determine the misorientation angle between two adjacent micro grains; we select only micro grain boundaries which represent - and - twist boundaries. A quantitative microstructural analysis was performed to determine the density of carbides on boundaries. Our results are discussed on the basis of general tendencies which were reported for grain boundaries in the literature.
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the evolution of dislocation density during heat treatment and creep of Tempered Martensite ferritic steels
Acta Materialia, 2003Co-Authors: J Pesicka, R Kužel, A Dronhofe, G EggeleAbstract:Abstract The evolution of dislocation density in two Tempered Martensite ferritic steels (a 12% and a 9% chromium steel, “X20” and “P91”) during heat treatment and creep is analyzed using transmission electron microscopy (TEM) and X-ray diffraction (XRD); both methods yield results which are in good agreement when rationalizing the XRD-data based on densities of free dislocations. It is shown that due to the intermediate formation of Martensite, standard heat treatments produce very high dislocation densities in Tempered Martensite ferritic steels (TMFSs). Long term tempering and creep are characterized by a decrease of dislocation density; but dislocation densities are still high as compared to alloys where the formation of microstructure does not involve a martensitic transformation. A heterogeneous microstructure after long term tempering and creep is a characteristic feature of TMFSs. Micro grains with high dislocation densities co-exist next to micro grains without dislocations. The XRD method yields average data and cannot account for this microstructural heterogeneity; but it supports the TEM results which in isolation suffer from providing insufficient statistics. The results of the present study are discussed in the light of earlier work published in the literature and contribute to a better understanding of the role of free dislocations in TMFSs during tempering, creep and high temperature low cycle fatigue.
