An extensive experimental study characterizing the sequence of events that lead to the formation of a very high density of Y–Ti–O solute nanoclusters (NC) in mechanically alloyed, hot isostatically ...pressed ferritic stainless steels is reported. Yttria dissolves in the Fe–14Cr–3W(0.4Ti) powders during mechanical alloying. The dissolved Y and O, and when present Ti, subsequently precipitate during hot consolidation. The number densities and volume fractions of the NC decrease, and their radii increase, with increasing consolidation temperature. The NC form at 850 and 1000
°C in milled alloys containing Y, both with and without Ti additions. The presence of Ti refines the NC, and both Ti and high milling energy are necessary for the formation of NC at the highest consolidation temperatures of 1150
°C. However, the precise structure and composition of the NC are not well understood. Indeed, their character varies, depending on the alloy composition and processing variables.
Nanostructuredferritic alloys (NFAs) have the potential to make transformational contributions to developing advanced sources of fission and fusion energy. NFAs are Fe-Cr based ferritic stainless ...steels that contain an ultrahigh density of Y-Ti-O nanofeatures (NFs). The NFs provide both outstanding high temperature properties and remarkable tolerance to irradiation induced displacement damage as well as the degrading effects of transmutation product helium. Indeed, NFs can transform helium from a liability to an asset by forming a high density of nm-scale bubbles that act as sinks for point defects and helium may provide near immunity to radiation damage. This article outlines recent progress on engaging the challenges facing NFA development.
Nanostructured oxide dispersion strengthened (ODS) steels are considered candidates for nuclear fission and fusion applications at high temperature and dose. The complex oxide nanoclusters in these ...alloys provide high-temperature strength and are expected to afford better radiation resistance. Proton, heavy ion, and neutron irradiations have been performed to evaluate cluster stability in 14YWT and 9CrODS steel under a range of irradiation conditions. Energy-filtered transmission electron microscopy and atom probe tomography were used in this work to analyze the evolution of the oxide population.
Oxide dispersion-strengthened (ODS) steels are being developed and investigated for nuclear fission and nuclear fusion applications in Japan, Europe, and the United States. In addition, commercial ...ODS products are available and have been used in niche applications. Microstructural and mechanical properties studies have been conducted at Oak Ridge National Laboratory and elsewhere on various commercial and experimental ODS steels. Tensile and creep properties have been obtained and collected from literature and commercial sources. These data are compared to show the differences and similarities of different ODS steels, and observations are explained in terms of the microstructures of the steels.
Irradiations to 1.5
dpa at 300–750
°C were conducted to investigate the changes in mechanical properties of an advanced nanocluster strengthened ferritic alloy, designated 14YWT, and an oxide ...dispersion strengthened ferritic alloy ODS-EUROFER. Two non-dispersion strengthened variants, 14WT and EUROFER 97, were also irradiated and tested. Tensile results show 14YWT has very high tensile strengths and experienced some radiation-induced hardening, with an increase in room temperature yield strength of 125
MPa after irradiation, while results for ODS-EUROFER show a 275
MPa increase following irradiation. Master curve fracture toughness analysis show 14YWT has a cryogenic
T
o reference temperatures before and after irradiation of about −188 and −176
°C, respectively, and upper-shelf
K
J
Ic
values between 175 and 225
MPa√m. The favorable fracture toughness properties and resistance to radiation-induced changes in mechanical properties observed for 14YWT are attributed to a fine grain structure and high number density of Y–Ti–O nanoclusters.
