In Situ SEM Push-to-Pull Micro-tensile Testing of Ex-service Inconel X-750

摘要

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.