Oxidation Behavior of Ni 3 Al-8Cr-0.1B Alloy Containing Zr Addition
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OXIDATION BEHAVIOR OF Ni3Al-8Cr-0.1B ALLOY CONTAINING Zr ADDITION* GUO JIANGTING, SUN CHAO, LI HUI, AND GUAN HENGRONG Institute of Metal Research, Academia Sinica, 2-6 Wenhua Road, Shenyang, P. R. China ABSTRACT The isothermal oxidation behavior of Ni3A1 + 8Cr + 0.1B alloy containing zirconium addition was studied over a temperature range 850 to 1150*C and for periods up to 100 h. The results show that isothermal oxidation kinetics of the alloys at various temperatures obey a parabolic law. The addition of zirconium to Ni3A1-8Cr-0.1B alloy improves oxidation resistance at 850 to 950°C. However, the additive results in a considerable weight gain at 1050 to 1150°C. The structure of scale and phase transformation of the surface zone beneath the scale have been investigated. The addition of zirconium leads to the formation of adherent scale at all temperatures, since the internal oxides, like "tree roots" between the oxide scale and the substrate, are developed during oxidation. INTRODUCTION In recent years the investigations of intermetallics for high-temperature structural applications by material scientists have included the study of Ni3Al. This intermetallic compound maintains long range ordered structure up to its decomposition temperature of -1390"C and possesses excellent high-temperature mechanical properties with an anomalous temperature dependence of the yield strength. Thus, it is entirely possible that Ni3A1 can be a new type high-temperature structural material. Since the ductility enhancement of boron additions to Ni3A1 was found [1], the study of Ni3A1 has entered a new stage of development, and the prospects of application of Ni3AI are becoming increasingly evident. Thus, the addition of 0.1-0.3 at. % B to Ni3A1 is generally adopted, and in order to improve room- and high-temperature strengths, zirconium additions to Ni3A1 are being used [2,3]. To retard or remove dynamic embrittlement, addition of moderate amounts of chromium to nickel aluminide have been successful [2]. However, if addition to excellent mechanical properties, use of Ni3A1 requires good high-temperature oxidation resistance. Effects of zirconium on oxidation resistance of Ni3A1-8Cr-0. 1B alloy have not been systematically studied. The present study deals with the influence of zirconium upon oxidation at 850 to 1150"C in order to understand its effect on oxidation mechanism. EXPERIMENTAL PROCEDURE The Ni3AI alloys studied were prepared by melting pure metals and Ni-29.5B in appropriate proportions in a 10-kg vacuum induction furnace, followed by precision casting into 6-mm-thick plates. The chemical compositions of the alloys are shown in Table I, which contain about 10% disordered y phase. The heat treatment schedules consist of 4 h at 1200°C, followed by furnace cooling to 1100(C for 1 h, followed by air cooling to room temperature. After homogenization treatment of the plates, specimens of 12 x 10 x 3 mm for isothermal oxidation studies were cut. Machined specimens were gradually wet ground on all six sides with 600 grit silicon carb
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