Fatigue Crack Initiation in Nickel-Based Superalloy Ren� 88 DT at 593�C

Abstract

Fatigue crack initiation in the polycrystalline nickel-based superalloy, René 88 DT, was investigated at 593 °C in air using an ultrasonic fatigue testing instrument. Within the testing stress range of 500 -760MPa, all fatigue failures initiated from internal regions. Large crystallographic facets were observed at the crack initiation sites in most failed fatigue samples. Critical microstructure features controlling fatigue crack initiation and the early stage of small crack growth were identified by the combination of serial sectioning, orientation imaging microscopy (OIM) and quantitative fractographic analysis. Large crystallographic facets at crack initiation sites were formed in regions close to Σ 3 annealing twin boundaries within favorably oriented large grains. The facet planes are parallel to Σ 3 annealing twin boundaries and are of {111} type. Cyclic deformation substructures were studied by transmission electron microscopy. At the low cyclic stresses and long lives examined, fatigue failure at 593°C is controlled by strain localization in large, favorably oriented grains and subsequent propagation of small cracks through neighborhoods of favorably oriented grains. Annealing twin boundaries, prevalent in the microstructure, appear to be important in cyclic strain localization.

High resolution mapping of strain localization near twin boundaries in a nickel-based superalloy

“…The size of the secondary c 0 phase is about 100-200 nm, while tertiary c 0 precipitates are several nanometers in diameter. Crystallographic features have been previously studied using electron backscatter diffraction measurements [29]: the material possesses very weak crystallographic texture, a large population of R3 grain boundaries (58% of the total boundary fraction), an average grain size of 26 lm, and a low fraction of large grains on the order of two to five times of the average grain size.…”

“…Each family is characterized by a tilt and twist angle as defined in Ref. [29]. The twist angle is sketched is Fig.…”

Microstructural statistics for fatigue crack initiation in polycrystalline nickel-base superalloys

“…The size of the secondary γ phase is about 100-200 nm, while tertiary γ precipitates are several nanometers in diameter. Crystallographic features were previously studied using electron backscatter diffraction (EBSD) measurements (Miao et al 2008) showing that the material possesses very weak crystallographic texture, a large population of Σ3 boundaries (46% by 2D measurements and 70% by 3D measurements of the total boundary length fraction, Lenthe et al 2016), an average grain size of 26 µm, and a low fraction of large grains on the order of two to five times of the average grain size.…”

Measurements of plastic localization by heaviside-digital image correlation

“…The size of the secondary g0 phase is about 100e200 nm, while tertiary g0 precipitates are several nanometers in diameter. Crystallographic features have been previously studied using electron backscatter diffraction (EBSD) measurements showing that the material possesses very weak crystallographic texture, a large population of S3 boundaries, an average grain size of 26 mm [5,46]. Single crystals and oligocrystals of Ren e 88DT were also investigated in this research.…”