Thermal-relaxation studies of the stability of amorphous structures in zirconium-based superconducting alloys

The superconducting transition temperature $T_c$ of amorphous ${\mathrm{Zr}}_{2}X (X=\mathrm{C}\mathrm{o},\mathrm{N}\mathrm{i},\mathrm{P}\mathrm{d})$ and ${\mathrm{Zr}}_{3}X (X=\mathrm{N}\mathrm{i},\mathrm{P}\mathrm{d},\mathrm{R}\mathrm{h})$ alloys annealed isothermally below their crystallization temperatures is studied as a function of annealing time (up to ∼ 20-100 h). Upon annealing, $T_c$ is always found to decrease below its value in the as-quenched state. The major portion of $T_c$ depression is attributed to the relaxation of "quenched-in" strains. Alloys of ${\mathrm{Zr}}_2$Co, ${\mathrm{Zr}}_2$Ni, ${\mathrm{Zr}}_2$Pd, and ${\mathrm{Zr}}_3$Rh which undergo polymorphous crystallization ($P$ alloys) show trends of saturation in $T_c$. On the other hand, for ${\mathrm{Zr}}_3$Ni and ${\mathrm{Zr}}_3$Pd alloys which crystallize eutectically ($E$ alloys), saturation in $T_c$ is only observed at relatively low annealing temperatures over rather short intervals of annealing time. The results are attributed to the stability and instability of the "equilibrium" amorphous structures in the $P$ and $E$ alloys, respectively. It is conjectured that instability in the $E$ alloys leads to initial microscopic phase separation, the homogeneity of which is evidenced by results on transition width and flux pinning measurements. All homogeneous samples exhibit very weak flux-pinning force of about ${10}^4$ N/${\mathrm{m}}^3$ at 1.5 K. Prolonged annealing of $E$ alloys leads to eutectic decomposition. Predictions derived from a simple "proximity-effect" model are consistent with the present findings.