Defect aggregates in neutron-irradiated β-Si3N4

Microstructural analysis of the defect aggregates formed in bulk samples of polycrystalline β-Si3N4 neutron-irradiated to a dose of approx. 2.0 x 1026 n/m2 at temperatures of 1100 K and 925 K has been carried out. This study has shown that the defect aggregates formed are faulted dislocation loops lying on the brace 101-bar0 brace planes with a Burgers vector of b = approx. 1/10<112-bar5>. The vector is non-rational but corresponds to the insertion of an extra layer of [SiN4] tetrahedra on the brace 101-bar0 brace planes plus an additional shear in the loop plane. The formation of these loops is dependent upon the temperature of irradiation. In the sample irradiated at 1100 K their formation is additionally dependent upon whether or not a particular grain contains pre-existing c-axis dislocations. If no c-axis dislocations are present then independent nucleation of the loops is apparent; if there are pre-existing c-axis dislocations then the loops form from an apparent dissociation between the arcs of the irradiation-induced helical c-axis dislocation. In the sample irradiated at 925 K only independent nucleation of the loops occurs, regardless of whether or not there are any pre-existing c-axis dislocations in the grains. (author)