Coupling multi-scale mechanical testing techniques reveals the existence of a trans-granular channel fracture deformation mechanism in high dose Inconel X-750

Recent testing has shown that Inconel® X-750 materials have lost strength and ductility following irradiation, which may raise concerns over the structural integrity of the material. Bulk component compression testing shows irradiation temperature has a clear influence on mechanical behaviour; material irradiated at higher temperatures exhibits less ductility compared to material irradiated at lower temperatures. This has been re-affirmed with the most recent large scale compression testing of material irradiated to doses between 70 dpa and 80 dpa. Measuring material yield strength from this data has proven to be challenging, due to the complex geometry of the material and the testing methodology. This work outlines the use of micro-indentation, and novel small-scale micro-mechanical testing to measure the mechanical properties of neutron irradiated Inconel X-750 material. The results and conclusions provide valuable insights into the mechanical property evolution, and embrittlement of the material. Microhardness and micro-mechanical testing have confirmed that material irradiated at lower temperatures has a lower yield strength, lower critical resolved shear stress (CRSS), and higher plasticity. The current results confirm the existence of an additional mechanism of crack initiation not previously considered for Inconel X-750 material. In addition to intergranular fracture, re-examination of component-scale test fracture surfaces also reveals that some portion of the component fails via transgranular, faceted pathways. Transmission electron microscopy (TEM) of deformed micro-tensile specimens has confirmed channel fracture. When subject to a slip band or dislocation channel, helium bubbles within the grain interior shear along the slip direction and coalesce prior to shearing the ligaments between the bubbles. The two deformation mechanisms are expected to interact in the bulk material. When discrete flow localization events (dislocation channels) intersect compromised grain boundaries decorated with helium bubbles, they will be expected to produce significant stresses which drives fracture propagation along the grain boundary.