12 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Electron backscatter diffraction | 4/7 | https://en.wikipedia.org/wiki/Electron_backscatter_diffraction | reference | science, encyclopedia | 2026-05-05T10:04:21.164305+00:00 | kb-cron |
The indexing results are used to generate a map of the crystallographic orientation at each point on the surface being studied. Thus, scanning the electron beam in a prescribed fashion (typically in a square or hexagonal grid, correcting for the image foreshortening due to the sample tilt) results in many rich microstructural maps. These maps can spatially describe the crystal orientation of the material being interrogated and can be used to examine microtexture and sample morphology. Some maps describe grain orientation, boundary, and diffraction pattern (image) quality. Various statistical tools can measure the average misorientation, grain size, and crystallographic texture. From this dataset, numerous maps, charts and plots can be generated. The orientation data can be visualised using a variety of techniques, including colour-coding, contour lines, and pole figures. Microscope misalignment, image shift, scan distortion that increases with decreasing magnification, roughness and contamination of the specimen surface, boundary indexing failure and detector quality can lead to uncertainties in determining the crystal orientation. The EBSD signal-to-noise ratio depends on the material and decreases at excessive acquisition speed and beam current, thereby affecting the angular resolution of the measurement.
== Strain measurement == Full-field displacement, elastic strain, and the GND density provide quantifiable information about the material's elastic and plastic behaviour at the microscale. Measuring strain at the microscale requires careful consideration of other key details besides the change in length/shape (e.g., local texture, individual grain orientations). These micro-scale features can be measured using different techniques, e.g., hole drilling, monochromatic or polychromatic energy-dispersive X-ray diffraction or neutron diffraction (ND). EBSD has a high spatial resolution and is relatively sensitive and easy to use compared to other techniques. Strain measurements using EBSD can be performed at a high spatial resolution, allowing researchers to study the local variation in strain within a material. This information can be used to study the deformation and mechanical behaviour of materials, to develop models of material behaviour under different loading conditions, and to optimise the processing and performance of materials. Overall, strain measurement using EBSD is a powerful tool for studying the deformation and mechanical behaviour of materials, and is widely used in materials science and engineering research and development.
=== Earlier trials === The change and degradation in electron backscatter patterns (EBSPs) provide information about the diffracting volume. Pattern degradation (i.e., diffuse quality) can be used to assess the level of plasticity through the pattern/image quality (IQ), where IQ is calculated from the sum of the peaks detected when using the conventional Hough transform. Wilkinson first used the changes in high-order Kikuchi line positions to determine the elastic strains, albeit with low precision (0.3% to 1%); however, this approach cannot be used for characterising residual elastic strain in metals as the elastic strain at the yield point is usually around 0.2%. Measuring strain by tracking the change in the higher-order Kikuchi lines is practical when the strain is small, as the band position is sensitive to changes in lattice parameters. In the early 1990s, Troost et al. and Wilkinson et al. used pattern degradation and change in the zone axis position to measure the residual elastic strains and small lattice rotations with a 0.02% precision.
=== High-resolution electron backscatter diffraction (HR-EBSD) ===
Cross-correlation-based, high angular resolution electron backscatter diffraction (HR-EBSD) – introduced by Wilkinson et al. – is an SEM-based technique to map relative elastic strains and rotations, and estimate the geometrically necessary dislocation (GND) density in crystalline materials. HR-EBSD method uses image cross-correlation to measure pattern shifts between regions of interest (ROI) in different electron backscatter diffraction patterns (EBSPs) with sub-pixel precision. As a result, the relative lattice distortion between two points in a crystal can be calculated using pattern shifts from at least four non-collinear ROI. In practice, pattern shifts are measured in more than 20 ROI per EBSP to find a best-fit solution to the deformation gradient tensor, representing the relative lattice distortion. The displacement gradient tensor (
β
{\displaystyle \beta }
) (or local lattice distortion) relates the measured geometrical shifts in the pattern between the collected point (
p
^
{\displaystyle {\widehat {p}}}
) and associate (non-coplanar) vector (
r
^
{\displaystyle {\widehat {r}}}
), and reference point (
p
{\displaystyle p}
) pattern and associate vector (
r
{\displaystyle r}
). Thus, the (pattern shift) vector (
q
{\displaystyle q}
) can be written as in the equations below, where
x
i
{\displaystyle x_{i}}
and
u
i
{\displaystyle u_{i}}
are the direction and displacement in
i
{\displaystyle i}
th direction, respectively.
q
=
β
r
−
(
β
r
.
r
^
)
r
^
{\displaystyle q=\beta r-(\beta r.{\widehat {r}}){\widehat {r}}}
β
=
(
∂
u
1
x
1
∂
u
1
x
2
∂
u
1
x
3
∂
u
2
x
1
∂
u
2
x
2
∂
u
2
x
3
∂
u
3
x
1
∂
u
3
x
2
∂
u
3
x
3
)
,
r
=
(
r
1
r
2
r
3
)
{\displaystyle \beta ={\begin{pmatrix}{\partial u_{1} \over x_{1}}&{\partial u_{1} \over x_{2}}&{\partial u_{1} \over x_{3}}\\{\partial u_{2} \over x_{1}}&{\partial u_{2} \over x_{2}}&{\partial u_{2} \over x_{3}}\\{\partial u_{3} \over x_{1}}&{\partial u_{3} \over x_{2}}&{\partial u_{3} \over x_{3}}\end{pmatrix}},\qquad r={\begin{pmatrix}{r_{1}}\\{r_{2}}\\{r_{3}}\\\end{pmatrix}}}