Effect of local anisotropy on fatigue crack initiation in a coarse grained nickel-base superalloy

Abstract

Abstract. In the present work, theoretical approaches, based on grain orientation dependent Young's modulus and Schmid factor are used to describe the influence of local grain orientation on crack initiation behaviour of the coarse grained nickel base superalloy René80. Especially for strongly anisotropic crystal structures with large grain size, such as the investigated material, the local elastic properties must be taken into account for assessment of fatigue crack initiation. With an extension of Schmid's law, the resulting shear stress amplitude, which triggers local cyclic plastic deformation, can be calculated depending on local Young`s modulus and Schmid factor. A Monte Carlo simulation with 100,000 samples shows how random grain orientation affects these parameters. Furthermore, the product of Young`s modulus and Schmid factor (called E•m) is used as a parameter to determine how grain orientation influences resulting shear stress amplitude for given total strain amplitude. In addition to the theoretical work using that approach, this model is also validated using isothermal LCF experiments by determining local grain orientation influence on the crack initiation site using SEM-EBSD analyses.

Probabilistic Modeling of Slip System-Based Shear Stresses and Fatigue Behavior of Coarse-Grained Ni-Base Superalloy Considering Local Grain Anisotropy and Grain Orientation

“…By considering the different grain orientation distributions in both batches it is possible to explain the observed crack initiation life differences qualitatively, as well as quantitatively (Sections 3.4 and 3.5). Note that LCF test data from Seibel [29] was used for probabilistic model calibration and its predictions are compared to the fine grain test data generated by Engel et al [25,31].…”

“…Plastic deformation of a metallic grain begins if one resolved shear stress within the slip systems of the crystal exceeds a critical value τ CRSS . As the E·m model in [25,31] proposed for coarse grained Ni-base superalloys, the resulting shear stress in the slip systems depends on the grains local Young's moduli and its Schmid factor m. According to [28,30] m is defined as…”

“…Due to the different cooling conditions, especially thin cross sections of components made of polycrystalline, Ni-base superalloys tend to form a crystallographic texture on edge layers, due to the preferred grain growth of <100> near orientations in direction of the temperature gradient [45]. The specimen slugs for the data from Engel [25,31] were casted as cylindrical bars of 12 mm in diameter for fine grain realization and 20 mm for coarse grain realization. It is important to note, that the fine grain batch, with an average grain size of about 1 mm, is still coarse with regard to material science, but in order to differentiate it from the material with an average grain size of about 3 mm it is called fine grained batch.…”

“…Due to the pronounced elastic anisotropy of nickel and Ni-base superalloys (anisotropy factor of up to 3), the effective value of Young's modulus for uniaxial loading of a single grain varies over a wide range. For room temperature it can vary between 130 GPa and 330 GPa and between 95 GPa and 260 GPa at 850 • C [24][25][26]. This aspect in combination with low grain numbers, leads to high scatter in the determined mechanical properties, which makes high safety factors in design inevitable.…”

“…Gottschalk et al [28,30] showed, that there is a correlation between the distribution of the Schmid factor and the lifetime of the Ni-base superalloy René80 tested in high temperature LCF tests. Engel [25,31] could also show coarse grained René80 that, besides the Schmid factor, also the local Young's modulus of the crack initiating grain has a huge influence on the resulting shear stress within the slip systems. With the proposed E·m model, it could be shown, experimentally and analytically, that cracks predominantly initiate in grains with Schmid factors between 0.35-0.45 and high corresponding Young's moduli and therefore high values in the product E·m.…”