1.What is a dislocation, C dislocation, C+A dislocation?
2.How to distinguish between the three types of dislocations?
It is well known that there are three distinct Berth's vectors for dislocations in HCP crystals:
A dislocation: 1 3< 11-20 >
c dislocation: <0001>
C+A dislocation: 1 3<11-23 >
The plasticity of a crystalline material is closely related to the number of movable slip systems in its structure.
When other external conditions are the same, the more slip systems there are, the more spatial orientations and the better the plasticity of the slip process.
In HCP alloys, the common slip systems can be divided into five types:
The cone slip can provide 5 independent slip systems, especially its slip direction <11-23 >, which can well adjust the deformation along the C-axis, and fully meet the von-mises criterion even when the base and prismatic slip systems cannot be activated.
Therefore, in transmission electron microscopy, HCP metal will pay more attention to the C+A dislocation starting.
As mentioned in the previous article, in TEM:
The type of dislocation can be judged by the extinction law.
First of all, it is necessary to make an extinction table according to different g and b.
Here's a dislocation extinction table I've put together:
Table 2 HCP G·B matting table.
Below are the standard electron diffraction patterns for HCP [2-1-10] and [01-10].
Fig.2. Standard electron diffraction pattern of HCP[2-1-10].
Fig.3. Standard electron diffraction pattern of HCP[01-10].
Therefore, according to the standard electron diffraction pattern table, two g vectors can be taken under the [2-1-10] or [01-10] belt axis, as shown by the red arrows in Figure 2 and Figure 3, to determine the three dislocation types.
After the g·b table and the selected g vector are available, let's take a look at how to determine the specific dislocation type.
The analysis steps are the same as in the previous article:
Fig. 4 G-vector brightfield images of [2-1-10] and [01-10] in mg.
01.Select the target dislocation
Select the dislocation: As shown in Figure 4, the yellow line segment - dislocation, the blue line segment - dislocation, and the green line segment - dislocation.
02.Determine the Burth vector
Determine the Burnell vector for each dislocation according to the extinction table:
As can be seen from the above table, the dislocation does not fade regardless of the g vector, so the yellow line segment-dislocation in the example diagram is a dislocation.
As can be seen from the above table, the dislocations are only visible at g=[0002], but not at g=[01-10] and g=[-2110], so the blue line segment-dislocation in the example figure is a dislocation.
As can be seen from the above table, the dislocations are visible under g=[01-10] and g=[-2110], but not at g=[0002], so the green line segment-dislocation in the example diagram is a dislocation.
From this method, it is possible to determine the type of dislocation for each dislocation.