Grain boundary microstructures and their effects on creep properties of base and modified bicrystal RR2086 superalloys

Abstract

In the field of aircraft engines, grain boundary defects lead to a high scrap rate when producing single crystal components such as turbine vanes. To improve the tolerance to grain boundary defects, grain boundary strengthening is still necessary. Rolls-Royce experimental superalloy RR2086 bicrystals (BXs) have been modified with the minor additions of grain boundary strengtheners C, B and Hf. Creep tests have been carried out at 950°C and 120MPa, using BXs of 18° boundary. It was found that the rupture life of a modified BX (589 hours) was remarkably longer than that of a base BX (2.4 hours). Electron microscopy observation showed that MC carbides were dominant precipitates at grain boundaries in the modified BXs before exposure. During exposure, modified BXs achieved a reasonable amount of M 23C 6 carbides and a γ layer (ductile and creep-resistant) along their grain boundaries. No cellular carbides appeared during static exposure and creep testing. Since carbide precipitates (including MC and M 23C 6) can effectively inhibit the grain boundary sliding, which is a main mechanism of creep rupture of superalloys, it is concluded that the observed structure is responsible for the significantly improved creep performance of the modified BXs.