The effect of cobalt content on the microstructure of liquid-phase sintered TaCCo alloys

A study has been made of the effect of the cobalt binder-phase content on the microstructure and certain mechanical properties of TaCCo alloys prepared by liquid-phase sintering. The investigation was limited to a composition range of 0 to about 65 vol.% cobalt within which the carbide grains formed a continuous network and beyond which the carbide became discontinuous and segregation of the phases occurred. The relationship between the volume fraction and the degree of carbide grain contact within this range was consistent with the existence of an equilibrium degree of contact determined by the ratio of the carbide grain-boundary energy to the carbide/binder interfacial energy. The rate of the carbide grain growth that occurs during sintering decreases slightly in the range from 7 to 60 vol.% binder phase. It is concluded that, in this range, the growth is controlled by a process of dissolution and reprecipitation at carbide/liquid interfaces. The hardnesses of alloys in the investigated composition range agree well with a semi-empirical equation that predicts that the hardness is dependent on the degree of carbide grain contact and on the mean free path through the binder phase as well as on the volume fraction of the phases. Results indicate that the resistance of the microstructure to cracking is also dependent on these parameters.