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The commercialization of nanocrystalline metals and alloys is currently entering a renaissance period. Many of the processing and consolidation challenges that have haunted them are now more fully understood, opening the doors for stabilized nanocrystalline metals to be produced on a bulk scale. While challenges remain, the increased volume at which these materials are being supplied is for the first time allowing for investigations into more traditional methods of metal working, such as extruding, rolling, forming, and forging. Recently, the manufacturing science has been developed to allow nanocrystalline Cu-Ta alloys to progress to this point. This article therefore builds upon the last decade of evolutionary progression within the family of stabilized nanocrystalline Cu-Ta alloys by presenting some of the first findings related to scaled powder synthesis, production of billets, thin sheets, and foils. The mechanical performance and physical properties relevant to forming electrical contacts and pins including, tensile, J-integral fracture toughness, Charpy Impact, bi-axial tension, and conductivity are reported. This introductory investigation into forming such a novel material, provides evidence to these alloys potential at bridging the gap between being a scientific curiosity to that of a real engineering material. • The commercialization of nanocrystalline metals and alloys is currently entering a renaissance period. Many of the processing and consolidation challenges that have haunted them are now more fully understood, opening the doors for stabilized nanocrystalline metals to be produced on a bulk scale. While challenges remain, the increased volume at which these materials are being supplied, is for the first time allowing for investigations into more traditional methods of metal working, such as extruding, rolling, forming and forging. Recently, the manufacturing science has been developed to allow nanocrystalline Cu-Ta alloys, to progress to this point. This article therefore builds upon the last decade of progression within the family of stabilized nanocrystalline Cu-Ta alloys by presenting some of the first findings related to scaled powder synthesis, production of billets, thin sheets and foils. The mechanical performance and physical properties relevant to forming electrical contacts and pins including, tensile, J-integral fracture toughness, Charpy Impact, bi-axial tension, and conductivity are reported. This introductory investigation into forming such a novel material, provides evidence to these alloys potential at bridging the gap between being a scientific curiosity to that of a real engineering material. [ABSTRACT FROM AUTHOR]

The investigation of the mechanical alloying (MA) conditions for producing the advanced oxide dispersion strengthened (ODS) 14YWT ferritic alloy led to significant improvements in balancing the strength, ductility and fracture toughness properties while still maintaining the salient microstructural features consisting of ultra-fine grains and high concentration of Y-, Ti- and O-enriched nanoclusters. The implemented changes to the processing conditions included reducing the contamination of the powder during ball milling, applying a pre-extrusion annealing treatment on the ball milled powder and exploring different extrusion temperatures at 850 °C (SM170 heat), 1000 °C (SM185) and 1150 °C (SM200). The microstructural studies of the three 14YWT heats showed similarities in the dispersion of nanoclusters and sub-micron size grains, indicating the microstructure was insensitive to the different extrusion conditions. Compared to past 14YWT heats, the three new heats showed lower strength, but higher ductility levels between 25 and 800 °C and significantly higher fracture toughness values between 25 °C and 700 °C. The lower contamination levels of O, C and N achieved with improved ball milling conditions plus the slightly larger grain size were identified as important factors for improving the balance in mechanical properties of the three heats of 14YWT. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The effects of neutron irradiation to a dose of 3dpa at 600°C and creep for 38,555h at 800°C on the microstructure of a commercial MA957 alloy were investigated by atom probe tomography. The size, number density and composition of the 2-nm-diameter Ti-, Y-, O-enriched nanoclusters were similar in the unirradiated, crept and neutron irradiated conditions indicating that the microstructure of this nanostructured ferritic alloy has remarkable tolerance to radiation damage. [Copyright &y& Elsevier]

Abstract: Thermal tests exposed V–4Cr–4Ti in static liquid lithium at 700 and 800°C for 250, 500, and 1000h. Post-exposure examination included chemical analysis of interstitial impurities in V–4Cr–4Ti to monitor impurity transfer, and tensile tests at room temperature and 500°C. Microstructures were characterized by room-temperature electrical resistivity measurements and transmission electron microscopy. Oxygen was not depleted from V–4Cr–4Ti when nitrogen pickup occurred during lithium exposures. In spite of a significant increase in interstitial impurity concentration, the matrix interstitial solute content was reduced due to precipitation. Plate-shaped precipitates in the matrix and globular precipitates at grain boundaries were formed during lithium exposures at 700°C, while only globular precipitates were observed at grain boundaries at 800°C. Increases in strength, decreases in ductility, and reduced dynamic strain aging resulted. Ductility remained high after 1000h exposures at both 700 and 800°C. [Copyright &y& Elsevier]

