In-Situ observations of oxidation and phase stability in cast nickel-based intermetallic alloys
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I. INTRODUCTION
IN the last two decades, extensive research has been performed to develop nickel-based intermetallic alloys with high volume fractions of L12 ordered phase, which are referred as Ni3Al alloys.[1] The applications for which these alloys have been evaluated include transfer rolls, heat-treating trays, rails for walking beam furnaces, radiant burner applications, grate bars in calcination furnaces, and forging of die-blocks. Most of these applications require good strength properties in the range of 700 °C to 1200 °C and resistance to thermal cycling, as well as resistance to either severely oxidizing or carburizing atmospheres. There has been extensive applied research to describe the performance of Ni3Al alloys for these conditions. Research on the oxidation of nickel-based superalloys has been done with reference to gas turbine applications.[2] The present work is part of the ongoing efforts to extend this knowledge to high-volume-fraction alloys close to the Ni3Al composition. Previous work on Ni3Al alloys has indicated that the oxidation resistance of these alloys is generally good compared to other nickel-based alloys.[3,4] However, the effects of processing, oxidation environment, and thermal cycling are yet to be quantified completely. Recent research has focused on the high-temperature corrosion behavior of these alloys under static and cyclic conditions in different atmospheres.[5] Klower et al. studied the stability of these alloys in carburizing, chlorinating, and sulfidizing environments. Their work showed that the Zr-containing alloys oxidized heavily through internal oxidation of Zr-rich regions. Lee et al. studied interactions between phase stability and the types of oxides that form in the oxidizing conditions at temperatures of 900 °C to 1100 °C.[6,7] Their results were in agreement with those of Klower.[6] In addition, Lee et al.[7] related the preferential formation of oxides to the solidification segregation. It is important to note that there exists a good knowledge base on the oxidation characteristics of intermetallic alloys.[8,9,10] The main S.S. BABU, formerly with the Metals and Ceramics Division, Oak Ridge National Laboratory, is with the Edison Welding Institute, Columbus, OH 43221. Contact e-mail: [email protected] E.D. SPECHT, M.L. SANTELLA, G.E. ICE, and S.A. DAVID are with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6096. Manuscript submitted April 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
interest in the present article is to evaluate the interaction between phase stability and the early stages of oxidation during rapid heating and cooling. Based on these previous studies, it was speculated that the early stages of oxidation under rapid thermal cycling might be influenced both by the primary mode of solidification and by the relative stability of various phases that form during solidification. For example, in alloys containing aluminum concentrations close to 20 at. pct, the primary solidification will occur
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