Microstructure and Oxidation of a Cast Nickel Aluminide Alloy
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Microstructure and Oxidation of a Cast Nickel Aluminide Alloy D.Y. Lee 1,2, M.L. Santella 2, I.M. Anderson 2, and G.M. Pharr 1,2 1
Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN, 37996-2200 2 Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831 ABSTRACT Specimens of the cast Ni3Al alloy IC221M were annealed in air at 900°C to examine the effects of oxidation and thermal aging on the microstructure. The alloy is comprised of a dendritically solidified γ-γ' matrix containing γ+Ni5Zr eutectic colonies in the interdendritic regions. Microstructures of aged specimens were examined by optical microscopy and energy dispersive X-ray (EDX) spectrum imaging in the scanning electron microscope (SEM). Two primary changes in the microstructures were observed: (1) there is considerable homogenization of the cast microstructures with aging , and (2) the volume fraction of the γ+Ni5Zr eutectic decreases. Oxidation products were identified using x-ray diffraction and EDX spectrum imaging with multivariate statistical analysis (MSA). During the initial stages of oxidation, the first surface oxide to form is mostly NiO with small amounts of Cr2O3, ZrO2, NiCr2O4, and θ-Al2O3. Initially, oxidation occurs primarily in the interdendritic regions due to microsegregation of alloying elements during casting. With further aging, a continuous film of α-Al2O3 forms immediately beneath the surface that eventually evolves into a double layer of α-Al2O3 and NiAl2O4. Although these oxides are constrained to the near surface region, others penetrate to greater depths facilitated by oxidation of the γ+Ni5Zr eutectic colonies. These oxides appear in the microstructure as long, thin spikes of ZrO2 surrounded by a thin sheath of Al2O3. INTRODUCTION IC221M (ASTM A1002-99) is a Ni3Al based intermetallic alloy designed for good castability and weldability and high room-temperature strength in the as-cast condition [1-3]. The alloy is comprised mostly of the L12 ordered phase γ′ (85-95 vol %) in a continuous solid solution of γ (solid solution of Ni saturated with Al) [1,2]. About 2 wt% zirconium, which reacts with nickel to form a γ+Ni5Zr lower melting eutectic (Tm=1170°C), is added to improve castability and weldability. This phase forms exclusively in the interdendritic regions of the cast microstructures. Although many studies of the physical and mechanical properties of this and other Ni3Al based intermetallic alloys have been performed [1-5], the oxidation behavior of IC221M in the as-cast condition has not been studied in detail. In this study, we examine the effects of thermal aging at 900°C in still air on the microstructures and oxidation behavior of the alloy. EXPERIMENTAL PROCEDURE The chemical composition of the alloy used in this study was: Ni, 8.21%Al, 7.61%Cr, 2.09%Zr, 1.52% Mo; (all compositions are expressed in wt.%). The material was prepared by centrifugal casting in air into a refractory-coated steel mold to produce a 6.1 m tube with an
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