Superconductivity near 20 K in ternary and quaternary alloys based upon Nb3Al

Ternary and quaternary A-15 type compounds, Nb
3
Al
1-y
M
y
(M=Be,B) and (Nb
1-x
Ta
x
)
3
Al
1-y
Ge
y
, were investigated in the composition range where the A-15 structure was stable. The superconducting critical temperatures of these compounds were measured as a function of composition (x and y), lattice constant, and condition of heat treatment. It was found that the critical temperature can be enhanced up to near 20 K by substituting Al for the quest elements in a stoichiometric compound Nb
3
Al. Magnetic field dependences of the critical current density were also discussed for as-cast and aged samples.