Diffusion coefficients of 63Ni in Fe40Ni40B20 metallic glass

Self-diffusion coefficients of ⁶³Ni tracer atoms in the metallic glass Fe₄₀Ni₄₀B₂₀ were measured in the temperature range 575–645 K by using the secondary ion mass spectrometry (SIMS) technique. The values of the diffusion coefficients were measured on the “air-side” as well as on the “wheel-side” of the ribbon and were found to lie between 5.6 × 10⁻²³ m²/s and 11.2 × 10⁻²¹ m²/s. In the quenched specimens, the diffusion was found to decrease considerably with increasing annealing time until a well-relaxed amorphous state of the material is reached. The diffusion coefficients in the relaxed state were found to obey an Arrhenius type temperature dependence with an activation enthalpy of 2.14 ± 0.16eV for the “airside” and of 2.29 ± 0.16eV for the “wheel-side”. The corresponding values of the pre-exponential factors were 4 × 10⁻⁴ m²/s and 8 × 10⁻³ m²/s, respectively. The time dependence of measured diffusivity during structural relaxation is interpreted in terms of elimination of excess free volume via quasi-vacancies annealing out at the specimen surface which leads to an additional transport of the tracer atoms. The activation enthalpy governing this relaxation process was estimated to be about 1 eV.