Abstract
The high-temperature oxidation behavior of Ni3Al (Ni-13.2 wt.% Al) with and without additions of 0.5 wt.% yttrium has been studied over the range of 900–1200°C in air. None of the commonly accepted rate laws were followed by the kinetics. Although the weight gains of samples containing yttrium were consistently 10–20% greater than those without yttrium, the steady-state scaling rates were identical. A quantitative x-ray diffraction technique was used to determine the kinetics of growth of the protective alpha-alumina layer (one of several oxides formed). The alumina growth followed the parabolic rate law under all conditions studied. The rate-controlling transport process in alumina was the enhanced diffusion of oxygen down grain boundaries. The presence of yttrium as nickel-rich intermetallics promoted the formation of nickel aluminate (spinel). A marked increase in scale adherence was observed for short times. At longer times, however, the outer layer of spinel and unreacted nickel oxide spalled off along with some of the inner alumina layer. Loss of adherence was caused by a complex yttrium-aluminum oxide which formed by the solid-state reaction of yttria and alumina. The poor scale adherence on Ni3Al was due to the formation of voids at the alloy-oxide interface. These voids concentrated the athermal stresses above the oxide-to-metal adherence strength. The voids were produced as a result of the selective oxidation of aluminum resulting from a “Kirkendall” effect in the substrate. During the selective oxidation process, a vacancy flux directed from the matrix to the metaloxide interface resulted in a supersaturation of vacancies. Equilibrium was maintained by the condensation of excess vacancies. The presence of yttrium as either nickel-rich intermetallics or internal oxide prevented the voids from forming. The yttrium-rich particles relieved the matrix of vacancy supersaturation by providing vacancy sinks. The chemical nature of the particles does not seem important. A necessary and sufficient condition for an effective vacancy sink appears to be the presence of an incoherent boundary between particle and matrix.
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This work is based on a portion of the dissertation of J. D. Kuenzly, accepted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering, University of California, Los Angeles, California, 1973.
Supported by the Advanced Research Projects Agency of the Department of Defense under Contract No. DAHC 15-70-G-15.
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Kuenzly, J.D., Douglass, D.L. The oxidation mechanism of Ni3Al containing yttrium. Oxid Met 8, 139–178 (1974). https://doi.org/10.1007/BF00612170
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DOI: https://doi.org/10.1007/BF00612170