High-Temperature Oxidation Behavior of Nb 3 Al Based Alloys
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HIGH-TEMPERATURE OXIDATION BEHAVIOR OF Nb 3 AI BASED ALLOYS TETSUO FUJIWARA, KEN YASUDA AND HIDEYO KODAMA Hitachi Research Laboratory, Hitachi, Ltd., 3-1-1 Saiwaicho Hitachi-shi 317 Japan
ABSTRACT Oxidation behavior of Nb 3Al has been studied in the temperature range from room temperature to 1500°C. It was revealed in this study that the oxide scale that formed at elevated temperatures was not protective against the propagation of the oxidation front. The products of oxidized Nb 3 Al alloys melted below 1500 °C, even though the melting point of Nb 3 A1 is 1960"C. We expected that the addition of third elements would improve oxidation resistance because atomic sizes smaller than Nb and Al might reduce the solubility and diffusivity of oxygen in the alloy. The elements V, Mn, Fe, Mo, Ta, W and Re were added as the third element for the examination of oxidation resistance. The oxidation resistance tests were carried out by measuring weight gain during temperature elevation and during isothermal heating in air. It was concluded that the addition of Re or Mo improved the oxidation resistance for Nb 3 Al alloys.
INTRODUCTION Intermetallic compound Nb3Al is a candidate for high-temperature structural applications because of its high melting point, 1960°C. This compound has attractive high-temperature strength even at 1500'C [1]. However, oxidation resistance is a matter of concern. The oxidation properties of Nb 3AI have been presented previously as one material among many Nb based intermetallic compounds [2]. The results indicated that Nb 3 AI did not show oxidation resistance at 1200"C in air. However, high-temperature oxidation behavior and oxidation resistance, especially above 1200°C, of this material is almost unknown. Furthermore, the effect of third elements on oxidation resistance of Nb 3AI based alloys has not been investigated by previous researchers. For oxidation protection, materials must form a slow-growing, dense, and well-adherent oxide layer, such as A12 0 3. According to Wagner's theory [3], formability of external A12 0 3 scale is increasing with decreasing of the solubility and diffusivity of oxygen in the alloy. We expected that the addition of third elements might improve oxidation resistance because the smaller atomic size than Nb and Al might reduce the solubility and diffusivity of oxygen in the alloy due to decreasing the lattice size of A15-Nb 3 AI. In this work, oxidation behavior at the temperature range from room temperature to 1500°C of binary Nb 3 AI based alloys was studied. Moreover, effects of some third elements, which have smaller atomic size than Nb and Al, on oxidation resistance were investigated.
Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
960
EXPERIMENTAL PROCEDURE The materials used in this study were prepared as 30 g button ingots by arc melting under argon atmosphere. The nominal alloy compositions in this study were the following: Nb20, 25 at.%Al (binary alloys) and Nb-25 at.% Al-5 at.% X (ternary alloy: X= V, Mn, Fe, Mo, Ta, W and Re). The ing
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