Sulfidation Characteristics of an Advanced Superalloy and Comparison with Other Superalloys Intended for Gas Turbine Use
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VELOPMENT of advanced materials and protective coatings for modern gas turbine engines is a major challenge. This is due to the fact the advanced materials should exhibit excellent high temperature strength properties in addition to high temperature corrosion resistance to enhance thermal efficiency of gas turbines. It is highly difficult to satisfy these requirements simultaneously since some alloying elements help to improve high temperature mechanical properties, while others promote high temperature corrosion resistance. Therefore, it is essential to evaluate the developed materials to meet the needs of mechanical and corrosion resistance at elevated temperatures. Nickel-based superalloys have been used for manufacture of gas turbine engine blades for a number of decades. Several failures of gas turbine engine blades were reported during service.[1–6] These failures were attributed primarily to high temperature corrosion of different types, but helped establish the relevant theories. An extensive body of work on hot corrosion of several superalloys and their established degradation mechanisms was provided by Gurrappa.[7–10] It was also shown that the hot corrosion of superalloys takes place through electrochemical I. GURRAPPA, Senior Scientist, is with the Aeronautical Materials Division, Defence Metalurgical Research Laboratory, Kanchanbagh PO, Hyderabad 500 058, India. Contact e-mail: igp1@rediffmail.com I.V.S. YASHWANTH, Student, is with the M.V.S.R. Engineering College, Nadargul, Hyderabad 501 510, India. J.S. BURNELL-GRAY, Team Leader, is with the Materials Expro North Sea Ltd., Stirling, FK79JQ, U.K. Manuscript submitted October 16, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
reactions.[10,11] Further, high performance protective coatings were successfully identified for the shielding of superalloys under hot corrosion conditions.[12–14] Efforts made by other researchers in developing protecting coatings helped in understanding degradation behavior.[15,16] Recently, an advanced nickel-based superalloy (ANS) was developed and excellent high temperature mechanical properties have been reported.[17] It is known that the high temperature capability of superalloys depends on their chemistry, i.e., nature and concentration of alloying elements. The major change is the addition of rhenium (Re) or of both ruthenium (Ru) and iridium (Ir)—at the cost of chromium (Cr) and aluminum (Al) which are helpful in enhancing high temperature strength properties. Depending on the addition of new alloying elements, the superalloys are named as fourth- or fifth-generation superalloys. Therefore, the chemistry of superalloys is greatly influenced by reducing the Cr and Al contents and increasing the Re, Ir, and Ru fractions. The fourth- and fifth-generation superalloys contain only about 3 pct Cr, but instead contain ~7 pct Re, 9 pct tantalum (Ta), and 2 pct Ru and/or Ir, which is a great contrast to the earlier generation superalloys containing about 10 pct Cr, 7 pct Al, and no Re, Ru, or Ir. Re, Ru, and Ir can increase high temperatu
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