Diffusion and thermal stability of amorphous copper zirconium

Measurements have been made of diffusion and thermal relaxation in amorphous Cu₅₀{Zr}₅₀$ samples prepared by melt spinning in vacuum. Diffusion of Ag and Au impurities over the temperature range 317--385 ^∘{}C was measured by Auger-electron spectrometry (for Ag) and Rutherford-backscattering spectrometry (for Au). Diffusivities of Ag and Au were found to be comparable to values for self-diffusion in crystalline Cu at the same temperatures, but activation energies and preexponential factors for the amorphous alloy are much smaller than those for the crystalline solid. These results suggest that the basic diffusion mechanism for the amorphous alloy may be an extended, liquidlike defect involving 13 or more atoms, rather than the monovacancy well established for crystalline Cu. Thermal relaxation measurements were made in amorphous Cu-Zr to 550 ^∘{}C by differential scanning calorimetry. The relaxation processes are characterized by a spectrum of activation energies, the lowest of which is essentially the same as that measured for Ag diffusion. The activation energy for crystallization is far larger, implying that the rate-limiting step in the amorphous-crystalline transition is the formation of finite crystallization nuclei, rather than only the diffusional motion of atoms.