Surface oxides and their effect on the oxidation behavior of amorphous and nanoquasicrystalline Zr-Pd and Zr-Pt alloys
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S. Murtya) Department of Metallurgical and Materials Engineering Indian Institute of Technology, Madras, Chennai 600 036, India (Received 16 December 2004; accepted 3 October 2005)
The oxidation behavior of melt-spun Zr75Pd25 and Zr80Pt20 alloys with nanoquasicrystalline phase embedded in amorphous matrix has been studied isothermally as well as nonisothermally in static air. The nature of oxides formed during oxidation has been studied by x-ray diffraction and scanning electron microscopy, and a transition in the structure of the oxides has been shown as one of the primary reasons for the difference in the oxidation behavior of the alloys.
I. INTRODUCTION
Amorphous and nanocrystalline alloys have gained a lot of attention over the past two decades.1,2 Oxidation resistance is an essential prerequisite for better performance of these materials for use at high temperatures or for catalytic applications. Amorphous alloys based on Zr have been successfully used as precursors for the preparation of highly active catalysts for CO2 and CO hydrogenation as well as for other catalytic reactions.3 Therefore, oxidation study of Zr-based alloys to be used as catalysts is very important. Earlier studies have cited either poor or strong oxidation of Zr-based amorphous alloys depending upon the alloying elements present in the alloy.4–8 Aoki et al.4 have reported that amorphous Zr67Ni33 hardly oxidizes below its crystallization temperature. Asami et al.5 have studied oxidation of amorphous Zr-rich Zr–Ni alloys. The oxidation kinetics was observed to follow a parabolic law with a rate constant Kp ⳱ 2.2 × 10−3 g2/m4s, which increased after cracking of the surface film.5 Oxidation products were tetragonal ZrO2 and NiO, and in the remaining matrix, intermetallic phases Zr2Ni and ZrNi were formed.5 Köster et al.6 have shown that oxidation is very rapid in case of amorphous Zr70Pd30 and Zr70Au30, but considerable oxidation resistance has been observed in case of Zr80Pt20 alloy. They have reasoned that a small amount of noble metal ions diffusing into the oxide increases the number of ion vacancies, thereby increasing the ion mobility. Kimura et al.7 have attempted to explain a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0084 J. Mater. Res., Vol. 21, No. 3, Mar 2006
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the high oxidation tendencies of Zr–Pd or Zr–Au amorphous alloys from the atomic size of the alloying elements. An increase in atomic size should lead to an acceleration of the oxidation due to an increase of the vacant space. Both these approaches, however, do not fully explain the high oxidation resistance of Zt–Pt alloy, and there is no report on the effect of the nature of the oxide formed on the different oxidation behavior of these alloys. In the present study, the oxidation behavior of meltspun Zr–Pd and Zr–Pt alloys has been studied by thermogravimetric analysis. Both isothermal and nonisothermal oxidation treatments have been carried out in static air. The effec
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