RECENT DEVELOPMENTS IN THE FIELD OF SILICIDES AND BORIDES OF THE HIGH-MELTING-POINT TRANSITION METALS

Abstract

The binary systems of silicides of the high-melting-point transition metals are now well understood, except for the hafnium-silicon system. Research since 1954 is reviewed, with particular reference to the compound Me₅Si₃ and its position in the silicide systems. Reference is also made to the pseudo-binary and pseudo-ternary silicide systems.

The structures of many of the intermetallic phases in the binary boride systems have now been determined, but complete equilibrium diagrams still remain to be established in some cases. New tentative diagrams are given for the systems vanadium–boron, niobium–boron, and tantalum–boron, and structures are suggested for the borides V₃B₂, Nb₃B₂, and Ta₃B₂ with the T2 structure (isostructural with U₃Si₂).

Ternary alloys of the systems Me–Si–B are of great interest, not only structurally but also for practical reasons. The complete systems Me–Si–B of Group VI (Mo–Si–B and W–Si–B) have accordingly been studied by X-ray, thermal-analysis, and micrographic methods. The system Cr–Si–B has been determined and attention is directed to the possible commercial applications of certain alloys containing additions of metals of the iron group, in particular nickel, for sprayed coatings resistant to liquid aluminium.

The question of cementing silicides and borides with metals and alloys is discussed theoretically. The character of the silicide or boride system in question and the behaviour of the intermediate phase in relation to the bonding material are of decisive importance for the selection of the latter. Only very limited data are to be found in the literature on the behaviour of silicides and borides in relation to metals and alloys. Alloys based on TiB₂, ZrB₂, MoSi₂, and WSi₂, impregnated with numerous metals and alloys, have been prepared. Their structures have been studied and the technical suitability of various combinations is discussed on the basis of their technological properties.