A low-cost process for casting directionally-solidified CDS) turbine blades of high stress-rupture strength has been successfully developed. The process employs an exothermically heated mold to produce DS castings. Using this process, a newly designed solid blade for the high-pressure turbine of the Garrett TFE731-3 turbofan engine was cast in the Mar-M 247 and Mar-M 2OOcHf alloys and engine-tested. The solid DS blade replaced a cooled, conventionally cast INIOO blade. An engine test in this research and development program! showed a 2.4-percent reduction in takeoff specific fuel consumption. An estimated 40-percent reduction in finished turbine blade cost was realized. As a result, the new DS Mar-M 247 blade has been selected for production in the TFE731-B-100, an advanced version of the TFE731-3 engine.
Single-crystal (SC) alloy derivatives of the MAR-M 247 alloy were developed for turbine airfoil applications. This development activity focused on casting process technology, SC alloy microstructure/mechanical property relationships, and coating-SC alloy interactions. Both the exothermic and withdrawal casting processes were evaluated for producing SC components and were found to produce equivalent material from a stress-rupture standpoint. Microstructural effects associated with the removal of grain boundary strengthening elements (B, C, Zr, Hf), microporosity, recrystallized grains, and mu phase precipitation are reviewed from the standpoint of mechanical properties. Coating-superalloy interactions were assessed from both mechanical and environmental standpoints. Thin coatings have not produced any reductions in SC alloy stress-rupture or high-cycle-fatigue properties. From an environmental standpoint, removal of the hafnium from the MAR-M 247 composition reduced the oxidation resistance of diffusion aluminide coatings. An overlay coating, SCC103, was consequently developed to provide significantly improved oxidation resistance and diffusional stability.
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