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D. Hesse Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany

M. Martina) Institute of Physical Chemistry I, Aachen University of Technology (RWTH), Templergraben 59, D-52056 Aachen, Germany (Received 8 February 2002; accepted 10 June 2002)

The crystallographic orientation plays an important role in high-temperature oxidation of the intermetallic compound CoGa. When CoGa is exposed to air at elevated temperatures, the oxide ␤–Ga2O3 is formed, and different scale growth rates are observed, depending on the crystallographic orientation of the CoGa grains. This dependence is a consequence of the anisotropy of the gallium diffusion rate through the ␤–Ga2O3 scale and of a topotaxial orientation relationship occurring between ␤–Ga2O3 and CoGa. The combination of ex situ techniques, such as transmission electron microscopy and electron backscatter diffraction with optical microscopy, applied in situ resulted in a thorough understanding of these relations and of the oxidation process in general. I. INTRODUCTION

The intermetallic compound CoGa shows an excellent high-temperature oxidation resistance. When exposed to air at elevated temperatures, Ga is selectively oxidized, and a compact and dense oxide scale of ␤–Ga2O3 forms. Through time-resolved, in situ x-ray powder diffraction it could be shown that after an induction period the scale growth follows a simple parabolic rate law.1–3 This means that the rate determining step is diffusion through the oxide layer. During the oxidation process pores are formed in the intermetallic CoGa at the metal/oxide interface. This observation gives clear evidence that ␤–Ga2O3 grows by diffusion of Ga ions from the metal/oxide to the oxide/gas interface. Diffusion of oxygen ions in the opposite direction can be excluded, since in this case ␤–Ga2O3 would grow at the metal/oxide interface and no pores could grow (they would be filled by the growing oxide). ␤–Ga2O3 is known to be a semiconductor;4,5 thus it can be concluded that ␤–Ga2O3 grows by ambipolar diffusion of Ga ions and electronic defects through the oxide scale,1–3 resulting in the observed parabolic rate law as described by Wagner’s oxidation rate theory.6 Since ␤–Ga2O3 has an anisotropic crystal structure as described below, an orientation dependence of the gallium diffusion rate through the oxide can be expected, which in turn should result in different rates of scale growth for different crystallographic orientations of the a)

Address all correspondence to this author. J. Mater. Res., Vol. 17, No. 10, Oct 2002

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oxide. The orientation of the forming oxide can in turn be expected to depend on the orientation of the underlying metal substrate, at least if a topotaxial oxidation mechanism occurs. This expectation is supported by studies of the very early oxidation states of CoGa.7–9 However, these experiments were performed under high vacuum conditions, and single-crystalline samples of CoGa were exposed to small amounts of oxygen correspondi