Effects of yttrium on microstructure and mechanical properties of a directionally solidified single crystal superalloy

Reactive elements such as yttrium are known to have beneficial effects on oxidation resistance of high-temperature materials. This paper focuses on the effects of yttrium on a single crystal nickel-based superalloy. The evolution of microstructure and mechanical properties are analyzed by comparing the alloys with different concentration of yttrium. The results show that most of yttrium atoms are solved at the roots of secondary dendrite arms as during directional solidification. After a solid solution heat treatment (1300 degrees C), some eutectic precipitates with N17Y2 are produced in the interdendritic areas through a eutectic reaction (L ->(Ni) + Ni17Y2). A small amount of yttrium addition will leads a rapid decrease of stress rupture life. However, the tensile strength shows an obviously drop when yttrium is added into the alloy, but turns to be enhanced when concentration of yttrium gets higher.