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

APFIM investigation of irradiated VVER 440-type reactor pressure vessel steels suggests the appearance of multi-component clusters. The effect is surprising from the thermodynamic point of view. Numerical estimations 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 from a Fe containing core and a vacancies-rich shell. Such structures can agree with the small angle neutron scattering curves measured. On the other hand, the scattering curves do not provide features which prove the existence of core-shell-structures.