Ni-W substrates for coated superconductors using the Rabits approach

 

Nickel and Ni–W alloys can be produced in long lengths with cube texture using the RABiTS technique (Rolling Assisted Biaxially Textured Substrates).

RABiTS approach (Rolling assisted biaxially textured substrates)

  • Induction melting and casting
  • homogenisation and hot forging or rolling
  • Cold rolling to 3 mm (6 mm, 1.5 mm)
  • Recrystallisation
  • Cold rolling to 80 µm
  • Final heat treatment for cube texture formation

In particular, Ni5W (i.e. a Ni alloy containing 5 at.% W) is commercially available at IFW's spin-off company =evico as a substrate material for coated conductors.  This Nickel containing 5 atomic per cent of tungsten has the following properties.

Properties of commercially available Ni5W

  • Magnetisation: 24 emu/g @ 77 K
  • 99 % of the surface are cube textured within 10°
  • Approx. 150 MPa yield stress at room temperature
  • Thermal stability up to 1000 °C in Argon atmosphere Fcc solid solution with small lattice mismatch to YBCO

 

However, Ni5W is ferromagnetic at 77 K, the operating temperature of high-Tc superconductors. Therefore, the alloy composition has been studied in order to reduce or eliminate the ferromagnetism at 77 K. At the same time the cube texture has to be obtainable. This alloy texture is obtained by annealing after intensive cold rolling of about 95% thickness reduction. To suppress ferromagnetism, increasing the concentration of non-ferromagnetic elements is an established approach and has already been applied for the Ni–W system. Both Ni-W and Ni-W-Cr Systems have been investigated in terms of texture formation during the deformation and magnetisation. In the following plot the magnetisation of the Ni-W-Cr and Ni-W alloys is displayed. A benchmark has been set at a magnetisation of 5 emu/g, however a further reduction leads to a further decrease of eddy current losses in an ac application of the coated conductor.

 

However the stacking fault energy also decreases with increasing solute content. Thus the dislocations in
the material split more easily, which affects the sharpness or even the type of the deformation texture. In
addition the formation of twins is supported both in the deformation texture and the recrystallization
texture. That results in special requirements of how to deform these materials in order to form a deformation texture which can then transform to the cube texture on annealing. An example of this texture transformaton upon annealing is shown in the next figure.

fig2pole figs.PNG

Using Electron back scatter diffration (EBSD) orientation maps can be obtained. From these maps an overall texture content can be extracted as well as grain boundary angles which are limiting the supercurrent passing through the deposited film on the substrate. Up to Ni9.5W and Ni4W8Cr a texture of min. 95 % could be obtained. In a EBSD color coded map  Ni9.5W looks as follows.

A worse example of the same Material with only slightly varied rolling and annealing parameters looks more colourful, but less usable for our purposes.