Special Issue "Nanostructured Ferritic Alloys"

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystalline Materials".

Deadline for manuscript submissions: closed (31 December 2018).

Special Issue Editors

Dr. Stuart A. Maloy
E-Mail Website
Guest Editor
Los Alamos National Laboratory, Los Alamos, United States
Interests: mechanical testing; irradiation effects; advanced characterization; alloy development; radiation tolerant materials; joining; nuclear reactors
Dr. David T. Hoelzer
E-Mail Website
Guest Editor
Oak Ridge National Laboratory, Oak Ridge, United States
Interests: alloy development; irradiation effects; microstructure analysis; mechanical properties and deformation; joining; mechanical alloying

Special Issue Information

Dear Colleagues,

Nanostructured ferritic alloys (NFA) emerged near the beginning of the 21st century, with much excitement in nuclear energy research communities around the globe as a hopeful solution for achieving high performance combined with tolerance to degradation of mechanical properties upon exposure to elevated temperatures and high neutron doses that are envisioned in future advanced nuclear reactor technologies. NFAs evolved from oxide dispersion strengthened (ODS) alloys, which have been around for many decades, due to refinement in the microstructure consisting of an ultra-fine grain structure and high concentration of nano-size (~2–5 nm) oxide particles. Recent studies have shown that NFAs possess remarkable high-temperature strength, resulting in outstanding creep performance combined with very low swelling rates as revealed in high-dose heavy-ion irradiation experiments. NFAs are traditionally produced using mechanically alloying iron-based alloy powders with a dispersoid powder, such as yttria-Y2O3, followed by a consolidation method to produce solid products. Unfortunately, this manufacturing method presents many challenges that still require better understanding via research and development. One area that needs greater understanding involves processing conditions and composition influences that affect the uniformity in grain size, as well as the dispersion of nano-size oxide particles that greatly impact the performance and the production reproducibility of NFAs. The successful development of NFAs will require novel ideas and greater understanding of fabrication and joining technologies that historically have hindered the acceptance of ODS alloys for high performance applications in the past. The fabrication of NFAs into complex products, such a fuel cladding, presents challenges due to the high strength properties that do not favor plastic deformation and another problematic area are advanced joining methods that do not degrade the salient microstructure features of NFAs. Therefore, the development of NFAs offering enormous potential in demanding applications of advanced nuclear reactors along with the significant challenges in processing, fabrication and joining technologies is a matter of intensive research interest and a hot topic.

In this Special Issue, we welcome original research and review articles covering the state of the art and the latest research results covering “Nanostructured Ferritic Alloys” that aim to provide improved understanding of key processing parameters for achieving the important microstructural traits that are linked to higher levels of performance and understanding of deformation and radiation effects in harsh environments of advanced nuclear reactors operating at high temperatures to high neutron doses.

Dr. Stuart A. Maloy
Dr. David T. Hoelzer
Guest Editors

Manuscript Submission Information

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Keywords

  • Nanostructured ferritic alloys
  • Irradiation
  • Tensile properties
  • Processing
  • Fracture toughness
  • Welding

Published Papers (6 papers)

