Improvement in High Temperature Oxidation Resistance of Co-Al-W Based Superalloys
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Improvement in High Temperature Oxidation Resistance of Co-Al-W Based Superalloys Takahiko Ito1, Shogo Ikeda1 and Katsushi Tanaka1 1 Department of Mechanics, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501, Japan. ABSTRACT Oxidation tests of Cr containing Co-based superalloys with compositions of Co20at.%Ni-9at.%Al-9at.%W-x at.%Cr (x = 2, 4, 6, 8 and 10) have been carried out at 1173 and 1273 K in air. Oxidation resistance is improved upon alloying with Cr not only at 1173 K but also at 1273 K. The weight gain of the 10at.%Cr alloy oxidized at 1173 K is similar to that of the 5th generation Ni-based superalloy of TMS-173. Alloying with Cr is efficient to improve oxidation resistance, however, the shape of γ’ precipitates is rounded and the alignment of the precipitates along the direction becomes less pronounced upon alloying with Cr. INTRODUCTION The discovery of a stable ternary L12 phase coexisting with a fcc solid-solution based on cobalt in the Co-Al-W ternary system [1] has stimulated extensive research activities [2-26]. Because the alloy has a so called cuboidal microstructure that resembles conventional Ni-based superalloys, where cuboidal preciptates are considered to have a potential to block the motion of dislocations resulting in superior mechanical properties at elevated temperatures. On the development of a new class of high-temperature structural material based on cobalt, ‘cobalt-base superalloys’, the effects of alloying elements on phase stability [5,7-10,14,19,23,24], mechanical and physical properties [2-4,11-13,15,16], creep strength at elevated temperature [20-22] and so on have been reported. The oxidation resistance is also an important feature for further improvement of these properties at elevated temperatures. In addition to the beneficial influence of the alloying elements of B, Cr and Si on the oxidation resistance [11,17,18,25], it has also been reported that surface coating with MCrAlY significantly improves the oxidation resistance as in the case of Ni-based superalloys [26]. However, since some oxidation resistance of the base alloy is required for a practical use, it is important to know the variation of oxidation resistance with the content of alloying elements. In the present study, the improvement in high temperature oxidation resistance upon alloying with Cr is explored systematically. In addition to isothermal oxidation test of the alloys with various Cr contents up to 10 at.% at 1173 and 1273 K, cyclic oxidation tests have been carried out to simulate a practical use. The oxidation resistance is evaluated by weight gain, thickness of oxide film and chemical composition of the oxide layer. EXPERIMENT Ingots of Co-based superalloys with nominal compositions of Co-20at.%Ni-9at.%Al9at.%W-x at.%Cr (x = 2, 4, 6, 8 and 10) were prepared by an induction melting from appropriate
mixtures of pure metals in an argon atmosphere. The ingots were homogenized at 1523K for 24h in a dynamically evacuated alumina tube, followed by a heat treatment for precipitating the γ' phase at
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