A study of constitutional equilibrium in niobium zirconium alloys, using the superconducting temperature transition

Abstract

The superconducting transition temperatures of various Nb-Zr alloys heat treated from 500° to 1050°C were used to determine the limit of solid solubility of Zr in Nb. It was found that a variation of oxygen concentration between 80 and 480 p.p.m. had no influence on the Zr solubility limit, nor on the transition temperature of the solid solution. The solubility limit thus determined was of the same form as that determined by Rogers and Atkins (1955) but at lower Zr concentrations. Constitutional equilibrium was reliably established only in specimens annealed for one week and then only in those alloys where there was a high degree of supersaturation of the Nb—Zr solid solution, despite all the alloys being deformed beyond 80%. Evidence in the nature of a proximity effect showed that after annealing for 4 hours between 800° and 900°C undeveloped precipitate nuclei were retained by cooling the alloys to room temperature.

Contamination from the specimen chamber environment was not observed in the specimens annealed for 4 hours, but it became a serious problem above 870°C in specimens annealed for 1 week. It is argued that the contamination was due to oxygen caused by the reduction of the silica specimen tube by the Nb-Zr alloy. Thus the alloy became a ternary system and a second precipitate, which appeared to be derived from ZrO₂, became observable after annealing at the higher temperatures.