Structure, residual stresses and mechanical properties of LaB6–TiB2 ceramic composites

Abstract Bulk and powdered LaB6-TiB2 composites obtained by the methods of the floating zone melting (FZM) and atomizing of heated ceramic material were investigated. Microscopic and X-ray diffraction methods were used to study and compare the phase and structural as-grown states and their changes after annealing. It is shown, that the different values of thermal expansion coefficients of the matrix and fibers form a complex stress state and a multilevel substructure in the matrix phase of the composites, which reduces their hardness and fracture toughness. In bulk composites, the structure at the macrolevel is disoriented by 1.5–2° disclinational boundaries into fragments, which in turn, contain less disoriented dislocation ensembles. In powdered composites, a structure of the matrix phase is less disoriented and is formed by subgrains with 0.5° boundaries and relatively uniformly distributed dislocations. The annealing at 1400–1600°С allows achieving an equilibrium structural state in bulk and powdered composites. An increase of the fracture toughness (1.6 times) and Vickers hardness (1.2 times), that happens due to a decrease in the dislocation density, is shown.