Abstract: The paper presents irradiation creep data for V–4Cr–4Ti irradiated to 3.7dpa at 425 and 600°C in the HFIR-17J experiment. Creep deformation was characterized by measuring diametral changes of pressurized creep tubes before and after irradiation. It was found that the creep strain rate of the US Heat 832665 of V–4Cr–4Ti exhibited a linear relationship with stress up to ∼180MPa at 425°C with a creep coefficient of 2.50×10−6 MPa−1 dpa−1. A linear relationship between creep rate and applied stress was observed below ∼110MPa at 600°C with a creep coefficient of 5.41×10−6 MPa−1 dpa−1; non-linear creep behavior was observed above ∼110MPa, and it may not be fully accounted by invoking thermal creep. The bilinear creep behavior observed in the same alloy irradiated in BR-10 was not observed in this study. [Copyright &y& Elsevier]

Abstract: Advanced nanostructured ferritic alloys (NFAs) containing a high density of ultra-fine (2–5nm) nanoclusters (NCs) enriched in Y, Ti, and O are considered promising candidates for structural components in future nuclear systems. The superior tensile strengths of NFAs relative to conventional oxide dispersion strengthened ferritic alloys are attributed to the high number density of NCs, which may provide effective trapping centers for point defects and transmutation products produced during neutron irradiation. This paper summarizes preliminary tensile and fracture toughness data for an advanced NFA, designated 14YWT, currently being developed at Oak Ridge National Laboratory. For this study, an alloy designated 14WT was manufactured using the same production parameters used to produce 14YWT but without the Y2O3 addition during ball milling required for NC formation in order to quantify the effect of the NCs on mechanical properties. Tensile specimens produced from both alloys were irradiated at 300, 580, and 670°C to 1.5 displacements per atom (dpa), while 14YWT fracture toughness specimens were irradiated at 300°C to 1.5dpa. Tensile strengths for 14YWT were found to be about two times greater than 14WT for both irradiated and unirradiated conditions, with yield strength for 14YWT decreasing from ∼1450MPa at 26°C to ∼700MPa at 600°C. Moderate radiation-induced hardening (50–200MPa) and reduction in ductility was observed for 14YWT for all irradiation conditions and test temperatures. In contrast, 14WT exhibited significant hardening (∼250MPa) for the 300°C irradiated specimens, while almost no hardening was observed for the 580 and 670°C irradiated specimens. Fracture toughness results showed 14YWT in the unirradiated condition had a fracture toughness transition temperature (FTTT) around −150°C and upper-shelf K JIc values around 175MPa√m. Results from irradiated 14YWT fracture toughness tests were found to closely mirror the unirradiated data and no shift in FTTT or decrease in K JIc values were observed following neutron irradiation to 1.5dpa at 300°C. [Copyright &y& Elsevier]

Abstract: Because of the high solubility and mobility of oxygen in vanadium, composition control during the fabrication of thin (0.25mm) wall tubing from vanadium alloys by cold drawing and annealing, presents a technological challenge. During intermediate annealing at 1000°C in the 10−4 Torr vacuum regime, oxygen penetration into the tube wall is controlled by the development of a semi-protective surface oxide (linear-parabolic oxidation conditions); oxygen-hardened surface layers lead to a high incidence of surface cracking during the final stages of cold drawing. In the 10−5 Torr regime, under linear kinetic oxidation conditions, rapid oxygen penetration results in unacceptably high levels of oxygen pick-up (∼1500wppm). In the 10−7 Torr vacuum regime, molecular impingement rates are reduced to the point where overall oxygen pick-up is reduced to <100wppm. Improved cleaning/gettering procedures also restrict carbon and nitrogen pick-up to very low levels. [Copyright &y& Elsevier]

Abstract: 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 1469MPa and 1261MPa while the yield strengths of the two oxide dispersion alloys were 819MPa (Y–Ti-oxides) and 583MPa (Ti-oxides). However, the ductility of the oxide strengthened alloys was better than that of the NC strengthened alloys. [Copyright &y& Elsevier]

Abstract: The oxide dispersion strengthened (ODS) steels are being developed and investigated for fission and fusion structural applications in Japan, Europe, and the United States. In this paper, the fracture toughness and tensile properties of an ODS steel with nominal composition Fe–12Cr–2.5W–0.4Ti–0.25Y2O3 (designated 12YWT) were investigated and compared to commercial reduced-activation ferritic/martensitic (RAFM) steels. Small, 1.6-mm thick and 3.2-mm wide, 3-point bend specimens were used for fracture toughness characterization of this steel. Specimens were fatigue pre-cracked to initial crack length (a) to width (W) ratio of 0.45 and tested quasi-statically in the temperature range from −50°C to 550°C. Specimens tested up to 50°C exhibited elastic–plastic cleavage fracture that was typical for the transition region in ferritic steels. The master curve transition temperature, T 0, for the 12YWT alloy was determined to be 102°C. Specimens tested at 100°C and higher exhibited ductile stable crack growth. In these cases, the J-integral at the onset of stable crack growth (J Ic) was determined from the J–R curves. Their equivalent values in terms of stress intensity factor, , were about 93MPa√m at 100°C and decreased to 53MPa√m at 550°C. This study showed that oxide dispersion strengthening resulted in significant decreases in the toughness properties compared to commercial RAFM steels, although appreciable level of toughness was still retained. Tensile tests were performed at temperatures between room temperature and 800°C. As expected, this material exhibited very high yield strength, ∼1300MPa, at room temperature. For comparison, the yield strength of commercial RAFM steels is about 550MPa. Yield strength of 12YWT decreases as test temperature increases and at 800°C it is about 323MPa. [Copyright &y& Elsevier]

Abstract: The ultrafine scale microstructure of three MA/ODS ferritic alloys (12YWT, 14YWT and MA957) have been characterized by atom probe tomography in the as-received condition and after isothermal heat treatments at temperatures of up to 24h at 1300°C (∼85% T m). Atom probe tomography revealed the presence of a high number density of 2–4nm diameter titanium-, yttrium- and oxygen-enriched particles. These ultrafine particles were found to be extremely resistant to coarsening during isothermal aging at 1300°C. Solute segregation of the alloying elements to dislocations and grain boundaries was also observed. [Copyright &y& Elsevier]

The microstructure of a commercial mechanically-alloyed oxide dispersion strengthened MA957 alloy has been characterized in the as-received condition and after annealing for up to 24 h at 1300 °C (∼0.85 Tm) by atom probe tomography and electron microscopy. Atom probe tomography revealed a high number density of ultrafine 2-nm-diameter Ti-, Y- and O-enriched particles in the ferrite matrix in the as-received condition. The size increased and the number density of these particles decreased during the annealing treatment for 1 and 24 h at 1300 °C. Some coarser (∼10 nm) Ti-, Y- and O-enriched precipitates were also observed at the grain boundaries. No significant grain growth or recrystallization was observed during the high temperature annealing treatment. [Copyright &y& Elsevier]

Sodium fast reactor cores present a truly challenging environment for the fuel cladding and core structural materials that limit achievable fuel burnup in these systems. Modern nanostructured ferritic alloys have the potential to deliver the desired high performance characteristics where historic austenitic and ferritic/martensitic alloys fall short. In this paper, a new nanostructured ferritic alloy, OFRAC (O ak Ridge F ast R eactor A dvanced Fuel C ladding), is developed and demonstrated in cladding geometry with a length > 1 m. The alloy composition and microstructure are tailored to deliver dramatically improved strength and creep resistance. At 600 °C the ultimate tensile strength is ∼600 MPa and the stress to induce a steady state strain rate of 10−6 s−1 is 500 MPa vs. 200 MPa for traditional ferritic/martensitic alloys. A very high defect sink density was engineered in these alloys that is expected to further enhance the traditional good swelling and irradiation creep resistance of ferritic/martensitic steels. These significant improvements along with the demonstrated viability for seamless cladding production indicate that this alloy is a compelling and viable choice for sodium fast reactor fuel cladding applications. [ABSTRACT FROM AUTHOR]

