Metallurgical considerations for optimizing the superconducting properties of Nb3Al

The effects of heat treatment and composition on the superconducting transition temperature of Nb₃Al were studied in conjunction with an X-ray diffraction investigation of ordering and other structural effects. It was determined that an ordering anneal of 750°C for at least 48 hr achieved a maximum midtransitionT _c of 18.5 K in arc-melted samples of Nb₃Al. The changes in lattice parameter and order parameter during heat treatment indicate that theβ-tungsten crystal structure becomes more ordered as the maximumT _c is approached. Further, theT _c of 18.5 K could not be reached in Nb₃Al material having nonstoichiometric composition. In such material, there was a direct correlation between increasing percentage of second-phase Nb₂Al present and decreasingT _c after the maximumT _c had been obtained. The information obtained during this investigation led to the development of a simple empirical model to explain the change in theT _c of Nb₃Al. Also based on this work, a method was evolved for producing conductor material based on the highT _c Nb₃Al alloy (Appendix I). J. G. KOHR, formerly Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass.     

The oxidation of ordered Zr3Al

Weight gain measurements have been made and metallographic examinations have been carried out on the oxidation of ordered Zr₃Al polycrystals in steam at 675 to 925 K and in pressurized water at 575 K. The study shows that the oxidation rate increases exponentially with increasing volume fraction, f _α, of second-phase αZr particles; that a change occurs in the nature of oxidation at 825 to 875 K; that at 675 to 825 K oxidation is characterized by a breakaway to a higher oxidation rate; that weight gain and oxide film thickening kinetics in pressurized water obey quartic to cubic rate laws, at least for times up to 716 days; and that short-circuit mass transport occurs along dislocations and along grain boundaries. The study also shows that in pressurized water and in steam at temperatures ⪝ 725 K, the oxidation resistance of Zr₃Al is greater than that of Zircaloy-2, provided that . The results are interpreted in terms of the microstructure of the alloys and of the oxide films and in terms of the decomposition phase transformation Zr₃Al → Zr₂Al + Zr. Formerly with Atomic Energy of Canada, Ltd., Clark River, Ontario.