Superconducting properties and microstructure of crystallized Hf-Nb-Si and Hf-V-Si amorphous alloys

Amorphous Hf-Nb-Si and Hf-V-Si alloys have been produced by rapidly quenching the melts using a melt-spinning technique. The silicon content in the amorphous alloys was limited to 14 to 20 at% and the niobium or vanadium content was limited to 0 to 45 at% and 0 to 35 at%, respectively. These amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The highest superconducting transition temperatures, T _c, attained were 8.9 K for the Hf₄₅Nb₄₀Si₁₅ alloy annealed for 1 h at 1273 K and 6.7 K for the Hf₅₀V₃₅Si₁₅ alloy annealed for 1 h at 1173 K. The upper critical magnetic field, H _c2, at 4.2 K and the critical current density, J _c, at zero applied field and 4.2 K were about 5.1×10⁶A m^–1 (6.4 T) and more than 1×10⁴ A cm^–2 for the Hf₄₅Nb₄₀Si₁₅ alloy and more than 8.0×10⁶ A m^–1 (10 T) and 5×10³ A cm^–2 for the Hf₅₀V₃₅Si₁₅ alloy. Detailed transmission electron microscopic studies of the annealed structure of these amorphous alloys established that, after crystallization, these alloys contain a body-centred cubic -Hf(Nb) solid solution and body-centred tetragonal Nb₃Si phases in the Hf₄₅Nb₄₀Si₁₅ alloy and hexagonal Hf₅Si₃, face-centred cubic HfV₂ and cubic V₃Si phases in the Hf₅₀V₃₅Si₁₅ alloy. Since Nb₃Si and Hf₅Si₃ are not superconducting above 4.2 K, it has been concluded that superconductivity in these crystallized alloys is due to the precipitation of -Hf(Nb) solid solution in the Hf-Nb-Si alloys and to the precipitation of HfV₂ and V₃Si compounds in the Hf-V-Si alloys.