Radiation-induced Segregation Of Cr Research Articles

Phase stability and microstructural evolution in neutron-irradiated ferritic-martensitic steel HT9

Ferritic Martensitic (F/M) steel HT9 specimens were irradiated in the Advanced Test Reactor up to 4.16 dpa in three temperature ranges (roughly from 300 to 600 °C). The post-irradiation microstructure, including dislocation structure , precipitation and radiation-induced segregation (RIS) was characterized using analytical scanning / transmission electron microscopy (S/TEM), and atom probe tomography (APT). Irradiation hardening was measured using nanoindentation . The results reveal a distinctive pattern of dislocation and precipitate evolution at high temperature, around 600 °C, where various defects and precipitates formed in the low dose regime followed by a recovering process with increasing dose. Dislocation loops formed in all temperature ranges, and the growth of dislocation loops is unconstrained above certain critical temperature, contributing to the increasing dislocation density even prior to doses of 0.5 dpa at 600 °C. Ni/Mn/Si clusters were identified in all temperature ranges and the compositions of these clusters converged to G phase stoichiometrically. Significant coarsening of G phase particles was observed at 600 °C, accompanied by the formation of G phase on grain boundaries. α ’ precipitates were only found in the medium and low temperature ranges (below 500 °C). The number density and volume fraction were higher in the low temperature specimens, while larger particles were observed in the medium temperature range. RIS of Cr, Ni, Mn, Si, P was identified at dislocation lines , grain boundaries and phase boundaries, and the temperature dependence is consistent with previous studies. The RIS of Cr to the existing VN particles was confirmed by APT and may accelerate the transition of VN to Cr-rich nitrides . The irradiation hardening contribution from dislocation loops, dislocation lines, G phase and α ’ phase was parsed based on a linear dispersed barrier hardening model. The results suggest that most irradiation hardening at high temperature is due to increasing dislocation density with dose.

Experimental and theoretical analysis of equilibrium segregation and radiation-induced segregation of Cr at grain boundaries in a reduced activation ferritic/martensitic (RAFM) steel

Helium ion irradiation at 350 °C was performed to study equilibrium segregation and radiation-induced segregation (RIS) of Cr at grain boundaries in reduced activation ferritic/martensitic steels. Cr concentration at grain boundary was measured by scanning transmission electron microscopy with an energy-dispersive spectrometer. The measured Cr concentration at grain boundaries in heat treated zone was 11.7 and 12.8 wt.% in irradiated zone, respectively, which matched well to the calculated results from Mclean and modified Perk model. Equilibrium segregation and RIS of Cr mechanisms were theoretically analysed. The analysis indicates that as temperature rises, equilibrium Cr segregation decreases monotonically, while RIS of Cr has a bell-shape profile, which increases first and then decreases. It is also shown that at low and high temperatures, equilibrium segregation of Cr is higher than that of RIS; at intermediate temperatures, equilibrium Cr segregation is lower than RIS.

Microstructure evolution of T91 irradiated in the BOR60 fast reactor

Microstructures of T91 neutron irradiated in the BOR60 reactor at five temperatures between 376 °C and 524 °C to doses between 15.4 and 35.1 dpa were characterized using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and atom probe tomography (APT). Type a<100> dislocation loops were observed at 376–415 °C and network dislocations dominated at 460 °C and 524 °C. Cavities appeared in a bimodal distribution with a high density of small bubbles less than 2 nm at irradiation temperatures between 376 °C and 415 °C. Small bubbles were also observed at 460 °C and 524 °C but cavities greater than 2 nm were absent. Enrichment of Cr, Ni, and Si at the grain boundary was observed at all irradiation temperatures. Radiation-induced segregation (RIS) of Cr, Ni and Si appeared to saturate at 17.1 dpa and 376 °C. The temperature dependence of RIS of Cr, Ni and Si at the grain boundary, which showed a peak Cr enrichment temperature of 460 °C and a lower peak Ni and Si enrichment temperature of ∼400 °C, was consistent with observations of RIS of Cr in proton irradiated T91, suggesting that the same RIS mechanism may also apply to BOR60 irradiated T91. G-phase and Cu-rich precipitates were observed at 376–415 °C but were absent at 460 °C and 524 °C. The absence of G-phase at 524 °C could be related to the minimal segregation of Ni and Si in that condition.

Modeling radiation induced segregation in Ni–Cr model alloys from first principles

In this paper, a rate theory model is employed to study radiation induced segregation (RIS) in Ni–Cr model alloys. In contrast with similar previous models, model parameters have been obtained from first principles via density functional theory (DFT) calculation. Qualitative model behavior is compared and contrasted with the physical mechanisms that underlie conventional RIS models, and the effects of Cr–Cr interstitial trapping and defect sink bias are investigated. It is concluded that: (1) the parameters required in this RIS model are too sensitive to be rigorously determined by present DFT approaches alone, and fitting to experimental RIS data is necessary to produce an accurate model. (2) In contrast with the emphasis of previous RIS models, the best fit DFT-based RIS model suggests that under radiation, the interstitial flux should drive Cr strongly toward enrichment near defect sinks, while the vacancy flux should drive Cr strongly toward depletion, and that the vacancy effect should be slightly stronger resulting in moderate Cr depletion. (3) The effects on RIS of Cr–Cr interstitial trapping and biased defect sinks are relatively small compared with the effects associated with variations in the species dependent diffusivities.