Temperature Dependence of Kr Precipitation in Ni
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TEMPERATURE DEPENDENCE OF Kr PRECIPITATION IN Ni* R. C. BIRTCHER AND A. S. LIU Materials Science Division, 9700 S. Cass Ave., Argonne National Laboratory, Argonne, IL 60439 ABSTRACT The precipitation of Kr injected into Ni at temperatures between 25 and 560 C has been studied with transmission electron microscopy. The Kr precipitates in cavities which increase in size with Kr fluence. Electron diffraction and dark-field imaging demonstrate that small Kr precipitates are solid, fcc crystals aligned with each other and the Ni lattice. The Kr is held in the solid state by the pressure generated by the cavity walls, and this pressure decreases with increasing cavity size. The mismatch between the Kr and Ni lattices is as large as 55%. The average Kr lattice parameter, determined from electron diffraction at room temperature, increases with increasing Kr fluence from 0.515 nm to an asymptotic value of about 0.545 nm for implantations at temperatures of 300 C or less. This increase towards an asymptotic limit is due to expansion of the Kr lattices as precipitates grow, until the decrease in cavity pressures allows melting of solid Kr in large cavities. Diffuse electron scattering was observed from 20 liquid Kr in large precipitates for fluences greater than 5.10 Kr+m-2. Implantations at temperatures of 400 C or higher result in a bi-modal size distribution containing small solid precipitates and an additional population of larger, faceted precipitates, a larger average Kr lattice parameter for a given Kr fluence, and a higher asymptotic lattice parameter of 0.550 nm. Solid Kr seems to inhibit motion of small precipitates, and Kr melting is a precursor to faceting and growth by coalescence. 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 material [1,2,3,4]. The concentration dependence of the precipitation was investigated in detail for Kr implanted at room temperature into Ni [4] and Al [3]. Kr precipitates in cavities whose size increased with Kr fluence. Smaller precipitates (diameters less than about 6 nm) are solid, fcc Kr epitaxically aligned with the Ni. Solid precipitates melt upon heating. At room temperature, large precipitates are liquid or gaseous Kr that freeze upon cooling. These results were discussed in terms of the Kr equation of state and the pressures expected for equilibrium cavities. In this work, the fluence dependence of Kr precipitation at higher temperatures 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, and electron diffraction and dark-field imaging have been used to
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