Sub-Grain Scale Digital Image Correlation by Electron Microscopy for Polycrystalline Materials during Elastic and Plastic Deformation

Abstract

Damage during loading of polycrystalline metallic alloys is localized at or below the scale of individual grains. Quantitative assessment of the heterogeneous strain fields at the grain scale is necessary to understand the relationship between microstructure and elastic and plastic deformation. In the present study, digital image correlation (DIC) is used to measure the strains at the sub-grain level in a polycrystalline nickel-base superalloy where plasticity is localized into physical slip bands. Parameters to minimize noise given a set speckle pattern (introduced by chemical etching) when performing DIC in a scanning electron microscope (SEM) were adapted for measurements in both plastic and elastic regimes. A methodology for the optimization of the SEM and DIC parameters necessary for the minimization of the variability in strain measurements at high spatial resolutions is presented. The implications for detecting the early stages of damage development are discussed.

High resolution mapping of strain localization near twin boundaries in a nickel-based superalloy

“…2 presents an example of a displacement field (along x -horizontal direction) and a strain map obtained between pairs of images over time (see Ref. [37]). Typical values below one pixel (<0.15% strain) are obtained with these imaging parameters.…”

“…The methodology used in this paper for the optimization of the SEM and DIC parameters necessary for the minimization of the variability in strain measurement at high spatial resolution is given in Ref. [37].…”

“…In the present paper, a high resolution DIC measurement approach [37] has been developed for a nickel based superalloy with an average grain size of 26 lm, to obtain insights to the straining processes at the grain-scale, but also at a finer scale where plastic strain localization can be directly correlated with physical slip bands. A strong effect of twin boundaries on the local strain field has been observed and related to deformation mechanisms observed by transmission electron microscopy.…”

Stable Speckle Patterns for Nano-scale Strain Mapping up to 700 °C

“…To illustrate this a TiAl specimen with a pattern using colloidal SiO 2 applied as described elsewhere [13] underwent cyclic loading for 10 6 cycles at a stress of 425 MPa, approximately equal to the measured threshold stress, and exposure to 700°C for 1 h, Fig. 1.…”

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

“…In the past five years, significant progress has been made to improve our understanding of plasticity in polycrystals through the development of new experimental techniques, e.g., cross-correlation based high angular resolution (HR-EBSD) 8,9 and high spatial resolution digital image correlation (HR-DIC), [10][11][12][13] which provide the means to measure elastic and plastic deformation gradients within polycrystals, including elastic strain, lattice rotation, plastic strain, and continuum rotation.…”