ELECTRONIC BASIS FOR OMEGA PHASE-STABILITY IN GROUP-IV TRANSITION-METALS AND ALLOYS

The point of view that the d-band occupancy determines the crystal structure has been applied to the problem of omega phase stability in Group IV transition metals Ti, Zr and Hf and alloys based on them. The free energy curves are drawn for an AB omega forming alloy at room temperature and pressure as a function of change in d-based occupancy for three competing crystal structures i.e. h.c.p. (α), b.c.c. (β) and simple hexagonal (ω). It is shown that such a representation gives a unified picture of omega phase formation during various pressure and thermal treatments in different systems. The dependence of composition of ω-phase, formed during ageing in alloys, on the alloying element is attributed to the breakdown of rigid band model. Pressure is shown to be equivalent to alloying, apart from an additional driving force of PΔV It is shown that application of pressure on omega forming alloy system will cause ω-transformation only in dilute alloys. © 1979.