Abstract: Friction stir welding was investigated for joining specimens of the ODS 14YWT ferritic alloy together and to an F82H tempered martensitic steel plate. The FSW run was performed using a polycrystalline boron nitride tool and resulted in good bonding between 14YWT/14YWT and 14YWT/F82H. Joints and interfaces were observed by light microscopy and SEM analysis to be narrow in width. The ultra-small grain size of 14YWT increased by a factor up to 4 while that of F82H decreased by a considerable amount in the weld zones. The TEM analysis showed no significant changes in the size of the oxygen-enriched nanoclusters in the weld zone of 14YWT. However, defects such as a wormhole on the advancing side of the weld zone in 14YWT and small pores associated with joints and interfaces were observed in the FSW sample. The hardness measurements from unaffected zone into weld zones showed ∼20% decrease in hardness for 14YWT (from ∼500VH to ∼380VH) and ∼100% increase in hardness of F82H (from ∼220VH to ∼440VH). [Copyright &y& Elsevier]

Abstract: The stress relaxation behavior of 14YWT and ODS-Eurofer97 was examined within the framework of an internal-variable theory of inelastic deformation. Stress versus strain rate curves obtained by stress relaxation tests for 14YWT were described well by the equations for grain-matrix deformation while those of ODS-Eurofer97 were fitted with the combined curves of grain matrix deformation and grain boundary sliding. The sudden drop of total elongation of 14YWT was discussed in light of fracture surface observations. Grain boundary decohesion at 900°C was identified. [Copyright &y& Elsevier]

Abstract: Stress relaxation behavior was investigated for the nanoclusters/dispersoids-strengthened steels including the nanostructured ferritic alloy 14YWT (SM10), oxide-dispersion strengthened (ODS) Eurofer97, and commercial ODS steel PM2000. The stress relaxation tests were carried out at high temperatures ranging from 600 to 1000°C. Overall, the relaxation rates of 14YWT and ODS-Eurofer97 were lower than that of PM2000. To analyze the strain rate sensitivity of the alloys, the load drop–time curves were converted to the stress–strain rate curves. In the log–log plots of these curves, no significant change in slope was observed in the strain rate range of 2×10−5–1×10−3s−1. At 600°C, 14YWT and ODS-Eurofer97 have similar activation values of ∼50b 3 while PM2000 has ∼100b 3. Above 700°C, the differences of the activation energy among alloys become more noticeable with increasing temperature. The activation energies of the three alloys were derived and compared. The rate-controlling mechanisms in the stress relaxation of the three nanoclusters/dispersoids-hardened alloys include dislocation glide and climb, and further study is necessary to clarify detailed contributing mechanisms. [Copyright &y& Elsevier]

Abstract: High temperature tensile fracture behavior has been characterized for the nanostructured ferritic alloy 14YWT (SM10 heat). Uniaxial tensile tests were performed at temperatures ranging from room temperature to 1000°C in vacuum at a nominal strain rate of 10−3 s−1. Comparing with the existing oxide dispersion strengthened (ODS) steels such as Eurofer 97 and PM2000, the nanostructured alloy showed much higher yield and tensile strength, but with lower elongation. Microstructural characterization for the tested specimens was focused on the details of fracture morphology and mechanism to provide a feedback for process improvement. Below 600°C, the fracture surfaces exhibited a quasi-brittle behavior presented by a mixture of dimples and cleavage facets. At or above 600°C, however, the fracture surfaces were fully covered with fine dimples. Above 700°C dimple formation occurred by sliding and decohesion of grain boundaries. It was notable that numerous microcracks were observed on the side surface of broken specimens. Formation of these microcracks is believed to be the main origin of the poor ductility of 14YWT alloy. It is suggested that a grain boundary strengthening measure is essential to improve the fracture property of the alloy. [ABSTRACT FROM AUTHOR]

