Low cycle fatigue in rene 88DT at 650 °C: Crack nucleation mechanisms and modeling

Abstract

Intermediate-temperature low-cycle-fatigue experiments were conducted on three microstructural conditions of the nickel-base superalloy Rene 88DT over several strain ranges. The most significant difference between the microstructural conditions was the grain size; two of the conditions had an average grain diameter of approximately 20 mm, while the third condition had an average grain diameter of 6 mm. Two dominant crack nucleation mechanisms were observed on the specimen fracture surfaces: crack nucleation from surface slip band damage accumulation and crack nucleation around subsurface inclusion clusters. The experimental results indicate that both the grain size and the applied strain range are contributing factors in the prevailing crack nucleation mechanism. The Fatemi-Socie parameter, a multiaxial fatigue damage parameter, was used to examine the ease and probability of surface slip band crack nucleation for these microstructural conditions. The parameter was modified to explicitly include grain size, fatigue R-ratio, and applied strain range, thus giving it a more physically meaningful basis. This modified Fatemi-Socie parameter was found to be a suitable parameter for characterizing the ease of surface slip band cracking for materials with different grain sizes and is able to explain the observed disparity in fatigue lives based on microstructural design.

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

“…Temperature [4,5], grain size [6,7], stress level [8], strain range [9], and size of non-metallic inclusions [10] have been found to affect fatigue performance. These microstructural features (grain boundaries, inclusions) introduce dislocation barriers and thus give rise to accumulation of damage, resulting in crack nucleation.…”

Plastic strain localization and fatigue micro-crack formation in Hastelloy X

“…On the modeling side, several approaches at different length scales have been put forward for local crack initiation predictions (crack initiation models) [41]. For example, researchers have proposed fatigue indicator parameters based on a critical accumulated slip [42,43], indicators based on the Fatemi-Socie parameter for multiaxial fatigue [44] which includes the plastic shear strain within slip bands and the normal stress to the plane of maximum shear strain [45,46], critical stress, or on energy considerations [47]. In general, the local strain or stress measurements required for these crack initiation parameters were obtained using finite element crystal plasticity simulations, e.g., [13,[41][42][43][47][48][49].…”

Crystallography of Interfaces and Grain Size Distributions in Sr‐Doped LaMnO3

“…However, it is not clear whether cathode degradation is correlated to average microstructural features or deviations from the average. Other important material properties, such as creep and fatigue, are significantly influenced by a few large grains, considered outliers from the average and existing only in the upper‐tail of the GSD . Here, we investigate the deviations from log‐normality in dense and porous LSM, highlighting the effects of pores on increasing the deviation of the GSD from log‐normal behavior, resulting in a statistical overpopulation of very large grains as compared to the log‐normal behavior.…”