Prediction of The High Temperature Oxidative Life of Interimetallics
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PREDICTION OF THE HIGH TEMPERATURE OXIDATIVE LIFE OF INTERMETALLICS
James A. Nesbitt and Carl E. Lowell NASA Lewis Research Center, Cleveland, OH 44135 ABSTRACT A method is presented to predict the oxidative life of intermetallics. The method is demonstrated by predicting the lifetimes of several aluminides undergoing cyclic oxidation at 1200 0 C. For NiAI and NiAI-Zr alloys, the lifetimes were predicted at several other temperatures as well. Using a critical surface recession failure criterion, it is shown that several aluminides (e.g., NiAI and FeAI) have long-term oxidation resistance (- 10,000 hours) to approximately 11 50 0 C. Aluminides containing reactive elements (e.g., NiAI-Zr alloys) have long-term oxidation resistance to approximately 1 200 0 C. The method is also applied to predict the oxidative lifetime of MoSi 2 at temperatures of 1200°-1400 0 C which shows that the oxidation resistance of MoSi 2 is significantly better than that for the aluminides. For the same failure criterion, it is shown that MoSi 2 can exhibit long-term oxidation resistance (- 10,000 hours) at temperatures in excess of 1400 0 C. Use of the method to predict the maximum use temperature is also demonstrated for NiAI and NiAI-Zr alloys.
INTRODUCTION Several intermetallics are of considerable interest for high temperature structural applications because of their relatively high specific strength (strength/density). However, as with most materials for use at high temperatures, oxidation resistance is a concern since fast growing surface oxides result in unacceptably high rates of metal consumption. Oxidation protection is afforded by the formation of oxide scales which grow at an acceptably low rate such that oxidation does not limit the useful life of a component far short of the mechanical life (e.g., fatigue or creep life). Very few oxides grow sufficiently slowly as to be considered acceptable in protecting components for high temperature applications in aero gas turbine engines. The parabolic rate constant for an oxide, kP, relates the weight gain during oxidation to time. Consequently, these rate constants give an indication of the growth rate of an oxide scale and can be used to rank the "protection" afforded by an oxide. An Arrhenius plot showing kP values for several oxides of interest is shown in Fig 1. This figure demonstrates the superiority of A12 0 3 and Si0 2 scales over other oxides. Thus, two intermetallics under current consideration, NiAI and MoSi 2, have excellent inherent oxidation resistance based on A120 3 and Si0 2 scales and have been the foundation of coating schemes to protect other materials for several decades (e.g., aluminide coatings for superalloys and MoSi 2 coatings for refractory metals). In contrast, other intermetallics of interest, such as Ti-24AI-1 1Nb" alloys, have poor inherent oxidation resistance due to significant Ti0 2 formation and will likely require All alloy compositions are given in atomic percent. Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
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