ELECTRONIC AND ATOMIC-SIZE EFFECTS ON THE OMEGA PHASE FORMATION IN TRANSITION-METAL BASED B.C.C ALLOYS

Two criteria for the omega phase formation in transition-metal based b.c.c. alloys have been proposed based on the electronic structure calculations and experiments on the omega phase. One is the electronic effect and the other is the atomic-size effect. The former effect is enhanced when the alloying transition element has more valence electrons than the mother metal. The latter effect is remarkable when the alloying transition element has a smaller atomic size than the mother metal. The importance of such a size effect has been confirmed from a series of experiments on the omega phase formation in Zr-Ti alloys. Following these criteria the compositional range for the omega phase to exist in various Ti and Zr alloys can be understood consistently. Further, the second-nearest-neighbour atomic interactions were shown to be large in metals and alloys where the omega phase is formed.