Ultrasonic study of the beta → omega phase transformation in Zr-Nb alloys

Abstract

Elastic constant, ultrasonic attenuation and X-ray measurements were made as a function of temperature on Zr-Nb single crystals in order to study the mechanism of the β → ω phase transformation. In the Zr-20 wt. % Nb alloy C 44 decreased with decreasing temperature starting at 330–340°K reaching a minimum at 160°K, while both C' and C 11 showed a positive deviation from linear quasiharmonic behavior starting at 250–280°K. These observations are reversible with temperature. X-ray measurements indicated that little if any ω phase was present in the 20 wt. % Nb alloy at 300°K and that upon cooling to 89°K a small amount of ω phase was formed. It was suggested that the positive deviation of C 11 and C' from the extrapolated values based on quasiharmonic theory is due to the presence of the ω phase and that the onset of the β→ω transformation is indicated by the first sign of deviation from linearity. The negative deviation of C 44 from quasiharmonic behavior is attributed to a softening of the b.c.c. structure which is a precursor to the actual transformation. The observations on the Zr-25 wt. % Nb alloy are in general agreement with these suggestions. In the Zr-30 wt. % Nb alloy the X-ray results indicated that no co phase is present over the temperature range 89–300°K and C 11, C' and C44 all showed a normal linear variation with temperature, in agreement with the above suggestions. The internal friction peak observed at 100–120°K in the Zr-20 wt. % Nb alloy was attributed to the lattice distortion associated with the β → ω transformation. The higher temperature internal friction peak, which is present in the 25 and 30 wt. % Nb alloys, but not in the 20 wt. % Nb alloy, was attributed to a process occurring entirely in the β phase.