Microstructural characterization of self-propagating high-temperature synthesis/ dynamically compacted and hot-pressed titanium carbides

Abstract

Titanium-Carbide produced by combustion synthesis followed by rapid densification in a high-speed forging machine was characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The density of the combustion synthesized/dynamically compacted TiC reached values greater than 96 pct of theoretical density, based on TiC_0.9, while commercially produced hot-pressed TiC typically exceeded 99 pct of theoretical density. The higher density of the hot-pressed TiC was found to be attributable to a large volume fraction of heavy element containing inclusions. The microstructure of both TiCs consists of equiaxed TiC grains with some porosity located both at grain boundaries and within the grain interiors. In both cases, self-propagating high-temperature synthesis (SHS)/dynamically compacted (DC) and hot-pressed, the TiC is ordered cubic (NaCl-structure,B ₁; Space Group Fm3m) with a lattice parameter of ≈0.4310 nm, indicative of a slightly carbon deficient structure; stoichiometric TiC has a lattice parameter of 0.4320 nm. For the most part, the grains were free of dislocations, although some dislocation dipoles were found associated with the voids within the grain interiors. In one SHS/DC specimen, a new, complex Ti-Al-O(C) phase was observed. The structure could not be matched with any previously published phases but is believed to be hexagonal, with a c-axis/a-axis ratio of ≈6.6, similar to the AlCTi₂ phase which has a point group 6 mmm. In all other SHS/DC TiC samples, the grains and grain boundaries were devoid of any second-phase particles. The hot-pressed TiC exhibited a greater degree of porosity than the SHS/densified specimens and a large concentration of second-phase particles at grain boundaries and within grains. The structure and composition of these second-phase particles were determined by con-vergent beam electron diffraction (CBED) and X-ray microanalysis.