Mechanical properties of neutron irradiated nanostructured ferritic alloy 14YWT

Advanced nanostructured ferritic alloys (NFAs) containing a high density of ultra-fine (2-5 nm) 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 deg. C to 1.5 displacements per atom (dpa), while 14YWT fracture toughness specimens were irradiated at 300 deg. C to 1.5 dpa. 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 ∼1450 MPa at 26 deg. C to ∼700 MPa at 600 deg. C. Moderate radiation-induced hardening (50-200 MPa) and reduction in ductility was observed for 14YWT for all irradiation conditions and test temperatures. In contrast, 14WT exhibited significant hardening (∼250 MPa) for the 300 deg. C irradiated specimens, while almost no hardening was observed for the 580 and 670 deg. C irradiated specimens. Fracture toughness results showed 14YWT in the unirradiated condition had a fracture toughness transition temperature (FTTT) around -150 deg. C and upper-shelf KJIc values around 175 MPa√m. Results from irradiated 14YWT fracture toughness tests were found to closely mirror the unirradiated data and no shift in FTTT or decrease in KJIc values were observed following neutron irradiation to 1.5 dpa at 300 deg. C.