Phase Equilibria and Oxidation Behavior of C40 Disilicides in the Nb-Cr-Si System
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Phase Equilibria and Oxidation Behavior of C40 Disilicides in the Nb-Cr-Si System Nobuaki Sekido 1, Ryoma Aizawa 2, Shunkichi Ueno 2 1 National Institute for Materials Science, Tsukuba 305-0047, Japan 2 College of Engineering, Nippon University, Koriyama 963-8642, Japan ABSTRACT The phase equilibrium and oxidation behavior of the disilicides that form in the Nb-Cr-Si ternary system have been investigated. Although NbSi2 and CrSi2 both exhibit a C40 crystal structure, they form separate ranges of compositional homogeneity in the ternary system. Their phase boundaries at 1300 oC have been experimentally determined in this study. The binary NbSi2 exhibited poor oxidation resistance, showing pest-like behavior during oxidation at temperature above 800 oC. In contrast, the alloys containing Cr showed much better oxidation resistance up to 1200 oC. INTRODUCTION Alloys based on Nb silicides are candidate materials that can be equipped with a superior temperature capability to Ni based superalloys. Since refractory metal silicides are generally brittle at ambient temperature, most alloy designs so far have been based on the development of multiphase alloys comprised of silicides and metallic solid solution phase, by which necessary toughness for structural applications is secured. Previous studies have demonstrated that a good combination of high temperature strength and room temperature toughness can be achieved by an optimal alloy design and a suitable selection of processing [1,2]. However, a particularly difficult challenge faced by these alloys is a lack in the oxidation resistance at high temperature. Historically, complex silicide coatings, such as a Si-Cr-Fe coating commercially known as R512E [3], have been used for oxidation protection coating on Nb alloys. The major reason why NbSi2 and its based alloy are not applicable to the coating materials is that, in contrast to MoSi2 or WSi2 that forms a SiO2 protective layer, NbSi2 exhibits poor oxidation resistance at temperature above 800 oC [4]. However, the R512E coating is practically limited for short period usage due to its low stability against long exposure to high temperatures. In fact, this kind of problem is common for various high temperature materials, and for such cases a coating being in equilibrium with its substrate is known to be advantageous. In this context, NbSi2 based multiphase coatings are worth pursuing, since they have potential opportunity to be in the equilibrium with the Nb based alloy substrates. For this, comprehensive knowledge of alloying effect on the phase stability and oxidation behavior of NbSi2 is an essential first step in realizing the effective coating designs. At the same time, it has been demonstrated that Cr is one of the effective alloying elements for improving the oxidation resistance of Nb silicides based alloys [1,5]. Therefore in this this study, the focus is placed on the effect of Cr addition on the oxidation behavior and the phase equilibria of NbSi2. EXPERIMENT
Alloys were prepared by arc-melting of high purity raw
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