Metallurgy and superconductivity in Nb-Al

The formation of the Nb
3
Al A15 phase under equilibrium conditions is studied on bulk, homogenized samples. Due to compositional effects, T
c
of samples annealed during long periods at temperatures below 900°C is found to be close to 12 K. It follows that the high T
c
values (15-16 K) observed on multifilamentary Nb
3
Al wires reacted at these temperatures cannot be interpreted on the basis of the equilibrium phase diagram.