Flux effects in precipitation under irradiation – Simulation of Fe-Cr alloys

Abstract

Radiation-enhanced precipitation of Cr-rich α' in irradiated Fe-Cr alloys, which results in hardening and embrittlement, depends on the irradiating particle and the displacement per atom (dpa) rate. Here, we utilize a Cahn-Hilliard phase-field based approach, that includes simple models for nucleation, irradiating particle and rate dependent radiation-enhanced diffusion and cascade mixing to simulate α' evolution under neutrons, heavy ions, and electron irradiations. Different irradiating particles manifest very different cascade mixing efficiencies. The model was calibrated using neutron data. For cascade inducing neutron/heavy-ion dpa rates at 300 °C between 10 -8 and 10 -6 dpa/s the model predicts approximately constant number density, decreasing radius, decreasing α' Cr composition, and lower α' volume fraction. The model then predicts a dramatic transition to no α'formation above approximately 10 -5 dpa/s, while electron irradiation, with weak mixing, had little effect at dpa rates up to 10 -3 dpa/s. These model predictions are consistent with experiments. We explain the results in terms of the flux dependence of the radiation enhanced diffusion, cascade mixing, and their ratio, which all vary significantly in relevant flux ranges for neutron and cascade inducing ion irradiations. These results show that both cascade mixing and radiation enhanced diffusion must be accounted for when attempting to emulate neutron-irradiation effects using accelerated ion irradiations. flux effects by triple-beam ions at 450 °C to 10 dpa [17]. Pareige et al. recently reported an APT study of Fe-12Cr under heavy-ion irradiation at 1×10 -4 dpa/s at 300 °C that observed a low density of dilute Cr clusters after irradiation to 0.5 dpa [18]. Likewise, Marquis et al. [19] found no α' in Fe-15Cr irradiated by Fe ++ ions at a dose rate of 1×10 -4 dpa/s up to 60 dpa at 300 °C, but observed a small density clusters with only 35-50 at.% Cr in in a Fe-15Cr at a lower dose rate of ≈ 1×10 -5 dpa/s, suggesting a pronounced dose rate effect [19]. Korchuganova et al. carried out a heavy-ion irradiation at room temperature on a Fe-22Cr alloy that was pre-aged at 500 °C to form α' precipitates with a distribution of sizes [20]. APT showed that the heavy-ion irradiation dissolved the small preexisting α' precipitates and made the larger ones more diffuse. A similar effect of heavy-ion irradiation was also shown to retard spinodal decomposition of Cr-rich α' in a duplex stainless steel with 20 wt.% Cr at 300 °C [21]. In contrast to the heavy-ion irradiation microstructures, Tissot et al. reported that a 3.9×10 -5 dpa/s electron irradiation at 300 °C accelerated precipitation of near equilibrium α' [22]. Accelerated α' precipitation under neutron irradiation has been attributed to radiation-enhanced diffusion (RED). RED depends on both temperature and dose rate due to the corresponding effect on vacancy and self-interstitial atom recombination mediated concentrations through a variety of mechanisms. However, a dose rate effect on RED does not full...

On the use of charged particles to characterize precipitation in irradiated reactor pressure vessel steels with a wide range of compositions

“…While increasing N at higher dpa rate has often been observed in lower dpa NI, Figure 10 shows that N actually decreases with dpa for the CPI in a way that is close to the extrapolated trend for a NI alloy at high  > 10 13 n/cm 2 -s (1.6x10 -8 dpa/s) and t > 10 20 n/cm 2 (0.15 dpa). While this trend is opposite to those observed for NI at lower dpa rates and dpa, it is qualitatively consistent with the effect of ballistic mixing by cascades [47]. Nevertheless, in view of these modest differences, CPI can be used as a convenient tool to assess the relative embrittlement sensitivity to various steel compositions, as well as a way to create RPV steel precipitate microstructures for scientific studies of mechanisms.…”

“…For example, for a fixed set of other variables, far fewer defects escape vacancy-SIA recombination at high dpa rates, thereby reducing the efficiency of RED and defect accumulation. RIS driven precipitation also depends on damage rate, as do the effects of ballistic mixing [46,47]. Further, there are differences between CPI and NI secondary atomic recoil spectra [41].…”

Microstructural examination of neutron, proton and self-ion irradiation damage in a model Fe9Cr alloy

“…We argue this only proves injected material also suppresses α' formation, and does not eliminate ballistic dissolution as occurring at all. Ballistic dissolution is shown to be important for dose rate effects from the phase-field modelling by Ke et al [98], and Kinetic Monte Carlo simulations by Soisson et al [40]. A 15Cr alloy is much more susceptible to α' formation than the 9Cr alloy studied here (which has a composition very close to the solubility limit of chromium [37,91]).…”

“…These will then continue to grow and become further enriched. Since no such precipitation was seen in the self-ion irradiated Fe9Cr, the large cascades, the injected iron-atoms, or both, could be suppressing this process [40,97,98].…”

“…A very large cascade could disrupt a local concentration of chromium that may otherwise become α'. Frequent bombardment of the lattice by such large cascades would mean this process occurs often [40,98]. Therefore, α' precipitates would struggle to nucleate due to this ballistic mixing.…”