The purpose of this paper is to make clear the mechanisms and the processes of formation of surrounding structure (s.s.) around TiC grains in sintered TiC-Mo₂C-Ni alloy. The partial 1300°C isothermal section of the quasi-ternary high carbon alloy was studied. Referring to this diagram, metallographic studies mainly on the coarse-grained alloys having various compositions and sintered under various conditions were made in detail.
Results obtained were as follows: (1) The fromation of s.s.occurred through two or three steps, according to the alloy composition, sintering temperature and time, sintering method, etc. For instance, when the alloys with comparatively high contents of Mo₂C and Ni were sintered as usual, there could be observed three steps in the following order: the step of precipitation of Mo₂C-25%TiC solid solution (accompanied by the dissolution in particular of Mo₂C-2%TiC solid solution) on the surface of TiC grains; the step of solution-reprecipitation of s.s.and TiC core (bared as a result of dissolution of s.s.); and the step of precipitation during cooling. (2) Interdiffusions in or through the carbide phase during sintering seemed in general not to associate with the growth of s.s.and its composition changes. (3) Therefore, the s.s.was generally consisted of two or three layers of different thickness and compositions.