Solid state reactions in transition metal diboride-based materials

Abstract

The variety of microstructural features of the layered structures of transition metal borides in precipitation-annealed ceramics in the quasi-ternary system TiB 2 -WB 2 -CrB 2 have been investigated by high-resolution transmission electron microscopy (HRTEM) and electron-filtering transmission electron microscopy (EFTEM). The combination of the methods allows to resolve structure and composition of various precipitates. The reaction of the starting borides to the (Ti, W, Cr)B 2 solid-solution phase could be analysed to be a diffusive solid state reaction across the grain or interphase boundaries. The annealing leads to a partly reacted specimen which shows a microstructure consisting of unreacted TiB 2 cores with a shell of the (Ti, W, Cr)B 2 solid solution phase, where high stress due to the misfit between the lattices is relaxed by formation of dislocations in the interface. Plate-shaped precipitates of a β-WB-type phase with thickness of only one lattice constant could be characterised chemically by electron spectroscopic imaging (ESI) and their structure was identified by HRTEM. Spinodal decomposition of the matrix phase during annealing in the two-phase field was proven by ESI and electron diffraction. Frequently occurring stacking faults in the solid solution phase and in the WB 2 phase with WEBS structure could be structurally characterised with HRTEM.