Improved performance rhenium containing single crystal alloy turbine blades utilizing PPM levels of the highly reactive elements lanthanum and yttrium

摘要

Turbine inlet temperatures have now approached 1650℃ (3000°F) at maximum power for the latest large commercial turbofan engines, resulting in high fuel efficiency and thrust levels approaching or exceeding 445kN (100,000 lbs.). High reliabilityand durability must be intrinsically designed into these turbine engines to meet operating economic targets and ETOPS certification requirements. This level of performance has been brought about by a combination of advances in air cooling for turbineblades and vanes, computerized design technology for stresses and airflow, and the development and application of rhenium (Re) containing, highγ' volume fraction nickel-base single crystal superalloys, with advanced coatings, including prime-reliantceramic thermal barrier coatings (TBCs). Re additions to cast airfoil superalloys not only improve creep and thermomechanical fatigue strength but also environmental properties, including coating performance. Re slows down diffusion in these alloys athigh operating temperatures 1. At high gas temperatures, several issues are critical to turbine engine performance retention, blade life, and integrity. These are tip oxidation in particular for shroudless blades, internal oxidation for lightly cooledturbine blades, and TBC adherence to both the airfoil and tip seal liner. It is now known that sulfur (S) at levels 0.2 ppm in these alloys reduces the adherence of a alumina protective scales on these materials or their coatings by weakening the Van der Waal's bond between the scale and the alloy substrate. A team approach has been used to develop an improvement to CMSX-4{sup}1 alloy which contains 3 percent Re, by reducing S and phosphorus (P) levels in the alloy to 0.2 ppm residual S in the alloy as very stable Y and La sulfides and oxysulfides, thus preventing diffusion of the S atoms to the alumina scale layer under high temperature, cyclic oxidising conditions. La also forms a stable phosphide. CMSX-4 (ULS) (La + Y) HP shroudless turbine blades will commence engine testing in May 1998.