First-principles investigation on the composition of Ni-Si precipitates formed in irradiated stainless steels

Recent atom probe tomography (APT) study has revealed the complicated nature of Ni-Si precipitates in irradiated stainless steels. Although Ni3Si gamma' phase has been confirmed under transmission electron microscopy (TEM), the Ni/Si ratio of the precipitates detected by APT is smaller than its theoretical value 3. An interpretation of the APT results is provided in this work by considering the lattice defects in the Ni3Si gamma' phase. Using first principles calculations, Si substitutions on Ni sites were found to be the most thermodynamically stable among all the single defects considered here. Although two such substitutional defects are repulsive to each other, the repulsion decreases quickly as their separation distance grows. By keeping a large enough distance between each other, multiple Si substitutions can appear at high densities in the gamma' phase, which can be one important contributor to the small Ni/Si atom ratio in Ni-Si precipitates observed by APT. (C) 2017 Elsevier B.V. All rights reserved.