A Comparison of Static and Cyclic Long-Term Oxidation of Two Nb-Cr-Mo-Si-B Alloys
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TRODUCTION
CURRENT high-temperature turbine engines utilizing nickel-based superalloys are operating within 200K of the alloy melting point. In order to increase the efficiency of these turbine engines, higher temperatures are required. Pushing the boundaries of the Ni-based alloys could introduce a condition where a liquid phase would be present.[1] Replacement alloys of several systems are under investigation with the goal of surpassing the temperature limitation. Advances in thermal efficiency of turbine engines require new alloys to be developed to surpass the limiting factor of melting temperature for current nickel base superalloys.[2] Focus has been placed on two refractory metal-based systems of molybdenum and niobium, both of which have melting points above 2273 K (2000 °C). Alloys of both systems have desirable properties in creep resistance and toughness, but are limited because of their oxidation responses at high temperatures. Several studies focused on the oxidation properties of Niobium-based systems[3–10] have shown that the catastrophic oxidation of the base material can be curtailed with the addition of alloying elements of molybdenum, chromium, silicon, and boron. In the Mo-Si-B system, a protective borosilicate layer is formed; however, it is dependent on the volatilization of Mo oxides that allow the silicate to flow.[11,12] The increase in oxidation resistance has been reported by Chan to be dependent on the Cr content in the solid solution and high volume fractions of the NbCr2 Laves phase.[4,13] The benefits of
KATHRYN S. THOMAS, Graduate Research Assistant, and SHAILENDRA K. VARMA, Professor, are with the Department of Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX, 79968-0520. Contact e-mail: [email protected] Manuscript submitted December 5, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
the CrNbO4 oxide have not been studied extensively. Most of the research on the CrNbO4 oxide has dealt with its dielectric properties or thermal expansion.[14,15] Isothermal studies up to 100 hours for several developmental Nb-based alloys have found large weight gains with a strong dependence of the volume fraction of the NbSS.[5,16] An initial goal to combine the benefits of the Nb-Cr and Mo-Si systems prompted earlier study[9,17] in the Nb-Cr-Mo-Si system, focusing on maintaining the Nb5Si3 silicide for structural purposes and Laves phase for its oxidation resistance. The 25Cr content was found as a maximum before which casting complications developed.[17] In 24-hour studies on the addition of five at pct boron to Nb-25Cr-20Mo-15Si system and the current alloys, results indicated the addition improved oxidation resistance up to 1473 K (1200 °C).[18,19] Several goals were identified in the development of the current alloys, (1) reduce the a solid solution, (2) promote the formation of a borosilicate layer, and (3) stabilize the microstructure. Upon addition of 10 at pct boron to the system, results of short-term 24 hours and long-term cyclic 7-day exposure, showed the base
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