Doctoral Dissertations

Date of Award

5-2004

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

George M. Pharr

Committee Members

Joseph E. Spruiell, Hahn Choo, Michael L. Santella, Thomas E. Shannon

Abstract

The thermal aging effects on the microstructure, oxidation behavior at 900⁰ and 1100⁰C, and mechanical properties of IC221M (Ni₃Al based intermetallic alloy, ASTM A1002-99) were investigated. The microstructure consists of dendritic arms of the [gamma] (nickel solid solution) phase containing cube-shape [gamma]ʹ (Ni₃Al) precipitates. The interdendritic regions are mostly [gamma]ʹ (Ni₃Al) with up to 8 vol.% [gamma] + Ni₅Zr eutectic constituents. Thermal aging effects on the microstructures and how microsegregation affects the oxidation behavior were examined. Four primary changes in the microstructures were observed: (1) there is considerable homogenization of the cast microstructures with aging, (2) the volume fraction of [gamma]ʹ increases with aging time and temperature, (3) the [gamma]ʹ phase coarsens, and (4) the volume fraction of the [gamma] Ni₅Zr eutectic constituents decreases.

During the initial stages of oxidation at 900⁰C, surface oxides form along the microsegregation patterns, revealing the cast microstructures. The first oxide to form is mostly NiO with small amounts of Cr₂O₃,ZrO₂,NiCr₂O₄, and [theta]-Al₂O₃. Initial oxidation occurs primarily in the interdendritic regions due to microsegregation of alloying elements during casting. With further aging, the predominant surface oxides become NiO and NiAl₂O₄ spinel, with a continuous film of [alpha]-Al₂O₃ forming immediately beneath them. Although these oxides are constrained to the near surface region, other oxides penetrate to greater depths, facilitated by oxidation of the [gamma] + Ni₅Zr eutectic constituents. These oxides appear in the microstructure as long, thin spikes of ZrO₂ surrounded by a sheath of Al₂O₃. They can penetrate to depths greater than 10 times that of the continuous surface oxide. The oxidation behavior at 1100⁰C is similar to that at 900⁰C, but the oxidation kinetics are faster, NiO dominates at all aging periods, and the surface oxides do not adhere to the matrix meaning that a protective oxide scale does not form. Energy dispersive x-ray techniques with multivariate statistical analysis (MSA) and Thermo-CalcTM simulations were used to develop an understanding of the microstructural changes and oxidation behavior.

Mechanical properties of IC221M were studied in tension tests and hardness tests. Two aging atmospheres (air and argon) and several temperatures were employed for tension testing. The results show that the yield strength at room temperature dramatically drops after 50 hours of aging at 900oC, but further aging at 900oC does not affect the strength: the dramatic decrease of the yield strength is presumably due to the coarsening of the [gamma] and [gamma]' phases. Interestingly, while the yield strength of tensile specimens decreases with increasing aging temperature in air because of oxidation, it increases with increasing aging temperature in argon. This strengthening is probably caused by dissolution of zirconium into the matrix at higher temperatures. Elevated temperature tension testing showed that the yield strength increases with testing temperature to a peak of about 650 MPa at a temperature in the range 600o~800oC, an anomalous behavior often observed in materials with the Ll2 ordered crystal structure. While the tensile properties of as-cast specimens aged in air or Ar at temperatures above 950oC are not acceptable for structural use at 900oC, the tensile properties of specimens aged below 950oC are relatively good. Evidence has been found that preoxidized specimens are less susceptible to the high temperature environmental embrittlement.

The hardness of as-cast specimens and specimens aged at 900o and 1100oC was measured by Vickers hardness testing and nanoindentation. Interestingly, while the Vickers hardness decreases sharply during the initial stages of aging at 900oC, the nanohardness does not. This appears to be a grain size effect due to the coarsening of the [gamma] and [gamma]' microstructure during aging ; nanoindentation measurements sample single grains while Vickers measurements sample larger areas containing multiple grains. The hardness and oxidation properties of IC221M aged in air at 900oC can be improved by solution treating at 1100oC. However, solution treatment may be detrimental to the tensile properties at elevated temperatures.

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