Precipitation of Kr In Ni at Room Temperature
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PRECIPITATION OF Kr IN Ni AT ROOM TEMPERATURE* R. C. BIRTCHER AND A. S. LIU** Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 ABSTRACT The fluence dependence of Kr precipitation in Ni at room temperature has been studied with the aid of Transmission Electron Microscopy. As in other metals, the Kr precipitates in small cavities. Electron diffraction demonstrates that the Kr precipitates are solid, fcc crystals aligned with each other and the Ni lattice. The trends are similar to those observed for Kr precipitation in Al at room temperature. The average Kr lattice parameter, determined from the electron diffraction, increases with
increasing Kr fluence from 0.515 nm to an asymptotic value of 0.545 nm. The asymptotic limit is due to the melting of the larger Kr precipitates. The mismatch between the Kr and Ni lattices is as large as 55%. Diffuse electron scattering was observed 16om lapge, liquid Kr precipita Ms.+ Tbis occurs for Kr fluences above 5.10 Kr m- in Ni and above 2.5.10UKr m- in Al. At room temperature, the largest solid Kr precipitate observed in dark field images was 8.3 nm in diameter compared to 4.7 nm in Al. The larger precipitates are liquid or gas. The solid Kr metals at the same lattice parameter in both Ni and Al suggesting that the melting is thermodynamic in nature and independent of the host material. INTRODUCTION Studies of rare gas precipitation in metals have been motivated by the use of rare gases in thin-film and materials-modification technologies and rare gas generation in reactor fuels. The rare gases have a low solubility in most materials and thus tend to precipitate. The results of several studies show that in general the heavy rare-gases (Ne,
Ar, Kr, Xe) will
precipitate in cavities as liquids or as crystalline solids aligned with the host materials [1,2,3]. The concentration dependence of the precipitation was investigated in detail for Kr implanted into Al at room temperature [3]. The Kr precipitated in cavities whose size increased with Kr fluence. The small precipitates (diameters less than 4.7 nm) were found to be solid, fcc Kr epitaxially aligned with the Al. The small precipitates melted upon heating. Larger precipitates were liquid Kr that froze upon cooling. The larger precipitates solidified at lower temperatures. These results were analyzed in terms of the Kr equation of state and the pressures expected for different cavity sizes. As an extension of the study of Kr in Al, the fluence dependence of Kr precipitation at room temperature in Ni has been studied with transmission electron microscopy (TEM). TEM bright-field images have been used to determine the evolution of the microstructure with Kr fluence. Electron diffraction and dark-field imaging have been used to study the solidification and epitaxy of the Kr precipitates. The average Kr lattice parameter was determined from electron diffraction patterns. These results are compared with similar changes found after Kr implantation into Al at room temperature.
"*Work supported by U. S. D
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