Powder-metallurgical processing and phase separation in ternary transition metal carbides

University dissertation from KTH Royal Institute of Technology

Abstract: Ternary transition metal cubic carbides have high hardness and are potential carbides in cemented carbide and cermet tools, as well as hard coatings used to improve metal cutting performance. In the present work, (Ti,Zr)C, (V,Nb)C, and (V,Ta)C ternary cubic carbides were synthesized using traditional powder-metallurgical methods. The effect of synthesis temperature and starting materials on synthesis is investigated, and the microstructure evolution during aging is studied. (Ti,Zr)C was found to decompose into lamellae upon aging at the temperature range from 1150 to 1800 °C. A similar microstructure was observed in (V,Ta)C and (V,Nb)C- 0.5 wt% Fe. All of these structures were found to form through discontinuous precipitation.The grain misorientation distribution of (Ti,Zr)C aged at 1400 °C is investigated. It was found that decomposition tends to occur at high-angle grain boundaries above 25°. The hardness of as-synthesized (Ti,Zr)C powder was found to be 41±6 GPa. Fully decomposed (Ti,Zr)C particles were found to be slightly harder than the undecomposed counterpart. On the other hand, in (V,Nb)C-0.5 wt% Fe, the decomposed structure formed upon aging at 1200 °C was found to have a hardness of 26±2 GPa, which is basically the same as the unaged alloy.Furthermore, the sintering behavior of (Ti,Zr)C with WC-Co is investigated. There are two γ-phases in the final microstructure, one TiC-rich and one ZrC-rich. (Ti,Zr)C was found to decompose at an early stage of sintering, and the final grain size of WC and the two γ-phases was found to be 10% smaller than that in a reference WC-TiC-ZrC-Co composite.