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 (␣ Ј) 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 ␣ Ј, ␤, and ⍀ phases are determined as functions of the V content in the composition range 3 Յ at. pct V Յ 70. This information is combined with a detailed analysis of the available experimental data on the ␣ Ј, ␤, 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.

Structural properties and stability of metastable phases in the Zr-Nb system: Part II. Aging of bcc (β) alloys and assessment of β-Zr

“…4.4 ϫ 10 Ϫ6 K Ϫ1 along the a ␣ axis and 3.6 ϫ 10 Ϫ6 K Ϫ1 along the c ␣ axis. [17] However, the present result compares very well with the ␣ ␤ (300 K) value recently proposed for the metastable ␤ phase of Ti by Aurelio et al, [30] viz (300 K) ϭ 7.14 ϫ 10 Ϫ6 K Ϫ1 . The ␤ phase becomes stable at T R in the elements of group V of the Periodic Table: V, Nb, Ta, and their ␣ ␤ (300 K) values are known from experiments, [14] viz 8.85 ϫ 10 Ϫ6 K Ϫ1 , 7.21 ϫ 10 Ϫ6 K Ϫ1 , and 6.62 ϫ 10 Ϫ6 K Ϫ1 , respectively.…”

“…This phase might be considered as a distorted bcc structure, which forms coherently from ␤ by a diffusionless phase transition. A recent analysis of crystallographic data in a related alloy system [30] suggests that the apparent LP for the ␤ phase coexisting with ⍀ might be lower than expected in the composition range where the ⍀ fraction is large enough. The results in Figure 1 indicate that the deviations from the dashed line are largest in the composition range 7 Ͻ at.…”

Theoretical modeling of molar volume and thermal expansion

“…Combining with previous data, they extrapolated to pure bcc Zr and obtained a value of the lattice parameter at room temperature. Aurelio et al (2002) used the same method to estimate the lattice parameter of metastable bcc Ti at room temperature. In another work, Aurelio et al (2003) determined the lattice parameters of bcc Zr at several temperatures below 900 K. The assessments of CLEs for bcc Ti and bcc Zr are based on the extrapolated lattice parameters as well as the measurements in their stable regions.…”

Studies in materials science and magnetism at Centro Atómico Bariloche using neutron powder diffraction

“…It is therefore of fundamental interest to study the structural properties and relative stability of metastable phases in these alloys. With such data, a series of issues were analysed related to the structure, relative stability, and phase relations in the alloys and its constitutive elements [6][7][8][9][10][11][12]. Neutron diffraction experiments were conducted over several years, studying the atomic ordering in the Ω phase, the composition dependence of interatomic distances in the [13].…”