Mechanical behavior of cold-worked, precipitation-hardened Inconel X-750 before and after helium-implantation

Inconel & REG; X-750 has been implanted with helium at a temperature of 400 degrees C to obtain an average concentration of [He] of about 3000 atomic parts per million (appm). The implantation resulted in the formation of a low density of point defect clusters in the form of dislocation loops and a high density of He-stabilized cavities. Uni-axial tensile tests were conducted at room temperature on both the helium-implanted and non-helium-implanted materials. There was a reduction in ductility and an increase in yield strength after helium implantation. The fracture surface of the helium-implanted-material exhibited a mixed failure mode of inter-granular and transgranular fracture. TEM observations showed that the trans-granular fracture is the result of failure along twin/martensite platelets that were present in the material prior to implantation. The interfaces of these platelets with the austenite matrix, which are bordered either by incoherent twinned material or thin layers of & epsilon;-martensite, are paths for crack propagation that may be enhanced by He segregation at the interfaces. The results are discussed in terms of the implications for observations of channel fracture in neutron irradiated austenitic stainless steels and Ni-alloys.