Long Term Oxidation of Model and Engineering TiAl Alloys
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Long Term Oxidation of Model and Engineering TiAl Alloys Ivan E. Locci1, Michael P. Brady2, James L. Smialek1
1
NASA-Glenn Research Center, Cleveland, OH 44135, [email protected] Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
2
ABSTRACT The purpose of this research was to characterize the oxidation behavior of several model (TiAl, TiAl-Nb, TiAl-Cr, TiAl-Cr-Nb) and engineering alloys (XD, K5, Alloy 7, WMS) after long-term isothermal exposure (~7000 h) at 704°C, and after shorter time exposure (~1000 h) at 800°C in air. High-resolution field emission and microprobe scanning electron microscopy were used to characterize the scales formed on these alloys. Similarities and differences observed in the scales are correlated with the various ternary and quaternary microalloying additions.
INTRODUCTION Titanium aluminides (TiAl) are of great interest for intermediate-temperature (600°C-850°C) aerospace, automotive, and power generation applications because they offer significant weight savings compared to today's nickel-based alloys. TiAl alloys are being investigated for low-pressure turbine (LPT) blade applications, exhaust nozzle components and compressor cases in advanced subsonic and supersonic engines [1-2], and exhaust valves in automobiles [2-4]. Significant progress has been made in understanding the fundamental aspects of the oxidation behavior of binary TiAl alloys [5-12]. However, most of this work has concentrated on shorter term (< 1000 hours), higher temperature (900°C-1000°C) exposures. Much less data is available in the literature regarding the oxidation behavior of the quaternary and higher order engineering alloys under the long term, low temperature conditions likely to be encountered in near term structural applications [5-6,11,13]. The present investigation was undertaken to characterize the long-term oxidation behavior of various model and advanced engineering titanium aluminides at 704°C in air. Some engineering alloys were also exposed to 800°C in air for 1000 h. Of particular interest for this study was the formation of nitrides, which have been linked to disruption of alumina scale formation in air [12,14], and the formation of brittle oxygen- and titanium- enriched, aluminum-depleted phase(s) at the metal scale interface, termed by various researchers as Z, X or NCP phase (new cubic phase) [14-18].
EXPERIMENTAL PROCEDURES The alloys included in this investigation are listed in Table 1. All compositions presented in the paper are reported in atomic percent. Alloys were cast, hot isostatically pressed and heat treated to produce a duplex microstructure of γ-grains and α2 + γ lamellae. Several alloys were isothermally exposed to 704°C for 7000 hours and 800°C for 1000 hours in static air. All samples had a 600-grit finish. Back-scattered electron (BSE) images and elemental maps of polished cross-sections were obtained by electron probe microanalysis (EPMA) equipped with a wavelength dispersive x-ray spectrometry (WDS) using pure element standards for Ti, Al, Cr, Nb, and a pur
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