Published online by Cambridge University Press: 31 January 2011
Alloys of Nb73Al12Si14.5B0.5 were rapidly solidified into amorphous ribbons using the melt spinning technique. These were isothermally annealed at temperatures ranging from 660 to 780 °C. The A15 phase began to crystallize at 700 °C and small amounts of second phases appeared at the higher temperatures. Crystallization was dependent on quenching rate as well as annealing conditions. Below 750 °C nucleation was nonuniform and was enhanced by surfaces and quenched-in nuclei. Above 750 °C nucleation became more uniform and completely crystalline ribbons with equiaxed grains ∼30 nm in diameter were obtained. These ultra fine grained ribbons had extremely high superconducting critical current densities of 8 × 1010 A/m2 and 5 × 1010 A/m2 at magnetic fields of 0.5 and 15 tesla, respectively, at 4.2 K.
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