The as-quenched form of the omega phase in Zr-Nb alloys

Abstract

The as-quenched Ω phase was examined in the Zr-Nb system using high resolution dark field electron microscopy and selected area diffraction. The morphology of the Ω phase was shown to be based upon (111) tl(0001) rows of particles 10 to 15Å in diameter with an interparticle spacing of 15 to 25Å. The row morphology is in agreement with previous observations on a Zr-Ti alloy. In the Zr-8 wt pct Nb alloy, clusters made up of many rows were observed and, as the solute content increased up to 15 wt pct Nb, the clusters became smaller and the length of the individual rows decreased. Sharp ω reflections and {111} planes of intensity in reciprocal space were observed in the 8 wt pct Nb alloy. As the niobium content increased, the ω reflections broadened and became more diffuse, while the thickness of the {111} planes of intensity increased. It was suggested that the sharp ω reflections result from scattering by the clusters while the {111} planes of intensity result from scattering by isolated rows of particles. Comparison of the ω morphology with the diffraction effects showed good agreement with these suggestions. In alloys with 15 wt pct and higher Nb content, the 0001 and 0002 broadened ω reflections were observed to be displaced toward each other alongω phase. It was suggested that these displacements are related to the mechanism of the transformation, which is thought to involve instability of the bcc structure relative to atomic displacement waves. The electron diffraction patterns were in agreement with predictions concerning this mechanism.