Abstract

Nb-20Mo-15Si-25Cr (25Cr alloy) and Nb-20Mo-15Si-25Cr-5B (25Cr/5B alloy) alloys have been subjected to oxidation in air for 24 hours from 973 K to 1673 K (700 °C to 1400 °C). Even though B additions do not improve oxidation resistance at temperatures higher than 1473 K (1200 °C), the lower temperature oxidation resistance is superior with B by influencing the microstructure. Porous oxide scale development at lower temperatures has been attributed to the dominant growth of Nb2O5 and the vaporization of MoO3. An intermediate oxidation layer is developed between the scale and the metal for the 25Cr/5B alloy at temperatures above 1173 K (900 °C). Scale densification at elevated temperatures results in higher stress development as a result of the mismatch of coefficients of thermal expansion, ultimately resulting in oxide spallation.

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