Effect of prolonged isothermal exposure on elevated-temperature, time-dependent fatigue-crack propagation in INCONEL all
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OVER the past 20 years, the initial successful development and application of low–coefficient of thermal expansion (CTE) iron-nickel–based superalloys such as the INCOLOY* Alloys 903, 907, and 909 have significantly *INCOLOY and INCONEL are the trademarks of Special Metals Corporation Family of Companies.
improved the efficiency of aircraft engines by controlling the clearance between turbine or compressor blade tips and outer seals and shrouds. Since Cr lowers the Curie temperature and increases the CTE, these low-CTE superalloys contain only a residual level of Cr. Consequently, these alloys are susceptible to stress-accelerated grain-boundary oxidation (SAGBO)[1–4] as well as a high rate of general oxidation. The recently developed INCONEL* Alloy 783 is a result of a development program aimed at improving the resistance to SAGBO while maintaining a low CTE. Alloy 783 is unique, since it contains a very high level of aluminum (5.4 wt pct) compared to conventional low-CTE superalloys such as the INCOLOY Alloys 903, 907, and 909. Aluminum results in precipitation of NiAl-type  in an austenite matrix in addition to Ni3Al-type ␥ ⬘. It was previously reported that thermomechanical processing could be used to tailor the LONGZHOU MA, formerly Graduate Student, Department of Mechanical Aerospace Engineering, West Virginia University Morgantown, WV 26506, is Research Scientist, Materials Engineering, Harry Reid Center for Environmental Studies, University of Nevada, Las Vegas, NV 89154. Contact e-mail: [email protected] KEH-MINN CHANG, Professor, is with the Department of Mechanical Aerospace Engineering, West Virginia University. SARWAN K. MANNAN, Metallurgist-Advanced, is with the Research and Development Department, Special Metals Corporation, Huntington, WV 25705. SHAILESH J. PATEL, Director of Quality and Technology, is with Special Metals Corporation, Huntington, WV 25705. Manuscript submitted August 2, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
morphology and distribution of  phase to improve the SAGBO resistance of Alloy 783, while providing low thermal expansion and useful mechanical properties up to 600 ⬚C.[1,4] Also, the high Al content reduces the density to 7.81 g/cm3, which is 5.0 pct lower than Alloy 718. Due to its low CTE, high strength, and good oxidation resistance, Alloy 783 has been specified by several commercial aircraft gas turbine industries. The alloy is currently specified for aircraft engine casings, shrouds, and seals and has been considered for use in similar industrial turbine components. Gas turbine components made of superalloys experience thermal exposure of up to 600 ⬚C for times up to 30,000 hours or more under service conditions. Therefore, the effect of prolonged thermal exposure on the properties of superalloys is of great interest to engine designers. Radavich and Fort[5] and Zheng and Ghonem[6] have pointed out that exposure of Alloy 718 at 650 ⬚C for 800 hours enlarged the ␥ ⬙ [Ni3AlNb] precipitate size, which remarkably improved intergranular cracking resistance. Mannan a
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