Oxidation behavior of niobium aluminide intermetallics protected by aluminide and silicide diffusion coatings
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INTRODUCTION
NIOBIUM aluminide (Nb3Al-xTi)–based intermetallics have attractive combinations of room- and elevated-temperature fatigue and fracture properties.[1,2] These include good room-temperature tensile ductility (,5 to 30 pct),[1] high room-temperature fracture toughness (,40 to 120 MPa!m),[2,3] and room- and elevated-temperature fatigue crack growth resistance comparable to existing high-temperature (Ti-, Nb-, and Ni-based) alloys.[3] However, there have been relatively few studies of the isothermal and cyclic oxidation behavior that may decide the high-temperature limit of niobium aluminide–based alloys.[1] Previous work on niobium aluminide intermetallics has shown that alloying with Cr can improve their isothermal oxidation behavior,[3] promoting an increase of ,50 8C in the potential service temperature limits to ,800 8C.[3] However, the levels of Cr of ,5 pct Cr used in those studies also resulted in the complete loss of room-temperature ductility, which is one of the attractive features of this new class of alloys. An alternative approach (alternative to alloying) to improve the oxidation resistance of niobium aluminide– based alloys involves the application of surface coatings. For example, surface coatings that form slow-growing Al2O3 or SiO2 scales during high-temperature service may be used to extend the temperature limits of high-temperature alloys.[4–9] Since Al2O3 or SiO2 scales may protect the surfaces of aluminized or silicided layers,[10] several researchers[11–14] have tried to increase the oxidation resistance of intermetallic alloys by depositing such coatings on the surfaces of titanium aluminides[11,12] and niobium aluminide– based alloys.[13,14] The current article discusses the effects of aluminide and YOULIN LI, Postdoctoral Research Fellow, WOLE´ SOBOYEJO, Associate Professor, and ROBERT A. RAPP, Professor Emeritus, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210-1179. Manuscript submitted July 15, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
silicide diffusion coatings on the isothermal and cyclic oxidation behavior of Nb3Al-xTi–based alloys. Aluminide coatings were produced by the direct aluminization of Nb3Al40Ti substrates or of Ni surface layers. Likewise, silicide coatings were produced by the direct siliciding of Nb3Al40Ti substrates or Mo surface layers on niobium aluminide substrates. Both the aluminide and silicide coatings were produced via the halide-activated, pack-cementation (HAPC) method. The isothermal and cyclic oxidation behaviors of coated and uncoated alloys were then examined in detail, and the oxidation kinetics were compared to those of other high-temperature materials.
II. MATERIALS AND EXPERIMENTAL PROCEDURES A. Materials The niobium aluminide intermetallic alloys used in this study were produced by Teledyne Wah Chang (Albany, OR). The compositions of the ingots were determined by wet chemical analysis. Nominal and actual alloy compositions are summarized in Table I. All compositions are quote
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