Influence of nitrogen Gas pressure on the miscibility Gap in the Ti–Zr carbonitride system

The microstructure of cemented carbides with a gradient structure at the surface consists of WC, cubic carbonitrides and a binder phase. The carbonitrides can, for example, consist of Ti(C,N)–Zr(C,N) where it is reasonable to believe that there is a miscibility gap with Ti-rich and Zr-rich carbonitrides. In the present work, the effect of the N2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. In the study, the carbonitride system is in equilibrium with: WC, liquid binder, graphite and, N2-gas of different pressures. Both Fe and Co are used as binder phase to study the effect of the binder phase. The results verify that there is a miscibility gap in the carbonitride system and that the region of the miscibility gap will change when N is introduced. There is a critical N2-gas pressure lower than 0.1 bar and above that pressure the compositions of the carbonitride are rather constant as a result of the formation of a surface rim.

► For the (Ti,Zr,W)(C,N) system, a miscibility gap is observed with two cubic carbonitride phases.
► There is only a miscibility gap when both Ti and Zr are present.
► Above a a critical N2-gas pressure lower than 0.1 bar the composition of the carbonitride phases is rather constant.
► The selected binder phase element has a small effect on the carbonitride composition.
► No classical spinodal-like structures are observed.