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Register a new userSolute-point defect interactions, coupled diffusion, and radiation induced segregation in fcc nickel
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Radiation-induced segregation (RIS) of solutes in materials exposed to irradiation is a well-known problem. It affects the life-time of nuclear reactor core components by favouring radiation-induced degradation phenomena such as hardening and embrittlement. In this work, RIS tendencies in face-centered cubic (fcc) Ni-X (X = Cr, Fe, Ti, Mn, Si, P) dilute binary alloys are examined. The goal is to investigate the driving forces and kinetic mechanisms behind the experimentally observed segregation. By means of ab initio calculations, point-defect stabilities and interactions with solutes are determined, together with migration energies and attempt frequencies. Transport and diffusion coefficients are then calculated in a mean-field framework, to get a full picture of solute-defect kinetic coupling in the alloys. Results show that all solutes considered, with the exception of Cr, prefer vacancy-mediated over interstitial-mediated diffusion during both thermal and radiation-induced migration. Cr, on the other hand, preferentially migrates in a mixed-dumbbell configuration. P and Si are here shown to be enriched, and Fe and Mn to be depleted at sinks during irradiation of the material. Ti and Cr, on the other hand, display a crossover between enrichment at lower temperatures, and depletion in the higher temperature range. Results in this work are compared with previous studies in body-centered cubic (bcc) Fe, and discussed in the context of RIS in austenitic alloys.
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In this work, we investigate the radiation-induced segregation (RIS) resulting from the coupling between the atomic and point defect (PD) fluxes towards the structural defects of the microstructure. This flux coupling depends on the migration mechanisms of PDs and atoms, including thermal diffusion mechanisms and forced atomic relocations (FAR) occurring in displacement cascades. We derive an analytic model of the PD and solute RIS profiles accounting for PD production and mutual recombination, the FAR mechanism, and the overall sink strength of the microstructure controlling the elimination of PDs at structural defects. From this model, we present a parametric investigation of diffusion and RIS properties in dilute Fe-$B$ ($B$ = P, Mn, Cr, Si, Ni, and Cu) binary alloys, in the form of quantitative temperature/radiation flux/sink strength maps. As in previous works, we distinguish three kinetic domains for the diffusion and RIS properties: the recombination domain, the sink domain, and the thermal domain. Both our analytical approach and numerical applications demonstrate that the diffusion and RIS behaviors of PDs and solute atoms largely differ from one kinetic domain to another. Moreover, at high radiation flux, low temperature, and large sink strength, FARs tend to destroy the solute RIS profiles and therefore reduce the overall amount of RIS by forcing the mixing of solute and host atoms, especially close to PD sinks. Finally, we provide quantitative criteria to emulate in-reactor RIS behaviors by ion irradiation.
We present a comprehensive set of first principles electronic structure calculations to study transition metal solutes and their interactions with point defects in austenite. Clear trends were observed across the series. Solute-defect interactions were strongly correlated to the solute size factors, consistent with local strain field effects. Strong correlations with results in ferrite show insensitivity to the underlying crystal structure in Fe. Oversized solutes act as strong traps for vacancy and self-interstitial defects and as nucleation sites for the development of proto-voids and small self-interstitial loops. The reduction in defect mobility and net defect concentrations explains the observed radiation-damage resistance in austenitic steels doped with oversized solutes. Oversized solutes remaining dissolved in oxide dispersion-strengthened (ODS) steels could contribute to their radiation-damage resistance. Ni and Co diffuse more slowly than Fe, along with any vacancy flux produced under irradiation below a critical temperature, which is 400 K for Co and their concentrations should be enhanced at defect sinks. Cr and Cu diffuse more quickly than Fe, against a vacancy flux and will be depleted at defect sinks. Oversized solutes early in the transition metal series form highly-stable solute-centred divacancy (SCD) defects with a nearest-neighbour vacancy. The vacancy-mediated diffusion of these solutes is dominated by the dissociation and reassociation of the SCDs, with a lower activation energy than for self-diffusion, which has important implications for the nucleation and growth of complex oxide nanoparticles containing these solutes in ODS steels. Interstitial-mediated solute diffusion is energetically disfavoured for all except Cr, Mn, Co and Ni. The central role that solute size plays in the results presented here means they should apply to other solvent metals and alloys.
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