The structure of a linear omega-like vacancy defect in Zr-Nb B.C.C. solid solutions

Abstract

Many Ti and Zr-base alloy systems in which a metastabile structure called the omega phase is formed, exhibit a characteristic diffuse scattering distribution from the untransformed b.c.c. solid solution. These diffuse scattering observations indicate that a localized defect is present in the b.c.c. solid solution. In the present work the structure of a new type of defect in the b.c.c. phase was determined using published diffuse scattering observations for a Zr-20 wt % Nb alloy. The defect was shown to consist of a vacancy about which are rows of atoms displaced along one 〈111〉 direction. The particular 〈111〉 displacements that occur on opposite sides of the vacancy give rise to structures that are similar to two subvariants of the omega phase. This sequence of subvariants and the additional small displacements normal to the 〈111〉 direction can explain the observed diffuse peak shifts characteristic of many Ti and Zr alloy systems. On the basis of the diffraction analysis alone it is not possible to state that the defect structure arrived at is unique. However, this structure does provide the best fit to the observations of all the models tried, and it explains in a physically reasonable manner many of the details of the diffuse scattering. The similarity of this new defect to the omega phase, suggests that it plays an important role in the mechanism of the omega phase transformation. What this role is, still remains to be determined.