In case of single crystals we obtain bright diffraction spots and in case polycrystalline materials a circular ring formation of the diffraction spots is seen but how do we associate the hkl-plane of the rings with an element or composite when the sample is a High entropy alloy.
Dear Bibek Singh ,
Selected area electron diffraction (SAED) is a technique in the TEM domain, aiming to obtain diffraction patterns that result from the electron beam scattered by the sample lattice.
There are two basic parameters in spot diffraction patterns. They are used to identity, and index such types of patterns. These parameters include R, corresponding to the distance between the diffracted and transmit (center spot) beams in the diffraction pattern screen. Also, this distance can be considered as a normal vector to the plane reflection. The angles such as θ between two vectors are drawn from the center to two adjacent points.
If the crystal structure is oriented in a favored and preferred direction, the SAED pattern will be formed from many partial rings.
If dealing with a single phase (an alloy or not), the SAED dot pattern reflects the only structure of the single-phase base on their reflection planes.
However, for phase segregation in any or in an alloy sample pattern, the SAED will be composed of a set of superimposed patterns related to each present phase.
Best regards,
WNM
In case of alloys, you will mostly end up with much more complicated, mixed and very confusing SADP's . It should not cause you to freak out on the other hand it takes decent time to index such DP's. Basic rules are not changing in terms of how to index a SADP in TEM specimens. You need to determine number of phases and try to simplify superimposed patterns of each phase. For the former, I may suggest you try to determine presence of any superlattice spots in case the crystal structure of one and/or more of the phases manifest ordered crystal structures or not. For the later, you can succesively get image pairs of BF/SADP images by putting SAA slightly inside and outside of your original selected area so that you can observe the variation in the DP's. It is better and helps a lot to analyse and simplify your TEM results by examining simultaneously with your general XRD results of bulk sample.
Kind regards
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