Metastable phases in the Ti-V system: Part I. Neutron diffraction study and assessment of structural properties

Abstract

This article presents the results of a neutron diffraction study of a series of quenched Ti-V alloys and an assessment of the composition dependence of the structural properties in the Ti-V system. Upon quenching to room temperature and atmospheric pressure, three metastable phases occur, viz., the hcp (α′) phase formed by a martensitic transformation, the omega (Ω) phase formed by a displacive transformation involving the collapse of the (111) planes of the bcc structure, and the untransformed bcc (β) phase. The lattice parameters (LPs) of the α′, β, and Ω phases are determined as functions of the V content in the composition range 3 ≤ at. pct V ≤ 70. This information is combined with a detailed analysis of the available experimental data on the α′, β, and Ω phases in pure Ti and Ti-V alloys and the β phase of V. New estimates for the LPs of β and Ω Ti and expressions describing the composition dependence of the LPs are presented. Using the assessed values, various open questions are discussed, i.e., the composition range where the hexagonal to trigonal symmetry change is observed in the Ω phase, the applicability of an approximation involved in the plane collapse model for the β → Ω transformation, and the extent to which the so-called Jamieson correlation for interatomic distances in the Ω phase holds for Ti.