Defect Microstructures and Deformation Mechanisms in Irradiated Austenitic Stainless Steels

Microstructural evolution and deformation behavior of austenitic stainless steels are evaluated for neutron, heavy-ion and proton irradiated materials. Radiation hardening in austenitic stainless steels is shown to result from the evolution of small interstitial dislocation loops during lightwater-reactor (LWR) irradiation. Available data on stainless steels irradiated under LWR conditions have been analyzed and microstructural characteristics assessed for the critical fluence range (0.5 to 10 dpa) where irradiation-assisted stress corrosion cracking susceptibility is observed. Heavy-ion and proton irradiations are used to produce similar defect microstructures enabling the investigation of hardening and deformation mechanisms. Scanning electron, atomic force and transmission electron microscopies are employed to examine tensile test strain rate and temperature effects on deformation characteristics. Dislocation loop microstructures are found to promote inhomogeneous planar deformation within the matrix and regularly spaced steps at the surface during plastic deformation. Twinning is the dominant deformation mechanism at rapid strain rates and at low temperatures, while dislocation channeling is favored at slower strain rates and at higher temperatures. Both mechanisms produce highly localized deformation and large surface slip steps. Channeling, in particular, is capable of creating extensive dislocation pileups and high stresses at internal grain boundaries which may promote intergranular cracking.