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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: 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]

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]