Abstract: Irradiations to 1.5dpa 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 125MPa after irradiation, while results for ODS-EUROFER show a 275MPa 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 values between 175 and 225MPa√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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: In the Generation IV Materials Program cross-cutting task, irradiation and testing were carried out to address the issue of high temperature irradiation effects with selected current and potential candidate metallic alloys. The materials tested were (1) a high-nickel iron-base alloy (Alloy 800H); (2) a nickel-base alloy (Alloy 617); (3) two advanced nano-structured ferritic alloys (designated 14YWT and 14WT); and (4) a commercial ferritic–martensitic steel (annealed 9Cr–1MoV). Small tensile specimens were irradiated in rabbit capsules in the High-Flux Isotope Reactor at temperatures from about 550 to 700°C and to irradiation doses in the range 1.2–1.6dpa. The Alloy 800H and Alloy 617 exhibited significant hardening after irradiation at 580°C; some hardening occurred at 660°C as well, but the 800H showed extremely low tensile elongations when tested at 700°C. Notably, the grain boundary engineered 800H exhibited even greater hardening at 580°C and retained a high amount of ductility. Irradiation effects on the two nano-structured ferritic alloys and the annealed 9Cr–1MoV were relatively slight at this low dose. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

The development of porosity at 1000°C in an oxide dispersion-strengthened ferritic alloy containing nanoscale (∼2–4nm) oxide particles is investigated. A comparison with an alloy fabricated by internal oxidation instead of mechanical alloying demonstrates that the porosity formation is associated with mechanical alloying of the alloy powder with Y2O3 in argon. The pores grow in spite of a submicron grain size suggesting that the grain boundaries are not effective paths for removing entrapped gas from the pores. [Copyright &y& Elsevier]

Abstract: This paper presents biaxial thermal creep results for the vanadium alloy V4Cr4Ti in liquid lithium using US Heat 832665 and two batches of tubing made from NIFS-Heat-2. The tests were performed at 700 and 800°C over a stress range of 30–120MPa using pressurized tube specimens. Lithium environments changed C, N and O concentrations in V4Cr4Ti. Oxygen removal from the alloy to lithium was small when there was a significant pickup of nitrogen during exposures. The creep response of V4Cr4Ti was characterized by an inverted primary creep followed by a secondary creep or an accelerating creep up to creep rupture. A normal primary creep was also observed in US Heat 832665 when tested at 700°C and 120MPa. Improved creep rupture properties were observed in the JP-NIFS-Heat-2 specimens compared to the US-NIFS-Heat-2 specimens. Creep response of V4Cr4Ti is apparently dependent on the heat, tubing production, and stress and temperature conditions. [Copyright &y& Elsevier]

Abstract: Mechanically alloyed, oxide dispersion strengthened (MA/ODS) ferritic alloys are attractive for fusion reactor applications because of their potential for operation at high temperatures and also because the dispersed oxide particles may provide a trap for transmutant helium. The microstructural stability of MA/ODS Fe–14wt% Cr–3% W–0.4% Ti–0.3% Y2O3 ferritic alloys was investigated by atom probe tomography. Atom probe tomography revealed that these MA/ODS 14YWT ferritic alloys contain a high number density of nanometer scale titanium-, yttrium- and oxygen-enriched nanoclusters in the as-extruded condition. These nanoclusters were found to be extremely resistant to coarsening at temperatures of at least 1000°C. [Copyright &y& Elsevier]

Ferritic alloys containing a high density of nanoscale clusters of Y–Ti–O exhibit superior creep strength and potential for high resistance to radiation damage. Small angle neutron scattering (SANS) was used to characterize the sequence-of-events and the necessary ingredients for the formation of nanoclusters (NCs) during processing, as well as their thermal stability during high temperature aging. Mechanical alloying (MA) dissolves Y2O3 in the master alloy Fe–Cr–W powders. A large population of 1–2 nm NCs precipitate during subsequent high temperature consolidation. The NC sizes increase and their volume fractions and number densities decrease with increasing the consolidation temperature. Both Ti and Y are necessary for NC formation at higher temperatures. The NCs in MA957 are stable during aging at 1150 °C for times up to 243 h, but systematically coarsen at 1200 °C. The NCs coarsen rapidly and become unstable at higher aging temperatures. Variations in the alloy hardness are consistent with differences in the NC sizes and number densities. [Copyright &y& Elsevier]