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Research

Open AccessArticle
The Effect of Internal Free Surfaces on Void Swelling of Irradiated Pure Iron Containing Subsurface Trenches
Crystals 2019, 9(5), 252; https://doi.org/10.3390/cryst9050252 - 15 May 2019
Abstract
We studied the effects of internal free surfaces on the evolution of ion-induced void swelling in pure iron. The study was initially driven by the motivation to introduce a planar free-surface defect sink at depths that would remove the injected interstitial effect from [...] Read more.
We studied the effects of internal free surfaces on the evolution of ion-induced void swelling in pure iron. The study was initially driven by the motivation to introduce a planar free-surface defect sink at depths that would remove the injected interstitial effect from ion irradiation, possibly enhancing swelling. Using the focused ion beam technique, deep trenches were created on a cross section of pure iron at various depths, so as to create bridges of thickness ranging from 0.88 μm to 1.70 μm. Samples were then irradiated with 3.5 MeV Fe2+ ions at 475 °C to a fluence corresponding to a peak displacement per atom dose of 150 dpa. The projected range of 3.5 MeV Fe2+ ions is about 1.2 μm so the chosen bridge thicknesses involved fractions of the ion range, thicknesses comparable to the mean ion range (peak of injected interstitial distribution), and thicknesses beyond the full range. It was found that introduction of such surfaces did not enhance swelling but actually decreased it, primarily because there were now two denuded zones with a combined stronger influence than that of the injected interstitial. The study suggests that such strong surface effects must be considered for ion irradiation studies of thin films or bridge-like structures. Full article
(This article belongs to the Special Issue Nanostructured Ferritic Alloys)
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Open AccessArticle
Numerical Simulation of Temperature and Fluid Fields in Solidification Process of Ferritic Stainless Steel under Vibration Conditions
Crystals 2019, 9(3), 174; https://doi.org/10.3390/cryst9030174 - 25 Mar 2019
Abstract
A three-dimensional model of a circular casting mold with a vibrating nucleus generator was established, and the characteristics of temperature and flow fields during the solidification process of ferritic stainless steel Cr17 in the casting mold were analyzed using finite element and finite [...] Read more.
A three-dimensional model of a circular casting mold with a vibrating nucleus generator was established, and the characteristics of temperature and flow fields during the solidification process of ferritic stainless steel Cr17 in the casting mold were analyzed using finite element and finite difference methods. A standard k-ε turbulent current model was adopted to simulate the temperature field, and a standard k-ε model in Reynolds-averaged Navier–Stokes equations (RANS) was employed to deal with the flow field. The temperature field diffuses outward with a positive temperature gradient. Low degrees of undercooling can prevent solidified shells from forming rapidly on the surface of the nucleus generator. The temperature perpendicular to the direction of vibration is lower than that in the direction of vibration. The flow field exhibits a heart-shaped distribution and spreads gradually outward. The uniform distribution of grains can be achieved at three different frequencies of vibration. The results show that the degree of undercooling affects the distribution of the temperature field while the frequency of vibration affects the flow field significantly. Under the conditions of undercooling of 540 K and vibration frequency of 1000 Hz, the region perpendicular to the vibration direction of the nucleus generator is the optimum area for equiaxed crystal formation. Full article
(This article belongs to the Special Issue Nanostructured Ferritic Alloys)
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Open AccessCommunication
Effect of High-Density Nanoparticles on Recrystallization and Texture Evolution in Ferritic Alloys
Crystals 2019, 9(3), 172; https://doi.org/10.3390/cryst9030172 - 24 Mar 2019
Cited by 1
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Nanostructured Ferritic Alloys)
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Open AccessCommunication
Radiation Tolerance in Nano-Structured Crystalline Fe(Cr)/Amorphous SiOC Composite
Crystals 2019, 9(3), 147; https://doi.org/10.3390/cryst9030147 - 13 Mar 2019
Abstract
The management of irradiation defects is one of key challenges for structural materials in current and future reactor systems. To develop radiation tolerant alloys for service in extreme irradiation environments, the Fe self-ion radiation response of nanocomposites composed of amorphous silicon oxycarbide (SiOC) [...] Read more.
The management of irradiation defects is one of key challenges for structural materials in current and future reactor systems. To develop radiation tolerant alloys for service in extreme irradiation environments, the Fe self-ion radiation response of nanocomposites composed of amorphous silicon oxycarbide (SiOC) and crystalline Fe(Cr) were examined at 10, 20, and 50 displacements per atom damage levels. Grain growth in width direction was observed to increase with increasing irradiation dose in both Fe(Cr) films and Fe(Cr) layers in the nanocomposite after irradiation at room temperature. However, compared to the Fe(Cr) film, the Fe(Cr) layers in the nanocomposite exhibited ~50% less grain growth at the same damage levels, suggesting that interfaces in the nanocomposite were defect sinks. Moreover, the addition of Cr to α-Fe was shown to suppress its grain growth under irradiation for both the composite and non-composite case, consistent with earlier molecular dynamic (MD) modeling studies. Full article
(This article belongs to the Special Issue Nanostructured Ferritic Alloys)
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Open AccessArticle
Effect of Cold Rolling on Recrystallization Behavior of Al-Free and Al-Added 15Cr-ODS Ferritic Steels
Crystals 2019, 9(3), 145; https://doi.org/10.3390/cryst9030145 - 12 Mar 2019
Abstract
The effect of cold rolling on the recrystallization behavior of Al-free and Al-added 15Cr-oxide dispersion strengthened (ODS) ferritic steels was investigated. The recrystallization of both steels are enhanced by cold rolling. The Al-free ODS steel with finer oxide particles is harder to recrystallize [...] Read more.
The effect of cold rolling on the recrystallization behavior of Al-free and Al-added 15Cr-oxide dispersion strengthened (ODS) ferritic steels was investigated. The recrystallization of both steels are enhanced by cold rolling. The Al-free ODS steel with finer oxide particles is harder to recrystallize than Al-added ODS steel with coarser oxide particles. The effect of Al addition on the recrystallization behavior is evident. It is estimated that the recrystallization temperature of Al-free and Al-added 15Cr-ODS ferritic steel is 900 °C and 1250 °C with the annealing period of 1 h. In Al-free ODS steel, a small hardening was observed in the temperature range between 850 °C and 1200 °C, while no such phenomenon was observed in Al-added ODS steel, which is indicative of retardation of recovery by finely dispersed oxide particles. Oxide particle growth is mostly dependent on annealing temperature, while recrystallization and grain growth are controlled by not only the temperature, but the cold rolling ratio, which alters the multiple factors such as dislocation density, initial grain shape and oxide particle dispersion morphology. The cold rolling direction also influences the grain morphology and grain orientation in Al-added ODS steel, and the second rolling in a perpendicular direction to the first cold rolling direction induces the rotation of the grains from <110> to <112>. The recrystallization temperature is not significantly changed by the cold rolling direction. Recrystallization after cold rolling appears to increase the {111} grain orientation on the cold rolled specimen surface. Full article
(This article belongs to the Special Issue Nanostructured Ferritic Alloys)
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Open AccessArticle
Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels?
Crystals 2018, 8(8), 306; https://doi.org/10.3390/cryst8080306 - 25 Jul 2018
Cited by 1
Abstract
Secondary cracks are known to absorb energy, retard primary crack propagation and initiate at lower loads than primary cracks. They are observed more often in hot-rolled than in hot-extruded ODS steels. In this work, the microstructural factors responsible for this observation are investigated. [...] Read more.
Secondary cracks are known to absorb energy, retard primary crack propagation and initiate at lower loads than primary cracks. They are observed more often in hot-rolled than in hot-extruded ODS steels. In this work, the microstructural factors responsible for this observation are investigated. Better understanding of these factors can lead to tailoring of improved ODS steels. Fracture toughness testing of two batches of 13Cr ODS steel, one hot-rolled and the other hot-extruded, was carried out. The fracture behaviour of secondary cracks was investigated using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Crystallographic texture and grain morphology play a predominant role in preventing secondary cracks in hot-extruded ODS steels. At lower temperatures, secondary cracks occur predominantly via transgranular cleavage. The fracture mode changes to ductile and intergranular at higher temperatures. Full article
(This article belongs to the Special Issue Nanostructured Ferritic Alloys)
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