Cameron Howard

A first of its kind small scale mechanical testing technique involving micro-three-point bending was invented, developed, and implemented on reactor irradiated, active Inconel X-750 components removed from service after approximately 53 and 67 dpa. These tests were performed at ambient room temperature in-situ using a scanning electron microscope in order to obtain live recordings of sample deformation and loading curves. Sample and testing apparatus preparation required novel lift-out and fabrication processes. Materials from two irradiation temperature regimes, low temperature (120-280 °C) and high temperature (300 ± 15 °C) were examined. Manufacturing and finishing (grinding) of this component create differences between its edge and center, so micro-specimens from both areas were extracted in order to study these differences. According to three-point beam bending theory, a 0.2% offset yield stress parameter is introduced and calculated for all specimens. Differences in mechanical properties due to irradiation temperature and dose effects were observed. Material irradiated at the higher temperature exhibited yield strength increases of ∼540 MPa after 53 dpa and ∼1000 MPa after 67 dpa. There was little difference (≤310 MPa) in yield strength between materials irradiated at the lower temperature at 53 dpa and 67 dpa compared with non-irradiated material. Differences in yield strengths between the edge and center of the component are retained after irradiation. The difference in yield strengths for the edge and center regions was ∼740 MPa for non-irradiated material. After irradiation to a dose of 67 dpa these differences were ∼570 MPa for the lower irradiation temperature and ∼710 MPa for higher irradiation temperature. There were no indications of grain boundary failures via cracking except for material irradiated to 67 dpa at low temperature.

The results from a novel, micro-tensile testing technique, employing a micro-electro-mechanical system, operated in a push-to-pull configuration, to study the effects of radiation damage on Inconel X-750, are presented. Non-irradiated material, along with material irradiated to 67 dpa and 81 dpa at two irradiation temperatures, 120–280 °C and 300–330 °C, is investigated. This testing approach implements a safe, lift-out, extraction method that enables the evaluation of material tensile behavior in specific locations of radioactive components with complex geometries outside of costly hot cell protective environments. Regional cold working and grinding manufacturing processes that go undetected in bulk component testing can be evaluated in parallel with radiation damage effects. Non-irradiated specimens on the order of 1 µm × 1 µm × 2.5 µm taken from center regions of the material unaffected by processing produced yield strengths of 938 MPa and 1043 MPa, in good agreement with the bulk yield strength of Inconel X-750: 972–1070 MPa. Mechanical strengths of material irradiated to 67 dpa decreased by ~75 MPa for material irradiated at 300–330 °C and ~176 MPa for material irradiated at 120–280 °C, compared to the non-irradiated material. However, material irradiated to 81 dpa has practically identical mechanical strengths at the two irradiation temperatures, and these strengths are ~100 MPa greater than those of non-irradiated material. Average ductility of the material decreases more quickly when irradiated at 300–330 °C, from an initial value of ~15% to ~5% after 67 dpa, whereas the ductility of the material irradiated at 120–280 °C remains close to the initial value at 67 dpa and decreases to ~2% after 81 dpa.

A novel lift-out, push-to-pull, micro-tensile , small scale mechanical testing (SSMT) technique was developed to assess the yield strength, failure strength, and failure mechanisms of activated ex-service Inconel X-750 removed from the CANDU nuclear reactor core after extended service. Neutron irradiated Inconel X-750 components fail in an intergranular manner. Because these ex-service components are less than 1 mm in thickness, conventional tensile specimens cannot be fabricated from them. Thus, large-scale testing is not possible, and specimens on the order of 1 μm × 1 μm × 2.5 μm (thickness × width × gauge length) containing individual boundaries were fabricated in order to assess the grain boundary strength of the material as a function of irradiation temperature and dose. The variability introduced by differences in thermo-mechanical processing during fabrication was also assessed. Application of this new micro-tensile testing technique to non-irradiated Inconel X-750 gives good agreement with the bulk yield strength of the nickel superalloy, 1070 MPa. From SSMTs, the measured yield strengths of non-irradiated specimens were 1001 MPa at the outer edge and 1043 MPa at the center of the component. Cold work, introduced by grinding of the outside surface of the component, reduces ductility, as does irradiation. Initial tests indicate that away from the surface in the center, the boundary strength was reduced by ~456 MPa after irradiation to 78 dpa at an average irradiation temperature of 180 °C; the corresponding ductility decreased from 16.6 to ≤2.3% total elongation. Testing is a work in progress and more tests are needed for higher precision with regards to grain boundary strength reduction.