Effects of aluminizing on the oxidation and hot corrosion behaviors of two-phase nickel aluminides

In this work, aluminizing treatment to improve the oxidation resistance of two-phase [NiAl + Ni3Al] Ni aluminides was performed and their surface characteristics were compared with the same alloys without aluminizing treatment in terms of microstructure, oxidation behaviors at 1100 degreesC and hot corrosion resistance at 900 and 950 degreesC. Two-step treatment of high Al activity low temperature (HALT) aluminizing was performed to form a NiAl compound layer of approximately 230 mum thickness on the surface. Isothermal oxidation test at 1100 degreesC showed that at the early stage of oxidation up to 2 h a metastable theta-Al2O3 phase was observed on the surface, which in turn would transform into dense equilibrium alpha-Al2O3 phase in the later stage of oxidation. Due to these dense phase formed on the surface, much improved oxidation resistance in the aluminized two-phase Ni aluminide, especially in the Ni-32 at. %Al-0.5 at. %B alloy could be obtained. Also improved resistance against hot corrosion up to 950 degreesC was resulted from aluminized two-phase Ni aluminides. (C) 2002 Elsevier Science Ltd. All rights reserved.