A new larger heat of a 14YWT nanostructured ferritic alloy (NFA), FCRD NFA-1, was synthesized by ball milling FeO and argon atomized Fe-14Cr-3W-0.4Ti-0.2Y (wt%) powders, followed by hot extrusion, ...annealing and cross rolling to produce an ≈10mm-thick plate. NFA-1 contains a bimodal size distribution of pancake-shaped, mostly very fine scale, grains. The as-processed plate also contains a large population of microcracks running parallel to its broad surfaces. The small grains and large concentration of Y–Ti–O nano-oxides (NOs) result in high strength up to 800°C. The uniform and total elongations range from ≈1–8%, and ≈10–24%, respectively. The strength decreases more rapidly above ≈400°C and deformation transitions to largely viscoplastic creep by ≈600°C. While the local fracture mechanism is generally ductile-dimple microvoid nucleation, growth and coalescence, perhaps the most notable feature of tensile deformation behavior of NFA-1 is the occurrence of periodic delamination, manifested as fissures on the fracture surfaces.
Four ferritic alloys based on the composition Fe–14Cr–3W–0.4Ti (nominal wt%) were developed with a predominant dispersion of either oxide particles or nano-size Y-, Ti-, O-rich clusters, or ...nanoclusters (NC). Tensile specimens machined from the alloys were tested at room temperature and at temperatures ranging from ∼360
°C to 800
°C in air using a strain rate of 10
−3
s
−1. The results showed that the high-temperature strength of the NC strengthened alloys was significantly better than that of the oxide strengthened alloys. The room temperature yield strengths of the two alloys containing the NC were 1469
MPa and 1261
MPa while the yield strengths of the two oxide dispersion alloys were 819
MPa (Y–Ti-oxides) and 583
MPa (Ti-oxides). However, the ductility of the oxide strengthened alloys was better than that of the NC strengthened alloys.
Because of their superior high temperature strength and corrosion properties, a set of Ni-base alloys has been proposed for various in-core applications in Gen IV reactor systems. However, ...irradiation-performance data for these alloys is either limited or non-existent. A review is presented of the irradiation-performance of a group of Ni-base alloys based upon data from fast breeder reactor programs conducted in the 1975–1985 timeframe with emphasis on the mechanisms involved in the loss of high temperature ductility and the breakdown in swelling resistance with increasing neutron dose. The implications of these data for the performance of the Gen IV Ni-base alloys are discussed and possible pathways to mitigate the effects of irradiation on alloy performance are outlined. A radical approach to designing radiation damage-resistant Ni alloys based upon recent advances in mechanical alloying is also described.
A nanostructured ferritic alloy (NFA), 14YWT, was produced in the form of thin walled tubing. The stability of the nano-oxides (NOs) was determined under 3.5 MeV Fe+2 irradiations up to a dose of ...∼585 dpa at 450 °C. Transmission electron microscopy (TEM) and atom probe tomography (APT) show that severe ion irradiation results in a ∼25% reduction in size between the unirradiated and irradiated case at 270 dpa while no further reduction within the experimental error was seen at higher doses. Conversely, number density increased by ∼30% after irradiation. This ‘inverse coarsening’ can be rationalized by the competition between radiation driven ballistic dissolution and diffusional NO reformation. No significant changes in the composition of the matrix or NOs were observed after irradiation. Modeling the experimental results also indicated a dissolution of the particles.
Ferritic–martensitic (F–M) steels with 9wt.%Cr are important structural materials for use in advanced nuclear reactors. Alloying composition adjustment, guided by computational thermodynamics, and ...thermomechanical treatment (TMT) were employed to develop high strength 9Cr F–M steels. Samples of four heats with controlled compositions were subjected to normalization and tempering (N&T) and TMT, respectively. Their mechanical properties were assessed by Vickers hardness and tensile testing. Ta-alloying showed significant strengthening effect. The TMT samples showed strength superior to the N&T samples with similar ductility. All the samples showed greater strength than NF616, which was either comparable to or greater than the literature data of the PM2000 oxide-dispersion-strengthened (ODS) steel at temperatures up to 650°C without noticeable reduction in ductility. A variety of microstructural analyses together with computational thermodynamics provided rational interpretations on the strength enhancement. Creep tests are being initiated because the increased yield strength of the TMT samples is not able to deduce their long-term creep behavior.
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