On high pressure β → ω phase transition in Ta and Zr-Nb

Abstract

Transition from ω(hexagonal) to β(bcc) phase under pressure is well established in group IVB elements. It is interesting, however, to explore if pressure induced β to ω transformation is possible. In shock wave experiments ω phase has been observed in polycrystalline Ta samples retrieved from shock pressure of ∼ 45 GPa (Hsiung and Lassila 1998), and also, in β stabilized Zr-Nb(Nb 20% by weight) quenched from shock pressure of ∼ 15 GPa (Dey et al. 2004). In static pressure measurements, on the other hand, β to ω transition is neither found in Ta up to 170 GPa(Cynn et al. 1998), nor in Zr-Nb up to 8 GPa. Structural stability of β and ω phases has been examined under hydrostatic compression up to 243 GPa in Ta and up to ∼ 18 GPa in Zr-Nb from first principles electronic band structure calculations. In agreement with experiments, present theoretical calculations suggest that the β phase is energetically favourable over ω phase at ambient conditions and remains stable throughout the compression range of calculations. Further, the calculation of activation barrier between β and ω phases at various pressures indicate that the ω phase is always unstable structure in Ta, however, the same phase is metastable Zr-Nb.