Multi-component clustering in VVER-type pressure vessel steels – thermodynamic aspects and impact on SANS

Atom probe field ion microscopic investigations of irradiated VVER 440-type reactor pressure vessel steels suggest the appearance of multi-component clusters. The effect is unexpected considering the phase diagrams of the concerning alloys. Numerical calculations of the negative minimum of the thermodynamic driving forces for a multi-component system are carried out considering a quasi-quaternary system consisting of Fe, Mn, Si and vacancies. A relative minimum was only found for a composite model of the multi-component clusters composed of an Fe containing core and a vacancies-rich shell. The ratio of their sizes is estimated from the condition that such structures agree with the small-angle neutron scattering (SANS) curves measured.