Abstract:A nanostructured ferritic alloy (NFA), in the form of thin-walled tubing, was fabricated by hydrostatic extrusion of a mandrel mounted mother tube cut from a plate. The plate was processed by high energy ball milling of Fe-Y-Ti-O gas atomized and FeO powders, consolidated by hot extrusion, followed by hot cross rolling. The tube microstructure is dominated by an ultrahigh density of nm-scale Y-Ti-O complex oxide precipitates. Here, we examine the stability of the nano-oxides (NOs) under 3.5 MeV Fe +2 irradiati… Show more
“…1b. Similar behavior was reported using atom probe tomography techniques 41 . It is important to note that there is no grain-size dependent distribution of NO in the small and medium size grains.Figure 1( a ) Band contrast map showing the microstructure of 14YWT alloy before irradiation; EFTEM Fe and Ti jump ratio maps obtained from ( b ) small (<0.5 µm) and ( c ) large grains (>2 µm).…”
Section: Resultssupporting
confidence: 81%
“…3c. Moreover, there is no considerable change in NO size, number density and distribution after 6 dpa neutron irradiation at 385–430 °C, which is similar to the ion irradiations on these alloys 41 . On the other hand, size distribution of oxide particles in large grains is not provided due to poor statistics.…”
Section: Resultssupporting
confidence: 66%
“…It has been also shown that this core-shell structure becomes more obvious with the increase in NO size 47,48 . However, detailed atom probe tomography analysis on 14YWT alloys before irradiation was not conclusive about the core/shell structure in NOs that contain Cr 41 . Thus, even though EFTEM maps in Fig.…”
Nanostructured ferritic alloys are considered as candidates for structural components in advanced nuclear reactors due to a high density of nano-oxides (NOs) and ultrafine grain sizes. However, bimodal grain size distribution results in inhomogeneous NO distribution, or vice versa. Here, we report that density of NOs in small grains (<0.5 µm) is high while there are almost no NOs inside the large grains (>2 µm) before and after irradiation. After 6 dpa neutron irradiation at 385–430 °C, α′ precipitation has been observed in these alloys; however, their size and number densities vary considerably in small and large grains. In this study, we have investigated the precipitation kinetics of α′ particles based on the sink density, using both transmission electron microscopy and kinetic Monte Carlo simulations. It has been found that in the presence of a low sink density, α′ particles form and grow faster due to the existence of a larger defect density in the matrix. On the other hand, while α′ particles form far away from the sink interface when the sink size is small, Cr starts to segregate at the sink interface with the increase in the sink size. Additionally, grain boundary characteristics are found to determine the radiation-induced segregation of Cr.
“…1b. Similar behavior was reported using atom probe tomography techniques 41 . It is important to note that there is no grain-size dependent distribution of NO in the small and medium size grains.Figure 1( a ) Band contrast map showing the microstructure of 14YWT alloy before irradiation; EFTEM Fe and Ti jump ratio maps obtained from ( b ) small (<0.5 µm) and ( c ) large grains (>2 µm).…”
Section: Resultssupporting
confidence: 81%
“…3c. Moreover, there is no considerable change in NO size, number density and distribution after 6 dpa neutron irradiation at 385–430 °C, which is similar to the ion irradiations on these alloys 41 . On the other hand, size distribution of oxide particles in large grains is not provided due to poor statistics.…”
Section: Resultssupporting
confidence: 66%
“…It has been also shown that this core-shell structure becomes more obvious with the increase in NO size 47,48 . However, detailed atom probe tomography analysis on 14YWT alloys before irradiation was not conclusive about the core/shell structure in NOs that contain Cr 41 . Thus, even though EFTEM maps in Fig.…”
Nanostructured ferritic alloys are considered as candidates for structural components in advanced nuclear reactors due to a high density of nano-oxides (NOs) and ultrafine grain sizes. However, bimodal grain size distribution results in inhomogeneous NO distribution, or vice versa. Here, we report that density of NOs in small grains (<0.5 µm) is high while there are almost no NOs inside the large grains (>2 µm) before and after irradiation. After 6 dpa neutron irradiation at 385–430 °C, α′ precipitation has been observed in these alloys; however, their size and number densities vary considerably in small and large grains. In this study, we have investigated the precipitation kinetics of α′ particles based on the sink density, using both transmission electron microscopy and kinetic Monte Carlo simulations. It has been found that in the presence of a low sink density, α′ particles form and grow faster due to the existence of a larger defect density in the matrix. On the other hand, while α′ particles form far away from the sink interface when the sink size is small, Cr starts to segregate at the sink interface with the increase in the sink size. Additionally, grain boundary characteristics are found to determine the radiation-induced segregation of Cr.
