Mechanisms of M23C6 Carbide Precipitation in Ni-Base Superalloy Single Crystals
- The demand for improved castability and low angle grain boundary (LAGB) resistance has led to the addition of low contents of e.g., B, Hf, Zr or C, into large industrial gas turbine components made of Ni-base superalloy single crystals (SXs). Due to the long-term application of Ni-base superalloy SX components in the temperature regime > 1000 °C, the formation of carbides is highly probable, which could jeopardize mechanical properties, such as high cycle fatigue. In the present contribution, the effect of internal and external stresses on the nucleation and growth characteristics of M23C6 carbides is investigated. Creep experiments are performed on the Ni-base superalloy SX LEK 94, which shows a low C concentration (= 0.1 at. %), at 1020 °C under parallel and circularly notched tensile specimens at a nominal stress of 160 MPa in the crystallographic direction [001]. The carbides are then characterized via scanning (S) and transmission (T) electron microscopy (EM). Nucleation isThe demand for improved castability and low angle grain boundary (LAGB) resistance has led to the addition of low contents of e.g., B, Hf, Zr or C, into large industrial gas turbine components made of Ni-base superalloy single crystals (SXs). Due to the long-term application of Ni-base superalloy SX components in the temperature regime > 1000 °C, the formation of carbides is highly probable, which could jeopardize mechanical properties, such as high cycle fatigue. In the present contribution, the effect of internal and external stresses on the nucleation and growth characteristics of M23C6 carbides is investigated. Creep experiments are performed on the Ni-base superalloy SX LEK 94, which shows a low C concentration (= 0.1 at. %), at 1020 °C under parallel and circularly notched tensile specimens at a nominal stress of 160 MPa in the crystallographic direction [001]. The carbides are then characterized via scanning (S) and transmission (T) electron microscopy (EM). Nucleation is enhanced in the dendritic cores, often as coalesced colonies, extending over micrometers within M-rich (M: Cr, Re, W, Mo) γ channels. Lath shapes with facets on {100} (parallel to growth direction) and {111} are common. These facets exist since early stages (Fig.1a) and later develop misfit dislocations (Fig.1b), preserving the orientation relationship {100}γ || {100}M23C6. Fig. 1c shows a region from the creep gage, where carbides interact with superdislocations in the γ’ phase. Possible mechanisms are discussed.…