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MHigh temperature oxidation behaviour of the single crystal superalloy CMSX-10 Hegde, Subray R.

Abstract

The gas turbine industry has sought to increase the operating temperature of turbines so as to achieve higher efficiency. The compositions of superalloys used in the critical components of engines are changing to improve their high temperature capabilities. Historically, efforts in improving the performance of superalloys, in general, have been focused only on the mechanical properties; predominantly, the creep resistance. Surface protection, though an important issue, has received less attention. The recent trend in the superalloy design is to reduce the chromium content and to increase additions of refractory metals such as Ta and Re. As a result, mechanical strength of the alloy has been considerably improved with little attention to corrosion resistance. The present work studies the high temperature oxidation behaviour of a 3rd generation single crystal superalloy, CMSX-10. In this work, a series of isothermal oxidation tests of the alloy were conducted in air using a thermo gravimetric analyser at temperatures ranging from 800 °C to 1250 °C. The mass gain versus time curve shows parabolic growth at short exposure, followed by sub-parabolic growth at longer times. The alloy exhibits a transition from parabolic oxidation to sub-parabolic oxidation behaviour at all temperatures. The mass gain versus temperature curve shows anomalous behaviour. Characterisation of the oxidised specimens revealed that this anomaly is attributable to the diffusion driven morphological changes of spinel and alumina phases in the subsurface. Chromium oxide (Cr₂O₃), which is considered to be a diffusion barrier, does not form in the oxidation of CMSX-10. However, the alloy forms continuous layers of spinel, NiAl₂O₄ and alumina when oxidised at temperatures above 1000 °C. Theses layers may act as a diffusion barrier at a later stage.

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