Microstructure and mechanical properties of ultrafine (Ti,Mo,W,Nb,Zr,Ta)(CN)–Ni composites prepared using the stabilized (Ti,Mo,W,Nb,Zr,Ta)(CN) phase

Abstract

Nanocrystalline (Ti,Mo,W,Nb,Zr,Ta)(CN) (crystallite size: approximately 30 nm, molar ratio of carbon and nitrogen = 7:3, 6:4, 5:5, and 3:7) powders were prepared via carbothermal reduction of a high–energy ball–milled mixture of oxides and graphite. When the molar ratio of carbon and nitrogen was 3:7, Mo2C phase, which was not combined with the carbonitride phase, was observed owing to the low affinity between Mo and nitrogen. However, the (Ti,Mo,W,Nb,Zr,Ta)(CN) phases were stabilized by the optimization of the ratio of carbon and nitrogen in the (Ti,Mo,W,Nb,Zr,Ta)(CN) powders without Mo2C. It was observed that the growth of the stabilized (Ti,Mo,W,Nb,Zr,Ta)(CN) was suppressed effectively during liquid–phase sintering, thus maintaining the grain size in an ultrafine scale (below 300 nm). The hardness (max. 16.4 GPa) of the (Ti,Mo,W,Nb,Zr,Ta)(CN)–Ni composites prepared using the stabilized (Ti,Mo,W,Nb,Zr,Ta)(CN) was superior to that (max. 13.2 GPa) of the conventional Ti(CN)–Mo2C–WC–Nb(CN)–Zr(CN)–TaC–Ni composite. Most of the fractures occurred along the interfaces between the (Ti,Mo,W,Nb,Zr,Ta)(CN) and Ni in the (Ti,Mo,W,Nb,Zr,Ta)(CN)–Ni composites whereas the transgranular fractures are primary in the conventional Ti(CN)–Mo2C–WC–Nb(CN)–Zr(CN)–TaC–Ni composite during failure. It is believed that the strengthening of the stabilized (Ti,Mo,W,Nb,Zr,Ta)(CN) grains by solid solution effect is the cause of the intergranular fracture occurring in the (Ti,Mo,W,Nb,Zr,Ta)(CN)–Ni composites during failure.