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).