Metallurgy, fabrication, and superconducting properties of multifilamentary Nb3Al composites

The control of metallurgical structure during fabrication that will improve the superconducting properties of multifilamentary, aluminium-stabilized, Nb
3
Al composites is described. Composites are fabricated by placing niobium rods in an aluminum matrix, and then drawing to wire. Nb
3
Al is formed at temperatures exceeding 1800°C for ∼5 s and ordered at 750°C for 48 h. A critical current, J
c
(H), in excess of 10
⁵
A/cm
²
(
F_{p} \sim 7 \times 10^{8}
dynes/cm
³
) at 7 T and a T
c
to 18.2 K are obtained. Attempts to improve J
c
(H) by controlling the grain size in the Nb
3
Al diffused layer are discussed. Precipitates, arising from the addition of carbon during Nb
3
Al layer growth, do not appear to be effective as grain-boundary or flux pinners. When 1% Zr is added to the Nb, the growth of the Nb
3
Al layer is accelerated, T
c
is lowered and J
c
(H) is not significantly improved. J
c
(H) rapidly decreases with an increase in Nb
3
Al or (Nb-Zr)
3
Al layer thickness, d. J
c
(H) is independent of d in composites with d ≳ 1.5 μm. In general, the Nb
3
Al grain size appears comparable to d for d ≤ 1 μm. Significant improvement of J
c
(H) for Nb
3
Al superconducting composites reacted at temperatures above 1800°C (to achieve T
c
> 17 K) is achieved only by maintaining the layer thickness well below d ∼ 1.0 μm.