Estimation of shear modulus of neutron irradiated induced precipitates in russian type of reactor pressure vessel steels

Neutron irradiation produces nano-metre-sized defects in reactor pressure vessel (RPV) steels, which causes deterioration of mechanical behaviour known as neutron embrittlement. This phenomenon can be characterized by irradiation induced change of the yield stress, δσ. Value of δσ depends on the dislocation-precipitate interaction mechanism, which is determined by the type of dislocation and properties of the nano-metre-sized defects relative to the matrix: chemical composition, size misfit, coherency to matrix, shear modulus and stacking fault energy. Estimation of the shear modulus of precipitates, G₁, relative to shear modulus of matrix, G₂, has been done in our study for neutron irradiated Russian type (RPV) steels VVER-440 BM 15Kh2MFAA and VVER-440 WM Sv10KhMFT. Both steels were irradiated at temperature of 255 ± 5 grad C with flux 3·10¹⁶ n/m²s (E > 0.5 MeV) up to fluencies 6.7·10²³, 14.6·10²³ and 22.8·10²³ n/m² (15Kh2MFAA) and 8.0·10²³, 17.0·10²³ and 27.0·10²³ n/m² (Sv10KhMFT). At the first step the response of the increase of yield stress caused by the maximal matrix damage (MD), δσ_MD, is determined by the combined analysis of positron annihilation spectroscopy (PAS) data on positron mean life time (MLT) and hardness data, δσ for irradiation doses great than the critical dose, Φ_c, after that positron MLT doesn't change. Has been concluded: (1) the ratios of shear modulus of nano-defects in iron matrix to shear modulus iron matrix in Russian type RPV steels can be found according to Russel-Brown model from the combined PAS, SANS and hardness data. (2) Shear modulus of precipitates are 76.9 GPa and 73.0 GPa in neutron irradiated VVER-440 BM 15Kh2MFAA and VVER-440 WM Sv10KhMFT Russian type of steels, respectively. (authors)