Crystallization characteristics of an amorphous Nb81Si19 alloy under high pressure and formation of the A 15 phase

Amorphous Nb-19 at% Si alloy, prepared by rapid quenching from the molten state, was annealed while being subjected to a pressure of 10 GPa. X-ray diffraction investigations on the alloy specimens quenched to ambient conditions have shown that pressure greatly alters the crystallization characteristics and the cubic A15 (Nb3Si)-phase forms in preference to the tetragonal Nb3Si phase at temperatures in the range from 7100C to 8000C. Up to 6800C, the component atoms do not show any tendency towards ordering upon crystallization and the body-centred tetragonal solid solution forms; while, at 8300C, niobium atoms diffuse to form the body-centred cubic Nb precipitates. Superconducting properties have been measured for the single-phase A15 structure with the lattice parameter a = 0.5155 nm with the results that the transition temperature, Tsub(c), is 3.4 K and the temperature coefficient of the upper critical field, Hsub(c2), is 1.2 MA m-1 K-1. (author)