Irradiation Hardening and Indentation Size Effect of the 304NG Stainless Steels After Triple Beam Irradiation

Abstract

The nuclear grade 304NG stainless steel (SS) has been developed in the past several decades as the new generation of internal material in light water reactors. The irradiation effects of domestic 304NG SS were simulated by the triple ion beam irradiation on the heavy ion, hydrogen and helium triple ion beam irradiation platform at China institute of Atomic Energy. The irradiation experiments were carried out with various doses (6, 15, 30 and 150 dpa at 300 ℃) and temperatures (300, 350, 400, 450 ℃ with 6 dpa). The depth-dependent hardness and elastic modulus of the specimens before and after irradiation were measured by nanoindentation with the continuous stiffness measurement technique. For the specimens irradiated at 300 ℃, the hardness generally increases with the increasing dose. The depth-dependent hardness in the micro-indentation region (indentation depth h > 100 nm) of those specimens with dose less than 30 dpa can be well explained by Nix & Gao formulae of the indentation size effect. For the specimens irradiated at different temperatures, the hardening effect can be observed for all specimens for indentation depth beyond 1 μm and the hardness decreases with increasing irradiation temperature. However, as the irradiation temperature elevates or the dose increases up to 150 dpa, the hardness for the indentation depth h < 500 nm deviates significantly from the projection of the Nix & Gao model. The surface morphology observed by SEM and the S parameters extracted from the slow positron annihilation Doppler broadening indicate that the drastic reduction of hardness those specimens with indentation depth h < 500 nm can be attributed to the change of surface morphology.