Cluster dynamics modeling of Mn-Ni-Si precipitates in ferritic-martensitic steel under irradiation

Abstract

Mn-Ni-Si precipitates (MNSPs) are known to be responsible for irradiation-induced hardening and embrittlement in structural alloys used in nuclear reactors. Studies have shown that precipitation of the MNSPs in 9-Cr ferritic-martensitic (F-M) alloys, such as T91, is strongly associated with heterogeneous nucleation on dislocations, coupled with radiation-induced solute segregation to these sinks. Therefore it is important to develop advanced predictive models for Mn-Ni-Si precipitation in F-M alloys under irradiation based on an understanding of the underlying mechanisms. Here we use a cluster dynamics model, which includes multiple effects of dislocations, to study the evolution of MNSPs in a commercial F-M alloy T91. The model predictions are calibrated by data from proton irradiation experiments at 400 °C. Radiation induced solute segregation at dislocations is evaluated by a continuum model that is integrated into the cluster dynamics simulations, including the effects of dislocations as heterogeneous nucleation sites. The result shows that MNSPs in T91 are primarily irradiation-induced and, in particular, both heterogeneous nucleation and radiation-induced segregation at dislocations are necessary to rationalize the experimental observations. We followed the kinetic equations developed by Slezov and Schmelzer [1-4] to describe nucleation-growth processes of diffusional phase transformations in multicomponent systems. The model assumes that the Mn-Ni-Si-rich phases can be treated as pure Mn-Ni-Si phases, with no other alloying elements. With the assumption that only monomers can migrate, the discrete cluster size distributions are governed by the coupled master equations: ( ) 1 , n n f n t J J t −

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

“…Visual examination of all of the APT maps in Figure 11 shows major to massive amounts of solute segregation to dislocation segments. The role of solute segregated dislocations, and dislocation loops, acting as heterogeneous precipitate nucleation sites, is both widely observed and important [4,11,27,38,46,70] [46]. It is also visually evident in Figure 11 that the CPI A-series alloys have high to very high dislocation densities.…”

“…However, there are many differences between NI and CPI as noted in the partial list and dpa rates can strongly affect RED accelerated precipitation [4][5][6]9,25,31,33,37,38,[43][44][45][46].…”

“…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].…”

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

“…We apply the radiation-enhanced diffusion (RED) model developed by Odette et al [57] to calculate the vacancy concentration under irradiation, r V X , and scale thermal diffusion coefficients with vacancy supersaturation. The method has been utilized to study radiation-induced precipitation in F/M steels [58]. At steady state, when defect production is balanced by annihilation at sinks as well as recombination in the matrix, particularly at solute trapped vacancies.…”

The mechanistic implications of the high temperature, long time thermal stability of nanoscale Mn-Ni-Si precipitates in irradiated reactor pressure vessel steels

“…Thermodynamic models predict that: a) MNSPs are nm-scale variants of equilibrium G and 2 phases in typical low alloy Mn (0.8 to 1.6 at.%), Ni (0.2 to 1.6 at.%) and Si (0.3 to 1.2 at.%) bearing RPV steels; and, b) MNSP formation is accelerated at low service temperatures, around 290°C, by the excess defect concentrations under irradiation, and corresponding radiation enhanced diffusion (RED) rates [4,14,[26][27][28]. Others propose that MNSPs are not thermodynamic phases, but rather are formed, grown and sustained by radiation induced solute segregation (RIS) to defect sinks [23][24][25]29], such as dislocation loops, vacancy clusters [23,24,30] and network dislocations [25,31]. Clarifying MNSP formation mechanisms is important, not only for developing advanced models to predict embrittlement over extended RPV operation, but also for guiding development of new irradiation tolerant alloys.…”