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 combi… Show more
“…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.…”
Section: Methodsmentioning
confidence: 99%
“…Each family is characterized by a tilt and twist angle as defined in Ref. [29]. The twist angle is sketched is Fig.…”
“…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.…”
Section: Methodsmentioning
confidence: 99%
“…Each family is characterized by a tilt and twist angle as defined in Ref. [29]. The twist angle is sketched is Fig.…”
“…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.…”
In advanced engineering alloys where inclusions and pores are minimized during processing, the initiation of cracks due to cyclic loading shifts to intrinsic microstructural features. Criteria for the identification of crack initiation sites, defined using elasticplastic loading parameters and twin boundary length, have been developed and applied to experimental datasets following cyclic loading. The criteria successfully quantify the incidence of experimentally observed cracks. Statistical microstructural volume elements are defined using a convergence approach for two nickelbase superalloys, IN100 and René 88DT. The material element that captures the fatigue crack-initiating features in René 88DT is smaller than IN100 due to a combination of smaller grain size and higher twin density.
“…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.…”
In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions.
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