Atom-Probe Study of Selective Oxidation of Ni from Cu-Ni Alloy
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ATOM-PROBE STUDY OF SELECTIVE OXIDATION OF Ni FROM Cu-Ni ALLOY K. HONO*, M. NAKAMURA%, H. W. PICKERING%, AND T. SAKURAI** *Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802
**The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
ABSTRACT The initial oxidation stages of Cu-Ni alloys have been studied by atom-probe field ion microscopy (APFIM). Oxide layers were formed in-situ in the FIM chamber on the clean alloy surfaces. The Cu-Ni alloys were oxidized in-situ at lxl0-4 Torr 02 at 970 K for less than 1 min. The atom-probe analysis of the {1111 and {100) surfaces of the oxidized alloys showed that the oxide was comprised of NiO. The concentration change at the oxide/metal interface was also analyzed by the atom-probe. INTRODUCTION Many modem surface techniques have been successfully employed to characterize oxidized metal, alloy, and semiconductor surfaces [1-3]. Although there are some shortcomings, atomprobe FIM is also a very good surface analytical instrument, mainly because of its superb spatial resolution. If the experiment is properly performed, layer-by-layer analysis is possible. In addition, the absolute concentration of any element including light atoms can be readily determined. There have been some studies which applied the atom-probe to the surface analysis of the oxide layer [4-8], although there have been few attempts to analyze the oxide layer formed on an alloy surface. Ni-Cu alloys are technologically important. They are used as catalysts in the chemical industry; hence, the surface properties of these alloys have been intensively studied [9,10]. CuNi alloys are known in the industry as "cupronickels" and widely used for heat exchanger or water pipes. They have excellent corrosion resistance; hence, their surface characterization is important for understanding such a beneficial surface property. A few studies of the initial stage of oxidation of Cu-Ni alloys showed that nickel oxide was solely formed on the alloy surfaces [11-12], whereas at later stages both NiO and Cu20 are present with Cu2O as the outer layer [1317]. There are some studies of chemisorption of oxygen and hydrogen on different Ni-Cu alloys [18,19], but not the transient stage of oxidation. Cu and Ni form a solid solution in the entire composition region of the phase diagram, whereas CuO and Cu20 are usually considered immiscible with NiO (2% solubility of Ni in Cu2O at 1073 K has been reported [ 13]). The present paper aims to characterize the initial selective oxidation of Ni from Cu-Ni alloys by atom-probe FIM. The reaction of alloys with their environment is well known to be more complicated than that of pure metals. Features that are unique to selective oxidation include enrichment of the more noble element on the alloy side of the oxide/metal interface and a loss of planarity of the oxide/metal interface [20]. Prior FIM observations of the NiO layer that forms on Ni surfaces are reported elsewhere [21]. Atom-probe FIM
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