Hardening of Ti(CN)–Fe composites by microstructural refinement and solid solution strengthening of metallic phase

Ti(CN)–Fe composites were hardened by refining their microstructure and strengthening their metallic phase. The microstructure of Ti(CN)–Fe composites fabricated by the spark plasma sintering of nanocrystalline Ti(CN)–Fe composite powders at a low temperature (1200 °C) for a short duration (5 min) was found to be finer and more uniform than that of conventional composites prepared by sintering at high temperature for a long duration. The composite powders contributed to the prevention of coalescence of Ti(CN) grains; the Ti(CN) grains in the ultrafine composites exhibited a smooth surface, unlike the rough surface of the Ti(CN) grains in the conventional composites. In addition, the mean free path of the Fe binder was shorter in the ultrafine composites than in the conventional composites. The amount of Ti in the Fe matrix was found to be approximately five times higher in the ultrafine composites than that in the conventional composites because there was insufficient time for the reprecipitation of the Ti dissolved in the Fe matrix during the cooling stage of the former. The uniform microstructure with reduced grain size, short Fe binder mean free path, and solid-solution strengthening of the Fe binder by the Ti solute contributed to the enhanced hardness (16.9–18.0 GPa) of the ultrafine composites compared to that of the conventional composites (less than 12 GPa). Finally, it was concluded that Ti(CN)–Fe composites can be hardened by low-temperature/short-time sintering of nanocrystalline composite powders, and the hardened composites could broaden the range of use of Ti(CN)-based composites in mechanical parts.