Ion-Beam Mixing in Amorphous and Crystalline Fe-Ti
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ION-BEAM MIXING IN AMORPHOUS AND CRYSTALLINE Fe-Ti
Udo Scheuer1, Lynn E. Rehn and Pete Baldo Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL 60439 lnow with Institut ffir Festk&irperforschung, KfA Jijlich, Postfach 1913, D-5170 J5ilich, Fed. Rep. of Germany
ABSTRACT Crystalline Fe and Fe-10at.%Ti and amorphous Fe-37at.%Ti films with Ag and Hf markers were produced by vapor deposition. Marker spreading during ion-beam mixing between 77 K and 580 K was measured using Rutherford Backscattering (RBS). Marker spreading was also measured between temperatures of 300 K to 700 K after full crystallization of the Fe-37at.%Ti films. Microstructural changes during ion-beam mixing were studied in situ, in a High-Voltage Electron Microscope. Homogeneous nucleation of a metastable bcc phase, was observed at high temperatures. The results are discussed in terms of their relevance to "radiation-enhanced" diffusion in amorphous materials. INTRODUCTION Atomic diffusion is a well-understood phenomenon in crystalline metals. Diffusion takes place via the motion of point defects, vacancies and interstitial atoms. Using energetic particle irradiation, point defects can be introduced in a controlled manner, and their role in the diffusion process can be evaluated. Under low-temperature irradiation, atomic mixing is observed as a result of recoil mixing, collisional mixing, and thermal spike diffusion. At higher temperatures, defects escape from displacement cascades and migrate long distances, giving rise to "radiation-enhanced diffusion" (RED). Despite that the concept of point defects loses its meaning in amorphous materials, irradiation of amorphous alloys has produced diffusion results remarkably similar to those obtained in crystalline materials. The observed temperature dependence has been interpreter [1],[2] as RED, in analogy to crystalline metals. However, the temperature range accessible for the study of RED in amorphous systems is very narrow. It is limited by cascade mixing at low temperatures and by crystallization at high temperatures. Diffusion in amorphous alloys at or above room temperature is clearly enhanced by irradiation. It is not clear, however, that this enhancement is due to the long-range diffusion of freely-migrating defects. In particular, the onset of the strongly temperature dependent regime between 300 K and 500 K is remarkably low when compared to single-crystalline metals [3]. In order to more closely compare ionbeam mixing and radiation-enhanced diffusion in the amorphous and the crystalline states, we have studied diffusion under irradiation in Fe-Ti alloys. The tracer diffusion coefficients under ion irradiation of Hf and Ag in an amorphous Fe-Ti alloy were measured at temperatures below the crystallization temperature. The Hf tracer diffusion coefficient under irradiation was also determined after crystallization of the films.
Mat. Res. Soc. Symp. Proc. Vol. 128. '1989 Materials Research Society
214
EXPERIMENTAL Amorphous Fe-Ti films were prepared by vapor-d
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