Aging of the surface of an Al-Cr-Fe approximant phase in ambient conditions: chemical composition and physical propertie
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Aging of the surface of an Al-Cr-Fe approximant phase in ambient conditions : chemical composition and physical properties D. Veys1, P. Weisbecker1, V. Fournée1, B. Domenichini2, S.Weber3, C. Rapin 4, J.M. Dubois 1. 1
Laboratoire de Science et Génie des Matériaux et de Métallurgie UMR 7584 CNRS – INPL – UHP, Ecole des Mines de Nancy, Parc de Saurupt, F–54042 Nancy cedex, France. 2 Laboratoire de Recherche sur la Réactivité des Solides, UMR 5613 CNRS – Université de Bourgogne, Sciences Mirande, 9 avenue Alain Savary, F–21078 Dijon cedex, France. 3 Laboratoire de Physique des Matériaux UMR 7556 CNRS – INPL – UHP, Ecole des Mines de Nancy, Parc de Saurupt, F–54042 Nancy cedex, France. 4 Laboratoire de Chimie du Solide Minérale UMR 7555, Université Henri Poincaré, F–54506 Vandoeuvre–les–Nancy cedex, France. ABSTRACT We have investigated the surface properties of quasicrystalline and approximant phases in the Al-(Cu)-Cr-Fe system upon aging in ambient conditions. We found that some of these properties (like the electrochemical behavior, wetting or friction) slowly evolves with the length of exposure to normal atmospheric conditions, reaching a stable state only after several days. This report essentially focuses on one of these alloys, an Al65Cr27Fe8 approximant phase with γbrass structure. In a first part, we describe the effect of aging on the electrochemical behavior of this alloy and we propose an interpretation based on a simple electrical model of the oxidized surface. In a second part, we present a model describing the surface as a stacking of several layers (oxides, oxy-hydroxides, contamination) whose thickness evolves with time. The model is supported by X-ray reflectivity, angle-resolved photoemission spectroscopy and secondary neutral mass spectroscopy measurements. INTRODUCTION It is well know that aluminum is always covered by a thin protective oxidized layer that forms instantaneously in contact with air. The chemistry of this surface layer has been characterized extensively in the past using a variety of techniques, including X-ray photoemission spectroscopy (XPS), vibrational spectroscopies (IR and Raman) [1] or secondary ion mass spectroscopy (SIMS) [2]. In reality, slight modifications of the chemistry occur in the topmost layers on a much larger time scale for continued air exposure, and this “surface aging” can affect some of the properties like wetting. Surface aging of aluminum was recently studied by Alexander et al. using XPS [3, 4]. They proposed that the air exposed aluminum surface actually consists of the stacking of three different layers, whose thickness evolves upon aging. Going from the bulk to the surface, one first finds an amorphous alumina layer with constant thickness, then a second layer containing amorphous aluminum oxy-hydroxides and finally a topmost layer identified as a contamination layer with high C content. These top two layers thicken with aging time. This detailed description of the surface of pure Al is actually quite recent, which is surprising regarding the te
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