Ferritic-martensitic Fe-Cr steels and their derivative alloys, including EUROFER, are currently viable candidates for DEMO first wall and structural applications. Such alloys have high corrosion resistance and low irradiation swelling; however, they exhibit radiation hardening at temperatures in the working range of fusion reactors. One of the hypotheses relates radiation hardening in Fe-Cr alloys to the formation of Cr-rich alpha’-phase precipitates. Proposed driving forces behind this process include segregation to dislocation loops, interfaces, and co-segregation with impurities under irradiation.

This work focuses on the effect of C and N interstitial solutes and non-equilibrium vacancies on the phase stability of Fe-rich Fe-Cr alloy. A purely theoretical approach was chosen for this work and consists of the following steps. The first step is the creation of a database of various combinations of defects (C, N, vacancies) in Fe, Cr and Fe-Cr matrices, and the calculation of mixing enthalpies for such structures using Density Functional Theory (DFT). The second step is fitting the obtained mixing enthalpies to the lattice-based model of concentration-independent Effective Cluster Interactions (ECIs) within the Cluster Expansion formalism. The last step is the application of ECIs to perform Monte Carlo simulations at finite temperatures and for sufficiently large supercells that allow for the investigation of the effects of small concentrations of defects corresponding to the experimental samples.

Using this approach, we have shown that the formation of Cr-rich precipitations is correlated with the presence of C, N and vacancies (see Figure 1), the effects of interstitial solutes and vacancies being opposite. C and N prefer to form stable short-range ordered phases with Cr, thus increasing its local concentration. When vacancies are present in the alloy, C and N prefer to segregate to vacancies, forming defect clusters free of Cr. Our theoretical results have been cross-checked with experimental studies of irradiated Fe-Cr alloy with C and N impurities, indicating qualitatively similar microstructure with quantitatively similar local Cr concentration in the Cr-rich precipitations.