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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: 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]

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]

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: 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]

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]

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]