Thermal dependent properties of LaB6–MeB2 eutectic composites

Abstract

The thermal expansion of polycrystalline LaB6–MeB2 ceramic composites (where Me denotes Ti, Zr, Hf) has been studied experimentally within the temperature range from 20 °C to 1500 °C. These ceramic composites were obtained by crucible-free float zone melting method. The first temperature interval with a negative thermal elongation (NTE) (~1000–1100 °C) coincided in the LaB6–MeB2 composites and LaB6 hexaboride and was associated with La–La atomic bond softening. The hierarchy of the interatomic Me–Me–B–B bonds in the LaB6–MeB2 composite compounds, their interface and residual stresses in the LaB6 matrix and the MeB2 reinforcing phase were identified to be the source of such phenomenon. The strong covalent component of interatomic bonds compresses the crystal lattice of the composites after softening ionic and metallic components and removing residual stresses. The composites exhibited several additional NTE intervals at a higher temperature (~1000–1250°С) which were associated with the of interatomic bonds softening in the TiB2, HfB2, ZrB2 reinforcing fibers. NTE intervals determines transition of the deformation mechanism of composites from low-temperature to high-temperature one. Insignificant plasticity and brittle fracture in the area of low-temperature deformation is replaced with an increase in both plasticity and strength with temperature rise.