Abstract Nanostructured ferritic alloys (NFAs) having sub-micron grain size with a high density of nano-oxides (NOs) (size of ∼2–3 nm) are one of the best candidates for structural components in Generation IV nuclear systems. In this study, 14YWT NFA cladding tubes were irradiated in BOR60 reactor up to 7 dpa at 360–370 °C. Detailed microstructural analysis has been conducted using bright field transmission electron microscopy, bright field scanning transmission electron microscopy, energy filtered transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy and transmission Kikuchi diffraction techniques. This revealed cavities, <100> and <111> type dislocation loops, and α′ precipitates forming after irradiation with relationships between cavities and NOs, and α′ precipitates and NOs. Cavities mostly form on the NOs; whereas, α′ precipitates form between the NOs where the point defect concentration is high. Moreover, α′ precipitates are distributed homogenously on and around the dislocation loops which is consistent with the observation that there is no Cr segregation on dislocation loops. Grain boundaries were found to be mostly depleted in Cr; however, the characteristics of each grain boundary determines the Cr behavior and the α′ denuded zone around the grain boundaries. Mechanical properties of the irradiated tubes have been determined by using both low force and high force nanoindentation techniques, resulting in 1.03 ± 0.33 GPa and 0.82 ± 0.20 GPa hardening, respectively. Dispersed barrier hardening calculations and nanoindentation measurements are in good agreement. In this study, 14YWT NFA has been systematically studied after neutron irradiation to better understand its superior performance: low α′ concentration, low swelling and low radiation-induced hardening. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Cr-stabilized nanostructured ferritic alloys (NFAs), dispersion strengthened by an ultra-high density of nanooxides, are attractive candidates for many nuclear energy applications due to their high-temperature strength, in-service stability and remarkable irradiation tolerance. However, typical NFA deformation processing paths lead to crystallographic texturing, formation of brittle microstructures and low toughness orientations, making fabricating components very difficult. Here, we characterize the dislocation-mediated deformation mechanisms that lead to the brittle texture component. The as-extruded bar is less brittle than the cross-rolled plate, which contains a large population of pre-existing cleavage microcracks. More generally, deformed ODS/NFAs are most often textured and have anisotropic low toughness orientations, even absent microcracks. However, cross-rolling produces a very high volume fraction of a plate normal {001}<110>-texture component, which constitutes the brittle cleavage system in iron. Microcracks propagate along {001} low angle deformation induced subgrain boundaries in <110> directions after nucleating by the Cottrell mechanism. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We irradiated four ferritic alloys with energetic Fe and He ions: one castable nanostructured alloy (CNA) containing Ti-W-Ta-carbides, and three nanostructured ferritic alloys (NFAs). The NFAs were: 9Cr containing Y-Ti-O nanoclusters, and two Fe-12Cr-5Al NFAs containing Y-Zr-O or Y-Hf-O clusters. All four were subjected to simultaneous dual-beam Fe + He ion implantation (650 °C, ∼50 dpa, ∼15 appm He/dpa), simulating fusion-reactor conditions. Examination using scanning/transmission electron microscopy (STEM) revealed high-number-density helium bubbles of ∼8 nm, ∼10 21 m −3 (CNA), and of ∼3 nm, 10 23 m −3 (NFAs). STEM combined with multivariate statistical analysis data mining suggests that the precipitate-matrix interfaces in all alloys survived ∼50 dpa at 650 °C and serve as effective helium trapping sites. All alloys appear viable structural material candidates for fusion or advanced fission energy systems. Among these developmental alloys the NFAs appear to sequester the helium into smaller bubbles and away from the grain boundaries more effectively than the early-generation CNA. [ABSTRACT FROM AUTHOR]

Casting of stainless steels is a promising and cost saving way of directly producing large and complex structures, such a shield modules or divertors for the ITER. In the present work, a series of modified high-nitrogen cast stainless steels has been developed and characterized. The steels, based on the cast equivalent of the composition of 316 stainless steel, have increased N (0.14–0.36%) and Mn (2–5.1%) content; copper was added to one of the heats. Mechanical tests were conducted with non-irradiated and 0.7 dpa neutron irradiated specimens. It was established that alloying by nitrogen significantly improves the yield stress of non-irradiated steels and the deformation hardening rate. Manganese tended to decrease yield stress but increased radiation hardening. The role of copper on mechanical properties was negligibly small. Analysis of structure was conducted using SEM-EDS and the nature and compositions of the second phases and inclusions were analyzed in detail. No ferrite formation or significant precipitation were observed in the modified steels. It was shown that the modified steels, compared to reference material (commercial cast 316L steel), had better strength level, exhibit significantly reduced elemental inhomogeneity and only minor second phase formation. [ABSTRACT FROM AUTHOR]

