Study of Ion Bombardment-Induced SubSurface Compositional Modifications in Ni-Cu Alloys at Elevated Temperatures by Ion
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STUDY OF ION BOMBARDMENT-INDUCED SUBSURFACE COMPOSITIONAL MODIFICATIONS Ni-Cu ALLOYS AT ELEVATED TEMPERATURES BY ION SCATTERING SPECTROSCOPY
IN
N. Q. LAM, H. A. HOFF, H. WIEDERSICH and L. E. REHN Materials Science and Technology Division, Argonne National Laboratory, Argonne, IL 60439
ABSTRACT Changes in the subsurface composition of Ni-40 at.% Cu alloys during 3-keV Ne bombardment at temperatures between 25 and 700*C were studied by means of ion scattering spectroscopy. Both the time evolution of the composition in the surface atom layer during ion bombardment and subsurface concentration profiles after rapid specimen cooling to room temperature were measured as a function of temperature. Radiation-enhanced diffusion coefficients were derived from the effective altered-layer thicknesses obtained. A comparison of the experimental measurements with theoretical calculations based on a phenomenological model enabled the identification of processes and kinetics responsible for subsurface compositional modifications.
INTRODUCTION Subsurface compositional modification of alloys during elevatedtemperature ion sputtering has been a subject of detailed investigations over the past few years. The interest in this phenomenon was motivated by recent concern about plasma contamination by sputtered particles in magnetic fusion devices and wide application of ion-beam processing in many areas of materials science. Changes in the near-surface composition due to preferential sputtering near room temperature have been observed in many alloy systems (extensive reviews have been given recently by Betz [11],Kelly [2], and Andersen [3]). At high temperatures where additional thermally-activated processes are expected, this phenomenon is more complex. In fact, it is now established that at least five distinct processes, including preferential sputtering (PS), displacement mixing (DM), Gibbsian adsorption (GA), radiation-induced segregation (RIS) and radiation-enhanced diffusion (RED), can contribute to subsurface compositional alterations of alloys during elevated-temperature ion sputtering. Recent model calculations of Lam and Wiedersich [4-6] have provided new insights into the simultaneous effects of these processes and helped understand a number of experimental observations. Most of the measurements of ion bombardment-induced alloy composition changes at high temperatures were made by means of Auger electron spectroscopy (AES) [7-12]. Unfortunately, owing to the finite escape depth of the Auger electrons, the AES technique cannot yield definitive information about the composition in the outermost atom layer, where the effects of PS and GA are known to be significant. In a recent work, Swartzfager et al. 1131 employed the ion-scattering-spectroscopy (ISS) technique, which is extremely sensitive to the first surface atom layer. However, the time
*Work supported by the U.S.
Department of Energy.
Mat. Res. Soc.Symp. Proc. Vol. 27 (1984) (Elsevier
Science Publishing Co., Inc.
80
evolution of the alloy composition in the first a
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