Effects of neutron irradiation on Ni-based alloys: a comparative study between PM-HIP and forging

Abstract This work presents a systematic and comprehensive investigation of neutron irradiation effects on Ni-based alloys manufactured by powder metallurgy with hot isostatic pressing (PM-HIP) compared to their conventional forged counterparts. PM-HIP is a leading candidate to replace forging as a manufacturing method for structural components in future nuclear reactors due to its more homogeneous microstructures and superior mechanical properties. However, such components can be exposed to elevated temperature irradiation in service, which will significantly alter material performance. Understanding PM-HIP material performance as compared to conventional forgings under realistic reactor operating conditions is therefore essential for certification and deployment. In this study, Ni-based Alloys 625 and 690 are investigated under neutron irradiation at target damage levels of ~ 1 and ~ 3 dpa, allowing for direct comparisons between PM-HIP manufacturing and forging. Uniaxial tensile tests evaluate the irradiation-induced changes in mechanical behaviour, while the irradiation-induced microstructural changes are investigated using transmission electron microscopy and atom probe tomography. Overall, PM-HIP Alloy 625 presents superior mechanical properties (e.g., less irradiation hardening, smaller reduction in ductility) compared to forged Alloy 625 under irradiation. This is primarily attributed to the fact that PM-HIP Alloy 625 has an order-of-magnitude lower void population than that observed in the forged counterpart at all damage levels due to its lower initial dislocation density. On the other hand, minimal differences in irradiation-induced microstructures are observed between PM-HIP and forged Alloy 690, resulting in suppressed differences in mechanical properties between them at corresponding damage levels. These findings consistently demonstrate comparable or greater irradiation tolerance in PM-HIP Alloy 625 and 690 than in their forged counterparts, providing crucial data to support the qualification of PM-HIP manufacturing of Ni-based alloys for future generation nuclear structural components.