Metastable phases in the Ti-V system: Part I. Neutron diffraction study and assessment of structural properties
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Metastable Phases in the Ti-V System: Part I. Neutron Diffraction Study and Assessment of Structural Properties ´ NDEZ GUILLERMET, G.J. CUELLO, and J. CAMPO G. AURELIO, A. FERNA 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 (␣ ⬘) phase formed by a martensitic transformation, the omega (⍀) phase formed by a displacive transformation involving the collapse of the (111) planes of the bcc structure, and the untransformed bcc ( ) phase. The lattice parameters (LPs) of the ␣ ⬘, , and ⍀ phases are determined as functions of the V content in the composition range 3 ⱕ at. pct V ⱕ 70. This information is combined with a detailed analysis of the available experimental data on the ␣ ⬘, , and ⍀ phases in pure Ti and Ti-V alloys and the  phase of V. New estimates for the LPs of  and ⍀ 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 ⍀ phase, the applicability of an approximation involved in the plane collapse model for the  → ⍀ transformation, and the extent to which the so-called Jamieson correlation for interatomic distances in the ⍀ phase holds for Ti.
I. INTRODUCTION
THE relation between structure, phase stability, and chemical bonding is a key ingredient of the physico-chemical approach to material systems. For stable phases, it is often possible to find reliable structural and thermodynamic data, as well as calculations of the electronic structure. However, metastable phases are generally poorly known, and there is a need for methods to produce and analyze new, reliable information. A natural starting point in the characterization of metastable alloy phases is the determination of lattice parameters (LPs), interatomic distances (IDs), and other structural properties as functions of composition. These data provide information of general interest in, i.e., the development of models for the relations between the various structures, the detailed description of the phase transitions, and the detection of changes in the bonding behavior. A class of materials which has attracted considerable attention, consists of the alloys formed by the elements of the group IV of the transition metal (TM) series, viz., Ti, Zr, and Hf, with other elements placed to the right in the Periodic Table. At high temperature, these alloys present a stable bcc ( ) phase, whereas on quenching to room temperature and atmospheric pressure (RTAP), three metastable phases occur, viz., the hcp (␣ ⬘) phase formed by a martensitic transformation from the parent phase,[1] the omega (⍀) phase formed by a displacive transformation involving the collapse of the (111) planes of the bcc structure,[2] hereafte
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