Modeling of yttrium oxide particle precipitation in iron

Full text of publication follows: Ferritic-martensitic steels are candidate materials for future fusion reactors. A promising way to improve mechanical properties of these steels at elevated temperatures is the strengthening with nano-size particles of Y2O3. The preparation of such oxide dispersion strengthened (ODS) steels is currently a two-stage process, involving mechanical milling of basic steel and oxide powders and subsequent hot (∼1100 deg. C) isostatic pressing (hipping). Understanding of the mechanisms of nano-particle formation is important for the efficient control of particle number densities and size distributions. According to the contemporary view, the parameters of oxide particles are determined primarily by the quality of milling and are not too much affected by the hipping, since the oxide melting temperature is well above the hipping temperature. However, the recent experimental findings indicate that the picture of oxide particle formation can be more complicated. In particular, there are evidences that after the milling at least a part of yttrium and oxygen atoms resides in the solid solution. If this is indeed the case, the formation of oxide particles can occur at the hipping stage as a result of yttrium-oxygen co-precipitation. Modeling of yttrium-oxygen co-precipitation is a way to clarify the details of the precipitation kinetics. Having in mind that the impurity (Y,O) concentrations are very high (parts of atomic percent) and the precipitation involves quite complicated interaction of particle constituents during the compound formation, the most appropriate tool is the direct simulation experiments using lattice kinetic Monte-Carlo (LKMC). Here we present the results of LKMC simulation of two-component compound precipitation during high-temperature annealing. As far as the simulation input parameters for the considered system are not well established, the unknown parameters were varied within reasonable ranges taking into account preliminary results of ab initio calculations. The simulations indicate that the parameters of resulting particle ensembles are very sensitive to the selection of processing parameters (temperature, annealing time, initial alloy composition) and the efficiency of compound elements binding, which opens a way to the control of the of the oxide particle distribution parameters through optimization of the hipping conditions. (authors)