Diffusion of 57 Co in amorphous Fe-RE (RE = Dy, Tb, and Ce) and Fe-Si-B alloys
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Tracer diffusion of 57 Co in amorphous Feioo-xDy* (x = 20-40), Fe75Tb25, Fe 67 Ce 33 , and Fe8oSi6Bi4 alloys prepared by dc sputtering has been studied at temperatures of 523 and 573 K. In the Fe-Dy alloys the diffusion coefficient of 57 Co shows a maximum at 33 at. % Dy. The magnitude of the diffusion coefficient of 57Co in Fe75Tb25 is nearly equal to that in Fe75Dy25, while those in Fe67Ce33 and Fe8oSi6Bi4 are about one order of magnitude less than the values in Fe67Dy33 and Fe8oDy2o. This suggests that the atomic size of the diffusant and the density of the matrix are dominant in the diffusion. Temperature dependence of the diffusion coefficient D of 57 Co in the amorphous Fe75Dy25 alloy has been determined in the range from 493-673 K. It shows a linear Arrhenius relationship expressed by D = 5.7 X 10~2 exp(—199 kJ mol~l/RT) m2 s" 1 . The magnitudes of the pre-exponential factor and the activation energy suggest that the cobalt tracer atoms in the amorphous Fe75Dy25 alloy diffuse by an interstitial-like mechanism.
I. INTRODUCTION Atomic diffusion in amorphous alloys is of interest for scientific as well as technological reasons. From a scientific point of view diffusion studies in amorphous alloys are attractive as they provide information on atomic transport in nonequilibrium structures with no long-range order and with less density than those of crystalline counterparts.1 The technological interest in diffusion in amorphous alloys is due to the fact that processes such as structural relaxation, phase separation, and crystallization, being of vital concern for the thermal stability of amorphous alloys, are often controlled by diffusion. An understanding of diffusion is, therefore, essential in the development of heat treatments to optimize amorphous alloy properties, as well as in establishing safe working temperatures to avoid crystallization and the associated degradation of properties. However, measurements of diffusivity in amorphous alloys have been limited so far because of the experimental difficulties of measuring the very small diffusion coefficients, usually less than 10~ 17 m2 s" 1 , which are typical of amorphous alloys below their crystallization temperatures.2 An additional difficulty in the diffusion measurements arises from the change of the diffusivity in the amorphous state as a function of the annealing time if structural relaxation takes place.3 Therefore, only careful experiments with adequate preannealings or in materials with negligible relaxations can provide reliable diffusion data.4 Furthermore, direct measurements using radioactive tracers ^Present address: Tokin Corporation, Taihaku, Sendai 982, Japan. J. Mater. Res., Vol. 8, No. 9, Sep 1993
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and the ion-beam sputter-sectioning technique are most suitable for these materials.4 So far, much attention has been paid on Fe-base metal-metalloid type and Zr-base metal-metal type amorphous alloys prepared by melt spinning.1-5 The latter type consists of atoms with very different sizes
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