A multiscale study on the morphology and evolution of slip bands in a nickel-based superalloy during low cycle fatigue

Abstract

Plastic deformation during low cycle fatigue in fcc materials with low stacking fault energy is accumulated in slip bands, which become preferential sites for crack initiation. Whilst these dislocation structures have been studied before, little has been done to assess the effect and evolution of the individual slip lines within them. In this study, samples of a γ precipitate strengthened nickel-based superalloy are fatigued at room temperature and 700 • C for 1, 40 and 500 cycles. The resulting dislocation structures are characterised via Electron Channeling Contrast Imaging and Transmission Electron Microscopy. We introduce a new methodology to measure slip band parameters such as the slip line spacing and shear step length by analysing the holes left by sheared precipitates in γ -etched secondary electron micrographs. Statistics of these parameters are obtained and compared for different conditions. Advantages of this technique include resolution at the scale of individual planes, acquisition of true three-dimensional data and applicability in the bulk of the material. The combination of these techniques provides a unique mechanistic and quantitative insight into the slip band and precipitate morphology evolution.

Stress response and microstructural evolution of nickel-based superalloys during low cycle fatigue: Physics-based modelling of cyclic hardening and softening

“…This is the area that the dislocations have to cross and it is delimited by the distance L AP B , schematically shown in Figures 8(a For L > 2d, one can see that as s increases, the distance L AP B decreases linearly from its initial value L AP B = d until it vanishes when s = d. However, once s = L−d the bottom half makes contact with the top half of the precipitate next to it, restoring part of the previously lost antiphase boundary. This behaviour has been observed before (Sundararaman et al, 1990;León-Cázares et al, 2020). Once s = L, these fully overlap and L AP B takes its initial value.…”

“…This detector was also employed in etched samples to observe shear steps in the precipitates. A methodology introduced in previous study (León-Cázares et al, 2020) was used to measure slip line spacings. This technique determines the orientation of the slip plane with respect to the sample surface from the elliptical markings left in the surface of etched holes.…”

“…Slip bands in superalloys start forming from the initial fatigue stages. In tests with a high strain amplitude these form during the first cycle (Petrenec et al, 2014;Mello et al, 2017), whereas they may develop gradually over multiple cycles for lower amplitudes (Risbet et al, 2003;León-Cázares et al, 2020). As the fatigue process continues, the dislocation density within the bands is expected to increase until a balance between their generation and annihilation is reached.…”

“…Gradual shearing of the precipitates during cyclic deformation occurs due to the to-and-fro glide of dislocations in the matrix and dislocation pairs within the precipitates Yashiro et al (2006); Huang et al (2012). The average shear step length increases as the cyclic test progresses (León-Cázares et al, 2020), resulting in a reduction of the precipitate cross sections. This behaviour arises from slip irreversibilities, by which dislocations glide in one direction but are unable to glide back.…”

Three-Dimensional Reconstruction of Planar Deformation Features from Single Electron Micrographs

“…These belonged to specimens that were low cycle fatigued at 20°C and 700°C; more information regarding these samples is discussed elsewhere. [17] The ECCI micrographs were taken with a 0 deg tilt in a FEI NOVA NanoSEM after standard sample preparation. Images of 11 grains where stacking faults, twin boundaries or slip bands appeared in four different slip plane orientations were selected for the analysis and the angle h of their intercepts with the sample surface were manually recorded.…”