The structure and decomposition of Zr and Ti b.c.c. solid solutions

Abstract

The literature concerning the decomposition of Zr and Ti alloys is reviewed with particular emphasis on the b.c.c. ( β)→athermal omega ( ω) transformation and the structure of Zr and Ti b.c.c. solid solutions. Illustrative examples are taken principally from the ZrNb and TiNb systems. High resolution dark-field electron microscopy, diffraction, and ultrasonic techniques have been used to follow the β→ ω phase transformation in Zr and Ti alloys. The ω morphology was shown to consist of 10–15 Å diameter particles arranged in 〈111〉 rows. As well, clusters of rows were presented in low solute content alloys and the cluster size decreased with increasing solute content. The observed morphology can explain the presence of {111} planes of diffuse intensity in reciprocal space and the change in appearance of the ω reflections with increasing solute content. The mechanism of the transformation is discussed in terms of instability or metastability of the b.c.c. structure relative to a 2 3 〈111〉 longitudinal displacement wave. This mechanism can be thought of as involving the 〈111〉 displacement of 2 neighboring close-packed rows of atoms relative to each other. Single crystal elastic constant measurements on ZrNb alloys show that C 44 decreases in the vicinity of the β→ ω transformation. This is attributed to a softening of the b.c.c. structure which is a precursor to the actual transformation. Alloys that are known to be well above the M s temperature for the β→ ω transformation, and hence must be single phase b.c.c. solid solutions, exhibit diffuse ω reflections. It is suggested that the diffuse intensity is due to the presence of a 〈111〉 linear fault (or disorder) in the b.c.c. phase. The atom movements associated with this fault are similar to the displacements required for the β→ ω transformation.