On the mechanistic basis of fatigue crack nucleation in Ni superalloy containing inclusions using high resolution electron backscatter diffraction

Abstract

This is an authors' copy of a manuscript published in Acta Materialia Volume 97, 15 September 2015, Pages 367-379. http://dx.doi.org/10.1016/j.actamat.2015.06.035 AbstractA series of interrupted three-point bend low-cycle fatigue tests were carried out on a powder metallurgy FHG96 nickel superalloy sample containing non-metallic inclusions. High resolution electron backscatter diffraction (HR-EBSD) was used to characterize the distribution and evolution of geometrically necessary dislocation (GND) density, residual stress and total dislocation density near a non-metallic inclusion. A systematic study of room temperature cyclic deformation is presented in which slip localisation, cyclic hardening, ratcheting and stabilisation occur, through to crack formation and microstructurally-sensitive propagation. Particular focus is brought to bear at the inclusion-matrix interface. Complex inhomogeneous deformation structures were directly observed from the first few loading cycles, and these structures were found not to vary significantly with increasing number of cycles. A clear link was observed between crack nucleation site and microstructurally-sensitive growth path and the spatially-resolved sites of extreme values of residual stress and GND density.

The effects of microstructure and microtexture generated during solidification on deformation micromechanism in IN713C nickel-based superalloy

“…This GND accumulation is partly due to the shape and sharp edges of carbides, see carbide 10,12,20 in Fig. 9b.…”

“…and LCF crack initiation and propagation in inclusion-containing superalloys [19,20]. According to Stroh [32], the dislocation pile-up leads to crack nucleation as they increase the local shear stress which facilitates nucleation of a micro-cracks.…”

“…Britton et al [18] confirmed the pile-up of dislocations at the intersection of slip bands and grain boundary, and further validate a dislocation mechanics by revealing elastic stress state ahead of the pile-up. By utilising HR-EBSD and/or High Resolution Digital Image Correlation (HR-DIC) technique, Jiang et al [19,20] revealed a direct relationship between dislocation density and fatigue crack nucleation in nickel-based superalloy containing non-metallic inclusions. Thus, HR-EBSD seems to be a promising tool in understanding the relationship between strain inhomogeneity, orientation gradients and GND accommodations as well as their interactions with microstructures during plastic deformation.…”

Microstructurally-sensitive fatigue crack nucleation in Ni-based single and oligo crystals

“…Accumulated slip, for example, was found to be suitable in one study [36]. However, a range of recent work [4,14,27,29,31,37] in nickel and other alloys addressing fatigue crack nucleation utilising high resolution DIC and EBSD has demonstrated that quantities including local stress, accumulated slip (within PSBs), and GND density are necessary but not sufficient indicators of fatigue crack nucleation. While one or more of these quantities is usually associated with the site of defect nucleation, any one of them is not in its own right predictive.…”

Toward Predictive Understanding of Fatigue Crack Nucleation in Ni-Based Superalloys

“…32,33 The initial dislocation density was low and homogeneously distributed through the microstructure, and the presence of residual stress gradients was low. 31 After two cycles, the GND density and residual stress gradient ''hot spot'' (red) maps are shown in Fig. 5, where points from the top 5% of GND density distribution from the entire map are overlaid on the EBSD image quality (IQ) map.…”