Irradiation damage in neutron-irradiated Fe-Cr model alloys

Ferritic-martensitic (F-M) steels are considered as lead candidate structural materials for Generation IV fission reactors and future fusion reactors. Compared to austenitic stainless steels, these steels have superior properties in thermal conductivities and thermal expansion coefficients. In addition, they have better resistance to swelling, helium embrittlement and irradiation creep at elevated temperature (T/Tm > 0.4). However, F-M steels exhibit low-temperature irradiation-induced embrittlement that leads to a substantial decrease in toughness at lower irradiation temperature (T < 500°C) even at very low doses. The underlying microstructral mechanisms and their dependence on the irradiation temperatures and chromium contents are not well understood. Body-centered cubic iron (Fe) and iron-chromium (Fe-Cr) (Cr = 10-16 at%) are used as model to study the irradiation-induced microstructural evolution and their relationship to mechanical properties. The irradiation effects as a function of Cr contents and irradiation temperatures were systematically investigated. Through using model materials, the effects of other substitutional alloying elements (e.g. nickel, tungsten and manganese), interstitial impurities (e.g. carbon and nitrogen) and secondary phases (e.g. carbides, nitrides and G-phase precipitates) commonly seen in commercial F-M steels can be reduced. Neutron irradiations were carried out at Advanced Test Reactor with target doses of 0.01, 0.1 and 1 dpa and target irradiation temperatures of 300 and 450°C. Following irradiations, the resulting microstructure were investigated with transmission electron microscopy (TEM), atom probe tomography (APT) and electron backscatter diffraction (EBSD). TEM was used to observe the crystallographic defect structures caused by irradiation damage, including dislocation loops and voids. APT was used to study the precipitation of Cr-rich α' phase under irradiation-enhanced diffusion process. EBSD is used to examine the grain size distribution and possible grain growth. The corresponding mechanical properties were evaluated with the hardness measurements. Both Vicker hardness test (microhardness) and high-load nanoindentation were used. The results of mechanical properties and microstructures were compared and related through Orowan model. In general, the increase in hardness in irradiated specimens can be attributed primarily to the formation of dislocation loops and α' precipitates. The dislocation loops appeared to results hardening at the lowest dose of 0.01 dpa. The addition of Cr in Fe greatly reduced the mobility of interstitials and small a/2 dislocation loops, leading to smaller loop size and more uniform distribution. The Cr effect on loop density is not clear in this study. Increasing irradiation temperature increased the mobility of point defects and small a/2 dislocation loops, resulting in larger loop size and lower loop density. In Fe, a/2 loops were sufficiently mobile at 300°C, leading to a dislocation decoration structure. Irradiation at 450°C predominantly produced immobile a dislocation loops in Fe, leading to a relatively uniform loop distribution. The addition of Cr caused a retention of a/2 loops in Fe-Cr irradiated at 450°C. The enhanced mobility of a/2 loops at higher temperature is related to the formation of some dislocation decoration in Fe-Cr. α' precipitate effects on hardening in Fe-Cr appeared at higher dose of 1 dpa. The precipitate density is higher with increasing Cr contents and decreasing irradiation temperatures. On contract, the size of α' precipitates was relatively invariant as ~2 nm (radii) to irradiation conditions. α' precipitates were identified to be the major reason that resulted in higher hardening in Fe-Cr with higher Cr contents at 1 dpa. Voids formation was observed in Fe irradiated at both 300 and 450°C to 1 dpa. The void size is larger in 450°C than in 300°C specimen. No voids formation was evidently observed in Fe-Cr. No grain growth can be detected in Fe irradiated at 450°C