TRACER DIFFUSION AND THERMAL-STABILITY IN AMORPHOUS CO-ZR AND THEIR RELEVANCE FOR SOLID-STATE AMORPHIZATION

Measurements have been made of tracer diffusion, thermal relaxation, and crystallization in amorphous Co89Zr11 samples prepared by melt spinning in vacuum. The diffusivity in as-quenched material is enhanced due to the presence of quasivacancies (excess free volume). During an annealing treatment, the quasivacancies are eliminated and the diffusivity approaches a constant value that depends on temperature only and pertains to a metastable equilibrium state. Tracer self-diffusion of Co-57 measured over the temperature range 513 - 693 K in relaxed amorphous material can be described by [GRAPHICS] Tracer diffusion of Au-195 in the temperature range 633-684 K is represented by [GRAPHICS] Co is a fast diffusor as compared to Au, which very likely diffuses at a similar rate as zirconium. Crystallization studies were made by means of differential scanning calorimetry. The diffusion of Co in amorphous Co-Zr can explain quantitatively the growth rates of amorphous interlayers by solid-state amorphization reactions (SSAR's) in the Co-Zr system reported in the literature. Significant atomic motion at temperatures at which SSAR occurs is restricted to Co diffusion, which explains that diffusion limited growth of an amorphous phase can take place without nucleation of the more stable intermetallic compounds. The activation enthalpy of crystallization (335 kJ mol-1) is much higher than the enthalpy of Co diffusion.