Microstructure and Oxidation of a Cast Nickel Aluminide Alloy
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Specimens of the cast Ni3Al alloy IC221M were annealed in air at 900C to examine the effects of oxidation and thermal aging on the microstructure. The alloy is comprised of a dendritically solidified - matrix containing +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 +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 -Al2O 3. Initially, oxidation occurs primarily in the interdendritic regions due to microsegregation of alloying elements during casting. With further aging, a. continuous film of -Al2O3 forms immediately beneath the surface that eventually evolves into a double layer of -Al2O3 and NiAl2O4. Although these oxides are constrained to the near surface region, others penetrate to greater depths facilitated by oxidation of the +Ni5Zr eutectic colonies. These oxides appear in the micro structure as long, thin spikes of ZrO2 surrounded by a thin sheath of Al2O 3.