Ion-Beam Mixing and Amorphization in Au/Zr Bilayers
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ION-BEAM MIXING AND AMORPHIZATION IN AU/ZR BILAYERS* FU-RONG DING**, P. R. OKAMOTO AND L. E. REHN Materials Science Division, Argonne National Laboratory, Argonne, IL 60439; **Permanent Address: Department of Technical Physics, Peking University, Beijing, China. ABSTRACT Au/Zr bilayer films with inert-gas markers were produced by low energy (< 4 keV) implantation. Mass transport was measured during ion-beam mixing with 1 MeV Kr at several temperatures between 330 and 540K. Two distinct regimes of apparent Arrhenius behavior were found with activation enthalpies of 0.06 and 0.9 eV in the temperature range 330-440K and 460-540K, respectively. Microstructural changes during ion-beam mixing were studied in situ, in a high voltage electron microscope. Heterogeneous nucleation of an amorphous phase was observed during mixing. The results are compared with similar studies reported previously in Ni/Zr bilayer specimens. INTRODUCTION It has been demonstrated previously that binary amorphous alloys can be formed in Ni/Zr bilayer polycrystalline specimens under 1 MeV Kr irradiation and during thermal annealing [1,2,3]. Both Ni/Zr and Au/Zr have large negative heats of mixing in the solid state. Calculations based on Miedema's model yield values of -0.5 eV/atom and -0.76 eV/atom for NiZr and AuZr, respectively. However, Ni is an anomalously fast diffuser in crystalline Zr and in amorphous NiZr, but Au is not. One of the authors has already investigated mass transport in Ni/Zr bilayer films during ion-beam mixing [1]. In this paper, measurements of mass transport during ion-beam mixing are extended to Au/Zr bilayers. The dominant moving species is identified by using inert-gas marker layers and subsequent RBS analysis, and activation enthalpies for mass transport are determined. These results are compared with the previous Ni/Zr results. The nucleation of the amorphous phase in Au/Zr bilayers is studied in situ during ion-beam mixing in a High Voltage Electron Microscope (HVEM). EXPERIMENTAL Markers including metals and inert gases are widely used in bilayer diffusion couples [4,5,6]. In general, inert gases such as Ne, Ar, Kr and Xe are effective markers because they are chemically inert. Au/Zr bilayer specimens with an embeded Xe marker layer were prepared in an oil-free UHV evaporator containing a low energy ion gun. First, thin films of Zr were evaporated onto oxidized Si substrates. Next, 2 to 4 keV Xe ions were implanted in the surface of the Zr to depths o[ about 4 to 6 1'm. This implantation produced concentrations of ~-xI0 Xe atoms/cm in the implanted region, and simultaneously sputter-cleaned the pre-deposited Zr surface. Finally, Au was vapor-deposited onto the Zr films. The pressure was about lxi0Torr during the evaporation and the implantation. Ion-beam mixing was performed with 1 MeV Kr at a nominal flux of 100 nA. Irradiation temperatures were measured with a Chromel-Alumel thermocouple attached to the specimen heating stage. To avoid forming compounds, *Work supported by the U. S. Department of Energy, BES-Mater
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