Phase Formation in Zr/Fe Multilayers During Kr Ion Irradiation
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researchers have used other techniques to study the problem and found evidence of phase separation in the amorphous phase, as well as demonstrated the importance of wavelength and composition in determining the final products [5], [9]. However no systematic study of these parameters, as well as a detailed study of the kinetics of the solid state reaction process have been attempted. To help elucidate these questions we have conducted an investigation of the influence of modulation wavelength and composition on phase formation in Zr-Fe multilayers. In a previous work we presented preliminary results which are here expanded and interpreted [10]. We have used in-situ irradiation in a transmission electron microscope (TEM), which allows us to directly measure the reaction kinetics and obtain the final reaction products for precisely defined conditions. We discuss the results in terms of previous work, and of the existing models for solid-state reaction and irradiation mixing. EXPERIMENTAL METHODS Metallic multilayers of Zr and Fe were prepared by vapor deposition onto a NaCl substrate, in a Balzers UMS 500P dual gun system at pressures of 5 x 10-9 torr. The overall thickness was kept around 100 nm to make the multilayers electron-transparent at 300 keV. Both the modulation wavelength X(the thickness of one Fe plus one Zr layer) and the overall composition were varied. These multilayers were characterized by Rutherford Backscattering spectroscopy (RBS), and Mossbauer Spectroscopy (CEMS) in addition to TEM. These samples were floated in de-ionized water onto Cu grids and examined in the Intermediate Voltage Transmission Electron Microscope (IVEM) at Argonne National Laboratory. This is a Hitachi 9000, microscope operated at 300 keV with an ion beam attachment that permits the irradiation of the sample in-situ [11]. The irradiation temperature can be controlled from 15 K to 973 K. We conducted two types of experiments: controlled heating and heating plus irradiation. All samples were annealed and the irradiation experiments conducted are listed in table I. Irradiations were conducted with 300 keV Kr ions at temperatures from 25 K to 623 K, at a current of approximately 1.7 x 1012 cm 2 sl to doses up to 2 x 1016 cm 2 (corresponding to about 60 dpa). Diffraction patterns were taken at regular intervals to detect the formation of the new phases. TABLE I: Irradiation Experiments Conducted on Zr-Fe Multilayers # of Thickness layers (nm) loX 105 7X 91 6X 90 5X 125 5X 95 4X 108 13X 96 "3X 100
Wavelength X,(nm) 10.5 13 15 25 19 27 7.4 33.3
Zr thickness (nm) 7 7.8 10 15.5 10.8 18 4 18
Composition (atom % Fe) 50 55 50 55 58 50 64 64
Irradiation Temperature (K) 25,293,373,473,573,623 17,293,373,473,573 None 293 None 293,373,473,573 293,373,573 17, 293,373,473,573
RESULTS Irradiation Experiments: The samples with near equiatomic composition underwent full amorphization under ion irradiation. In both equiatomic and Fe-rich multilayers the intensity of the Zr rings weakens faster than that of the Fe rings. This observation is con
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