Fast reactor irradiation effects on fracture toughness of Si3N4 in comparison with MgAl2O4 and yttria stabilized ZrO2

Fracture toughness of silicon nitride (Si3N4), magnesia-alumina spinel (MgAl2O4) and yttria stabilized zirconia (8 mol%Y2O3-ZrO2) was evaluated by the Vickers-indentation technique after the fast reactor irradiation up to 55 dpa (displacement per atom) at about 700 °C in the Joyo. The change of the fracture toughness by the irradiation was correlated with nanostructural evolution by the irradiation, which was examined by transmission electron microscopy. The observed degradation of fracture toughness in Si3N4 is thought to be due to the relatively high density of small-sized of the irradiation induced defects, which should be resulted from a large amount of transmutation gases of hydrogen and helium. Observed improvement of fracture toughness in MgAl2O4 was due to the blocking of crack propagation by the antiphase boundaries. The radiation effects affected the fracture toughness of yttria stabilized zirconia at 55 dpa, suggesting that the generated high density voids would affect the propagation of cracks.