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, Nb3Al composites is described. Composites are fabricated by placing niobium rods in an aluminum matrix, and then drawing to wire. Nb3Al is formed at temperatures exceeding 1800°C for ∼5 s and ordered at 750°C for 48 h. A critical current, Jc(H), in excess of 10⁵A/cm²( dynes/cm³) at 7 T and a Tcto 18.2 K are obtained. Attempts to improve Jc(H) by controlling the grain size in the Nb3Al diffused layer are discussed. Precipitates, arising from the addition of carbon during Nb3Al 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 Nb3Al layer is accelerated, Tcis lowered and Jc(H) is not significantly improved. Jc(H) rapidly decreases with an increase in Nb3Al or (Nb-Zr)3Al layer thickness, d. Jc(H) is independent of d in composites with d ≳ 1.5 μm. In general, the Nb3Al grain size appears comparable to d for d ≤ 1 μm. Significant improvement of Jc(H) for Nb3Al superconducting composites reacted at temperatures above 1800°C (to achieve Tc> 17 K) is achieved only by maintaining the layer thickness well below d ∼ 1.0 μm.