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 ≈10 mm-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. [ABSTRACT FROM AUTHOR]

Texture and microstructure of tubes and plates fabricated from a nanostructured ferritic alloy (14YWT), produced either by spray forming followed by hydrostatic extrusion (Process I) or hot extrusion and cross-rolling a plate followed by hydrostatic tube extrusion (Process II) have been characterized in terms of their effects on texture and grain boundary character. Hydrostatic extrusion results in a combination of plane strain and shear deformations which generate low intensity α- and γ-fiber components of {001}<110> and {111}<110> together with a weak ζ-fiber component of {011}<211> and {011}<011>. In contrast, multi-step plane strain deformation by hot extrusion and cross-rolling of the plate leads to a strong texture component of {001}<110> together with a weaker {111}<112> component. Although the total strains are similar, shear dominated deformation leads to much lower texture indexes compared to plane strain deformations. Further, the texture intensity decreases after hydrostatic extrusion of the alloy plate formed by plane strain deformation, due to a lower number of activated slip systems during shear dominated deformation. Notably, hot extruded and cross-rolled plate subjected to plane strain deformation to ~50% engineering strain creates only a modest population of low angle grain boundaries, compared to the much larger population observed following the combination of plane strain and shear deformation of ~44% engineering strain resulting from subsequent hydrostatic extrusion. [ABSTRACT FROM AUTHOR]

Abstract: The effects of oxygen variations on 14(Cr)YWT(Ti) nanostructured ferritic alloys (NFA) containing high densities of Y–Ti–O nano-features (NFs) were investigated using transmission electron microscopy, atom probe tomography and small angle neutron scattering. The alloy with the lowest O had the fewest and coarsest NFs. The microhardness increase with higher O content is consistent with the corresponding increasing NF volume fraction and variations in grain size. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: A present topic of high interest in magnetic fusion is the “gap” between near-term and long-term concepts for high heat flux components (HHFC), and in particular for divertors. This paper focuses on this issue with the aim of characterizing the international status of current HHFC design concepts for ITER and describing the different technologies needed in the designs being developed for fusion power plants. Critical material and physics aspects are highlighted while evaluating the current readiness level of long-term concepts, identifying the design and R&D gaps, and discussing ways to bridge them. [Copyright &y& Elsevier]

Abstract: In order to investigate the effect of the environment on the irradiation creep properties of highly purified V–4Cr–4Ti alloys, neutron irradiation experiments with sodium-enclosed irradiation capsules in Joyo and lithium-enclosed irradiation capsules in HFIR-17J were carried out using pressurized creep tubes (PCTs). It was found that the creep strain rate exhibited a linear relationship with the effective stress up to 150MPa at 458 and 598°C in the Joyo irradiation experiments. For HFIR-17J irradiation at 425°C, the creep strain rate also exhibited a linear relationship with the effective stress up to 150MPa. The activation energy of the irradiation creep and irradiation creep stress factor were estimated to be 46kJ/molK and 1–2, respectively. No significant difference in the irradiation creep behavior between liquid-sodium and liquid-lithium environments could be seen. [Copyright &y& Elsevier]

Abstract: Recent progress in the development of low activation vanadium alloys and MHD insulator coatings for a Li-self cooled blanket is reviewed. Research progress in vanadium alloys is highlighted by technology for fabricating creep tubes, comparison of thermal creep in vacuum and Li, understanding impurity transfer between vanadium alloys and Li and its impact on mechanical properties, behavior of hydrogen and hydrogen isotopes, low dose irradiation effects on weld joints, and exploration for advanced vanadium alloys. Major remaining issues for vanadium alloys are thermal and irradiation creep, helium effects on high-temperature mechanical properties and radiation effects on low-temperature fracture properties. Er2O3 showed good compatibility with Li, and is promising as a MHD insulator coating on vanadium alloys. Significant progress in coating technology for this material has been made. Recent efforts are focused on multi-layer and in-situ coatings. Tests under flowing lithium conditions with a temperature gradient are necessary for quantitative examination of coating performance. [Copyright &y& Elsevier]

