Assessment of the structural relation between the bcc and omega phases of Ti, Zr, Hf and other transition metals

AURELIO, G.; A. FERNÁNDEZ GUILLERMET

ZEITSCHRIFT FUR METALLKUNDE

Carl Hanser Verlag

Año: 2000 vol. 91 p. 35 - 42

0044-3093

The name omega ($Omega $) phase refers to a high-pressure structural modification of the transition metals (TMs) Ti, Zr, and Hf. In alloys of Ti, Zr and Hf with other TMs, the $Omega $ phase can be formed and retained metastably at room temperature by quenching the bcc structure, which is usually the stable high-temperature phase in these alloy systems. As a part of a systematic investigation of the structural and bonding properties of the bcc and $Omega $ phases, and of the b$cc ightarrow Omega $ phase transformation in TMs and alloys, we present in this paper a detailed analysis of the structural relations between these phases in Ti, Zr, Hf and in other TMs. The approach is based on recognising that $Omega $ and bcc are particular cases of a trigonal P$overline{3}$m1 structure, and exploiting in depth the geometrical relations which connect them. In particular, we focus on the ratio between the relevant interatomic distances (IDs) of these phases, which are assessed on the basis of an extensive database with experimental and theoretical information. Both stable and metastable structures are considered, and various remarkable regularities in ID ratios are discussed. Finally, in the light of the systematics of ID ratios stablished in the present work, a discussion is made of the probable lattice parameters for the $Omega $ phase of Hf, which are not yet accurately known from direct measurements.