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

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 (alpha') phase formed by a martensitic transformation, the omega (Omega) phase formed by a displacive transformation involving the collapse of the (111) planes of the bee structure, and the untransformed bee (beta) phase. The lattice parameters (LPs) of the alpha', beta, and Omega phases are determined as functions of the V content in the composition range 3 less than or equal to at. pct V less than or equal to 70. This information is combined with a detailed analysis of the available experimental data on the alpha', beta, and Omega phases in pure Ti and Ti-V alloys and the beta phase of V. New estimates for the LPs of beta and Omega 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 Omega phase, the applicability of an approximation involved in the plane collapse model for the beta --> Omega transformation, and the extent to which the so-called Jamieson correlation for interatomic distances in the Omega phase holds for Ti.