Precipitation of Ar, Kr and Xe in Ni at Room Temperaturey
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PRECIPITATION OF Ar, Kr AND Xe IN Ni AT ROOM TEMPERATURE* A. S. LIU AND R. C. BIRTCHER Materials Science Division, Argonne National Laboratory, Argonne, IL
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ABSTRACT Transmission electron microscopy (TEM) has been used to study the precipitation of inert gases (Ar, Kr and Xe) injected into Ni at room temperature. The nucleation and size distribution of precipitates were found to be insensitive to the type of inert gas atom but dependent on gas concentration (ion fluence). The precipitate density decreases and the sizes increases with increasing gas concentration. In all cases, the inert gases precipitate as solid fcc crystals with their axes aligned with those of the host Ni lattice. For the same inert gas concentration, the lattice parameter is larger for the higher atomic mass in accord with bulk results. The lattice parameter of each type of inert gas precipitate increases with increasing gas concentration owing to the pressure relaxation that accompanies precipitate growth. This results in melting of large precipitates in accord with the behavior of bulk quantities of the gases at low temperatures. INTRODUCTION Studies of inert gases injected into metals have shown that the inert gas precipitates into cavities under very high pressures [1,2,3,4]. These pressures are sufficient to solidify the gases at temperatures greatly in excess of their freezing temperatures at 1 atm. In all cases studied, the crystal axis of the solid precipitates aligned with the crystal axis of the host metal. Detailed investigation [3,4] of the precipitation of Kr as a function of implantation fluence reveals that cavity growth leads to a lowering of the cavity pressure to the point that melting of the precipitates occurred. Melting and freezing is also observed during thermal cycling. Experimental Thin single-crystal Ni films (70 nm thick) were prepared by evaporation of high purity Ni. Coated NaCl was cleaved into small pieces, and specimens were floated on a water-methanol mixture onto Cu TEM grids. Implantations were performed with 100 keV Ar+, 180 keV Kr+ or 200 keV Xe+ ions at fluxes less than 2.1016 m-2 sec-l. During implantation, the TEM grids were clamped at their periphery against a Cu plate. Implantation energies were selected on the basis of results from the TRIM computer code [5] so that the gas-concentrations depth profile within the Ni films peaked near the foil center and decreased to near zero at both foil surfaces. Estimates calculated with the TRIM computer code [5] are given in Table I for ion range, peak gas concentration, damage production and sputtering. Sputtering causes the peak in the inert gas concentration to shift closer to the back specimen surface. Estimates of sputtering given in Table I may be high due to reduced sputtering of the surface oxide. Sputtering results in a more uniform gas concentration through the thickness of the Ni thin film and reduces the maximun gas concentration. In addition, removal of the surface layer allows implanted gas to escape and causes inert gas precipitates to ruptur
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