Abstract: A novel in situ 59Ni(n,α) reaction He-implanter technique is described, that can be used to characterize the effect of the He/dpa ratio on microstructural evolution and changes in the flow properties of various materials at fusion relevant conditions. Irradiations in the high flux isotope reactor resulted in α-implantation from 1 to 5μm thick NiAl coatings on TEM discs producing a uniform He concentration of 5–50appm He/dpa to a depth of 5–8μm. Initial post-irradiation TEM observations of MA957 irradiated to 9dpa and 380appm He at 500°C provide strong evidence for the existence of a high density of small ⩽1nm size bubbles trapped on nano-scale YTiO clusters. In contrast, similar irradiations of Eurofer 97, described in a companion paper, produce a lower number density of larger cavities, which may be a mixture of bubbles and voids. [Copyright &y& Elsevier]

Abstract: Oxide dispersion strengthened ferritic steels are being considered for a number of advanced nuclear reactor applications because of their high strength and potential for high temperature application. Since these properties are attributed to the presence of a high density of very small (nanometer-sized) oxide clusters, there is interest in examining the radiation stability of such clusters. A novel experiment has been carried out to examine oxide nanocluster stability in a mechanically alloyed, oxide dispersion strengthened ferritic steel designated 12YWT. Pre-polished specimens were ion irradiated and the resulting microstructure was examined by atom probe tomography. After ion irradiation to ∼0.7dpa with 150keV Fe ions at 300°C, a high number density of ∼4nm-diameter nanoclusters was observed in the ferritic matrix. The nanoclusters are enriched in yttrium, titanium and oxygen, depleted in tungsten and chromium, and have a stoichiometry close to (Ti+Y):O. A similar cluster population was observed in the unirradiated materials, indicating that the ultrafine oxide nanoclusters are resistant to coarsening and dissolution under displacement cascade damage for the ion irradiation conditions used. [Copyright &y& Elsevier]

High temperature deformation properties of Nb–1%Zr alloy were evaluated with a special emphasis on the texture effect. Fully recrystallized Nb–1%Zr samples with different crystallographic textures were prepared by annealing 80% cold-rolled foil at 1500°C for 2h. Subsequent tensile tests at 1100°C showed that the sample oriented in the rolling direction is about 20% stronger than that oriented 30° to the rolling direction. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

The varying temperature irradiation experiment in the HFIR was carried out in order to investigate the performance of vanadium alloys subject to temperature variation during operation. No significant differences of irradiation hardening between steady 340 °C irradiation and variable 225/340 °C irradiation could be seen in any alloys. In the case of the irradiation at 520 °C, the temperature variation to 360 °C influenced the formation process of voids in the unalloyed and dilute vanadium alloys compared to isothermal irradiation at 520 °C. It is contributed to the change of irradiation hardening behavior between steady irradiation and temperature variable irradiation at 520 °C. It could not be seen any effect of varying irradiation temperature in vanadium alloys containing >1 wt% titanium in this study. It is caused by the insensitivity of formation process of Ti(OCN) precipitates against the effect of irradiation temperature variation. [Copyright &y& Elsevier]

Considerable progress has been made toward development of vanadium alloys for fusion. Much of the recent research has focused on vanadium alloys containing 4–5% Cr and 4–5% Ti, but a number of alternative compositions and processing routes have been explored in an effort to achieve improved performance. The goal of this paper is to review significant new results and to highlight critical issues that remain for future research. Progress in understanding the influence of interstitial impurities on microstructural evolution in both thermal and radiation environments are covered. The current state of knowledge of hardening and embrittlement of vanadium alloys in response to neutron irradiation is reviewed. Atomic-scale computer simulations to elucidate fundamental irradiation damage mechanisms are presented. The thermal and irradiation creep behavior of V–4Cr–4Ti is summarized along with an overview of the effects of He on tensile properties. [Copyright &y& Elsevier]

An atom probe tomography characterization of the microstructure of as-processed and crept mechanically-alloyed, oxide-dispersion-strengthened (MA/ODS) ferritic alloys has been performed. The significant enrichments of Cr, W, Ti, Y, O, C and B in the vicinity of dislocations and the presence of ultrastable 4-nm-diameter Ti-, Y- and O-enriched particles appears to be responsible for their improved high temperature mechanical properties. [Copyright &y& Elsevier]