Microstructural Changes to Xenon Naonoclusters in Aluminum under 1 MeV Electron Irradiation
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Microstructural Changes to Xenon Naonoclusters in Aluminum under 1 MeV Electron Irradiation. S. E. Donnelly,1 R. C. Birtcher,2 C. W. Allen,2 K. Furuya,3 M. Song,3 and K. Mitsuishi.3 1Joule Physics Laboratory, University of Salford M5 4WT, UK. 2Materials Science Division, Argonne National Laboratory, IL 60439, USA. 3National Research Institute for Metals, Sakura, Tsukuba 305, Japan. ABSTRACT Aluminum films containing solid Xe precipitates have been subjected to 1 MeV electron irradiation in a high-voltage electron microscope. High-resolution images have been recorded on videotape in order to monitor the changes to the system resulting from the passage of electrons through the film. Inspection of the video recordings reveals that complex, rapid processes occur under the electron beam. These include shape changes, the creation and movement of extended defects within the Xe lattice, movement of small clusters, coalescence of neighboring clusters and the apparent melting and resolidification of the Xe. An interpretation of many of the observations is presented in terms of the interaction of the nanoclusters with defects created in the aluminum by the high-energy electrons. INTRODUCTION Inert gases in metals have been under study since the 1950s, driven initially by interests in the mechanisms of ion pumping and subsequently by the desire to solve technological problems, associated with nuclear reactor materials. More recently, the wide use of inert gas ion beams for various surface engineering processes including thin film deposition and surface cleaning has maintained a high level of interest in this system. Inert gases are generally insoluble in metals as a result of the repulsive interaction between the gas atoms and the conduction electrons of the metal; calculations yield a negative heat of solution whose magnitude increases linearly with electron density. When introduced into a metal lattice (generally by ion implantation) an inert gas atom will thus “seek out” regions of low electron density such as grain boundaries, dislocations and lattice vacancies. As the concentration of gas builds up in the metal lattice, small bubbles become visible in transmission electron microscopy (TEM), using defocus contrast, when they reach sizes of the order of 1 nm and, on continued ion implantation, these grow by acquisition of additional gas atoms and vacancies and also by coalescence. The end result of continued implantation may be swelling, blistering and even exfoliation of the irradiated surface. An important parameter in elucidating bubble growth mechanisms is the pressure or density of the inert gas in the bubbles. Theoretically, a spherical cavity in a metal whose collapse due to surface tension forces is balanced by an internal pressure would be expected to have an equilibrium pressure Peq given by: Peq = 2γ /R ................................(1)
R1.1.1
where R is the radius of the cavity and γ is the surface tension (surface free energy) of the cavity/gas interface, which in the case of a cavity containing inert gas i
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