Unusual morphology of the omega phase in a zr-1.75 at. Pct Ni alloy

Abstract

The observations reported in the present communication were made in the course of a microstructural investigation on dilute Zr-Ni alloys. The alloys were prepared from nuclear-grade sponge zirconium and high-purity nickel by nonconsumable arc melting. Repeated melting was carried out to enhance homogeneity. A master alloy was used in the preparation of very dilute alloys. The observations reported here clearly indicate that the formation of the [omega] phase on [beta] quenching, can occur in the binary Zr-Ni system at very low solute concentrations, suggesting that nickel is a strong [omega] stabilizer. Such a situation is known to obtain in the binary Zr-Mo system also. However, the unusual [omega] morphology observed in the present work has not been reported in the context of dilute Zr-Mo alloys. The appearance of sharp [omega] reflections and the absence of streaking and diffuse distributions in the selected area electron diffraction (SAD) patterns suggest that the Zr-1.75 at. pct Ni composition lies toward the solute lean end of the composition range associated with the formation of the athermal [omega] phase in the binary Zr-Ni system. In a more dilute Zr-Ni alloy (Zr-1.30 at. pct Ni), no athermal [omega] could be observed in [beta] quenched specimens. Themore » absence of streaking is also consistent with the fact that the [omega] phase in the [beta] quenched Zr-1.75 at. pct Ni alloy did not occur in the form of very fine precipitates. The unusual [omega] morphology encountered in this work merits further investigation.« less