“…The FeCrAl alloy has a low fraction of second-phase particles having sizes ranging between~100 nm and~3 µm. On the other hand, 14YWT alloys have a high density (~6 × 10 23 m −3 ) of NO (Y-Ti-O) particles [18,[20][21][22]. The red arrows in these figures indicate the second-phase particles.…”
Ferritic alloys are important for nuclear reactor applications due to their microstructural stability, corrosion resistance, and favorable mechanical properties. Nanostructured ferritic alloys having a high density of Y-Ti-O rich nano-oxides (NOs < 5 nm) are found to be extremely stable at high temperatures up to ~1100 °C. This study serves to understand the effect of a high density of nano-particles on texture evolution and recrystallization mechanisms in ferritic alloys of 14YWT (14Cr-3W-0.4Ti-0.21Y-Fe wt %) having a high density of nano-particles and dispersion-free FeCrAl (13Cr-5.2Al-0.05Y-2Mo-0.2Si-1Nb wt %). In order to investigate the recrystallization mechanisms in these alloys, neutron diffraction, electron backscattered diffraction, and in situ and ex situ transmission electron microscopy have been utilized. It has been observed that even though the deformation textures of both the 14YWT and FeCrAl alloys evolved similarly, resulting in either the formation (in FeCrAl alloy) or increase (in 14YWT) in γ-fiber texture, the texture evolution during recrystallization is different. While FeCrAl alloy keeps its γ-fiber texture after recrystallization, 14YWT samples develop a ε-fiber as a result of annealing at 1100 °C, which can be attributed to the existence of NOs. In situ transmission electron microscopy annealing experiments on 14YWT show the combination and growth of the lamellar grains rather than nucleation; however, the recrystallization and growth kinetics are slower due to NOs compared to FeCrAl.
“…Oxide dispersion-strengthened (ODS) ferritic steels are being developed in the nuclear industry because of their excellent irradiation resistance, [1][2][3] good mechanical properties, and corrosion resistance. [4][5][6] At present, there are many types of ferritic ODS steels that can be divided into Al-free and Al-containing types.…”
Herein, two kinds of Fe-9Cr-8Al oxide dispersion-strengthened (ODS) steels (prealloyed and postalloyed) are fabricated via mechanical alloying (MA), hot isostatic pressing (HIP), and subsequent hot forging. Microstructures of the milled powder and forged bulk materials are carefully characterized. The results show that the adding sequence of Al has a significant impact on the microstructure. For the postalloyed sample (adding Al during ball milling), a dual-phase structure composed of reticular Al-rich regions and a steel matrix is formed in the milled powder due to the highly mismatched deformability between the Al powder and the Fe-9Cr powder during ball milling. For the prealloyed sample (adding Al prior to ball milling), Al is solid solutionized into the steel matrix before ball milling, and there is no dual-phase structure in the milled powder. In the final bulk materials, the average grain size of the prealloyed sample is much larger than that of the postalloyed sample. Moreover, the matrix of the postalloyed sample has a bimodal grain structure consisting of coarse grains closely surrounded by fine grains, whereas the prealloyed sample has a relatively uniform distribution of coarse grains. The coarsening mechanisms of Al addition on the microstructure are also discussed.
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