Tracer diffusion of 60 Co and 63 Ni in amorphous NiZr alloy
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H. Hahnb) and S. J. Rothman Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 14 July 1987; accepted 11 September 1987) Tracer diffusion of 60Co and 63Ni in the amorphous alloy NiZr near the equiatomic composition has been measured in the temperature range between 486 and 641 K using the ion-beam sputter-sectioning technique for serial sectioning. The temperature dependence for the difFusivities of Co and Ni in a-NiZr exhibit Arrhenius behavior; these can be expressed as follows:D* o =3.6XlCT 7 exp[ - (135 ± 14) k J m o r V ^ r ] m V s a n d Z ) ^ = 1.7xl(T 7 exp[ - (140 + 9) kJ mol^/RT] m 2 /s. The values of D & are in good agreement with those measured by the Rutherford backscattering technique. The measured diffusivities were independent of time, indicating that no relaxation took place during diffusion.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
A knowledge of defects and atomic transport in amorphous metallic alloys is indispensable for the understanding of basic phenomena such as the tendency to glass formation and the stability and structural relaxation of the glass. In particular, when amorphous materials are formed by solid-state reactions using diffusion couples,1"3 the diffusion behavior is very important. 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 n m 2 /s, which are typical of metallic glasses below their crystallization temperatures.4 In order to measure such a small diffusivity, the Rutherford backscattering technique (RBS) or secondary ion mass spectrometry (SIMS) has been used.5'7 Only a few direct measurements using radioactive tracers and the ion-beam sputter-sectioning technique have been carried out on amorphous alloys.8'10 In this article we report measurements of the impurity diffusion of ^Co and the self-diffusion of 63Ni in an amorphous NiZr alloy near the equiatomic composition using the ion-beam sputter-sectioning technique, and measurements of the activation enthalpy and the preexponential factor for the self-diffusion of Ni, which has not been measured so far. Since the metallic radii of Co and Ni are small, high diffusivities for these atoms are expected.l!
Amorphous thin layer samples were prepared by the codeposition of Ni and Zr from two rate-controlled electron guns onto a SiO2 substrate in an ultrahigh vacuum system as described previously.6 The thicknesses of the amorphous layers were typically 450 nm. The samples were cut into ~ 6 X 6 mm squares. The isotopes ^Co and 63Ni were purchased from Oak Ridge National Laboratory in the form of 0.5 M HC1 solutions. ThepH of the solution containing the radioactive ^Co or 63 Ni was adjusted to ~ 7 by the addition of NH 4 OH, and the isotope was electroplated on the sample surface. The specific activity of the radioactive tracers was sufficiently high so that the thickness of the tracer deposit fulfilled the thin layer condition. The samples were
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