CM 186 LC® ALLOY
SINGLE CRYSTAL TURBINE VANES

ABSTRACT

There is a need to introduce advanced turbine technology at reduced cost. SX superalloy vanes demonstrate excellent engine performance and durability benefits compared to their polycrystalline counterparts. However, their manufacturing cost can be prohibitive due to low casting and solution heat treatment yields due to rejectable grain defects. High purity (carbon and boron free), ultra high creep and fatigue strength SX alloys are limited to low angle boundaries (LABs) normally not exceeding 6° in critical airfoil locations. Carbon (C) and boron (B) containing SX superalloys (Ross, et al., 1996) can accommodate low angle boundaries in the 9° - 12° range with an overall sacrifice in creep and fatigue properties. Aero engine vane segments with complex configurations, can result in not only LAB defects exceeding 9° - 12° but also high angle grain boundary (HAB) defects ³ 15° occurring during the SX solidification process. This is further excaberated by recrystallised grains occurring during solution heat treatment from residual casting stresses and associated strain.

CM 186 LC^® is a hafnium (Hf) containing nickel-base superalloy developed for directionally solidified (DS) columnar grain turbine airfoils. SX casting experience - development and production - has shown the alloy can be readily cast into aero turbine multi-airfoil segments. Mechanical property and turbine engine testing show the alloy can accommodate grain boundaries at least up to 30° resulting in high SX casting yields. The SX vane components are either used as-cast or approximately 50% partial solutioned which avoid any recrystallisation (Rx) problems. Component costs can be < 50% of that of a conventional high purity SX alloy.

Mechanical property, oxidation and coating performance characterisation studies on SX CM 186 LC (including DS test pieces) and turbine engine test and application experience show a 72°F (40°C) metal temperature capability improvement (thin wall) over DS MAR M 002 alloy.