The spotted structures were confirmed by the present authors to develope in WC-Co alloys containing small amount of tantalum and niobium carbide, respectively. And, phenomena relating to the structures were investigated. The fine and coarse-grained WC-(10-20)%Co alloys having additional carbides of 0.5-2.0% were used as specimens.
The results obtained were as follows : (1) The spotted structures appeared to be caused by the precipitation from the liquid phase enriched with tantalum carbide or niobium carbide. The enriched phase was formed probably due to the separation of liquid binder phase during the cooling of specimens. (2) Spotted phase in each alloy was identified by x-ray or electron diffraction and x-ray microanalysis as (Ta, W) C_1-x or (Nb, W) C_1-x, i.e., nonequilibrium phase containing abnormally large amount of tungsten. (3) The strength of the alloys was lowered markedly by the appearance of these structures.