Related Films
The following related films are available in our free Lectures on Superconductivity

Nb3Al and the RHQT process

Niobium aluminide, Nb3Al, is a superconductor of the A15 (Cr3Si) structure. Its Tc of 18.7 K is only a little above that of the more established and easier to manufacture Nb3Sn, but its good critical current performance in high magnetic fields shows potential for applications in fields exceeding 20 T.

However, the achievement of stoichiometry is crucial for producing high Tc and hence high Bc2 in Nb3Al conductors. Low temperature processing by solid-state diffusion that takes place at temperatures of less than 1000 °C results in the formation of aluminium-deficient Nb3Al and hence the Tc is low. The rapid heating, quenching and transformation (RHQT) technique, pioneered at NIMS in Japan, is a good candidate process to avoid these probelms. Nb/Al multifilamentary composites are rapidly heated to obtain a body-centred cubic (bcc) supersaturated solid solution (Nb(Al)ss) and then transformation annealed at 800 °C to form stoichiometric, fine-grained Nb3Al showing excellent superconducting properties. However, the RHQ conditions and the transformation heat treatments greatly influence the final microstructure and so the superconducting properties. Therefore, attention is being focussed on the optimisation process and the effect of the RHQT conditions on the superconducting properties and microstructure of the Nb3Al superconductors.

Research in Cambridge

Work in Cambridge on this process has applied our in situ resistometry technique to RHQT samples from NIMS to better understand and optimise the RHQT processing conditions.

The most important parameter of the RHQ treatments is the value of Joule heating, which corresponds to the maximum heating temperature (Tmax). When Tmax is around 2000 °C, stoichiometric bcc Nb(Al)ss phase is formed, which is the optimum condition. However, if Tmax is below about 1900 °C, direct synthesis of some compound phases such as disordered A15 (Nb3Al) and σ (Nb2Al) occur. On the other hand, if Tmax is too high, two-phase separation into Al-deficient Nb(Al)ss and Al-rich Nb(Al)ss phases occur on quenching from the partial melting region, while near stoichiometric Nb(Al)ss is quenched from the complete melting region.

During the transformation heat treatment, many parallel stacking faults form in the A15 sub-grains with a spacing of 10-20 nm, with higher Al concentration around the stacking faults. The formation of stacking faults shifts the A15 phase to Nb-rich compositions and thus lower Tc and Bc2 than for the stoichiometric case. Ordering of the bcc Nb(Al)ss structure occurring just before transformation to the A15 phase is believed to be the cause of the stacking faults. Therefore, control of the temperature ramp rate during the transformation heat treatment is essential as a slower ramp rate will promote the ordering of bcc Nb(Al)ss and so the formation of the stacking faults. However, deformation of the Nb(Al)ss is effective in making the superconducting properties insensitive to the ramp rate.

In-situ resistance measurements have been performed during the transformation heat treatment on wires quenched under different RHQ conditions and for mechanically deformed Nb(Al)ss wires. In addition, in-situ resistance measurement was also performed on the JR-processed Nb-Al precursor wires in which phase changes during solid-state diffusion are much more pronounced.

Related publications

Showing 3 publications related to Nb3Al in reverse date order.
Links to online copies of these papers may require a journal subscription (personal or institutional).
1 K. S. Tan, I. Pong, S. C. Hopkins, B. A. Glowacki, A. Kikuchi, T. Takeuchi and K. Inoue, Monitoring of intermetallic phase formation in jelly-rolled Nb/Al multifilamentary conductors by in-situ resistometry
Intermetallics, 14 (4) 450-455 (April 2006) | DOI: 10.1016/j.intermet.2005.08.002 
2 K. S. Tan, S. C. Hopkins, I. Pong, R. J. Stearn, B. A. Glowacki, A. Kikuchi, T. Takeuchi and K. Inoue, In-situ resistance measurements of RHQT processed Nb3Al Superconductors
IEEE Transactions on Applied Superconductivity, 15 (2) 3532-3535 (June 2005) | DOI: 10.1109/TASC.2005.849352 

Applied Superconductivity Conference '04, Jacksonville, Florida, USA, 2004

3 K.S.Tan, S.C.Hopkins, B.A.Glowacki, A.Kikuchi, T.Takeuchi and K.Inoue, In-situ resistance measurements of Nb-Al multifilamentary conductors by solid-state diffusion and RHQT processes
Proc. Int. Workshop on Progress of Nb-Based Superconductors, eds. K. Inoue, T. Takeuchi and A. Kikuchi, NIMS, Tsukuba, Japan, 162-170 (2004) 

International Workshop on Progress of Nb-Based Superconductors, NIMS, Tsukuba, Japan, 2-4 February 2004

Related processes and techniques

External links

Extensive information about the Rapid Heating, Quenching and Transformation (RHQT) technique, and the development of Nb3Al superconductors, is available on the website of the group who pioneered the RHQT technique:

Also, the development of Nb3Al superconductors in Ohio State University and an extensive list of scientific papers published on Nb3Al can be found at the link below: