Only alpha phase(hexagonal close-packed) in Mg-Li alloy can analyse with EBSD, but beta phase in Mg-Li alloy cannot. Why?
Why do you think the beta phase cannot be analysed by EBSD? As long as it not unstable in SEM I do not see any reason that you cannot do it.
I would like to address your question: why is it possible to analyze alpha phase in Mg-Li alloy and not the beta phase using EBSD? Attached is Mg-Li phase diagram. It is seen that alpha is the result of solid clustering during solidification, whereas beta is the result of liquid clustering. I am assuming that your alloy has lithium in the range of 6-9% so that on cooling to ambient temperature you will have a combination of alpha and beta in the microstructure. It is my postulation that magnesium with atomic number of 12 has a major role in determining the structure of alpha whereas lithium with an atomic number of 3 has the major role in regard to beta phase. I believe that the electron diffraction from magnesium-rich alpha phase is amenable to analysis of EBSD, whereas the signals from lithium-rich beta are too weak for EBSD analysis.
I am not an expert on sample preparation and it is best to follow the advice of experts who have answered your question.
Best regards,
Malur Srinivasan
Use colloidal silica or ion milling for polishing the sample.The phase can be easily identified.
I reread your question and found that your alloy is either in the range pure magnesium to about 15% lithium-which will have only alpha phase (magnesium-rich) or in the range 30-100% lithium, which will have beta phase(lithium rich), as shown in the phase diagram. Thus my original answer considering the respective atomic numbers being primarily responsible for the feasibility of EBSD analysis addresses your question well. I noticed also that in my previous answer the label for the attachment is Al-Li phase diagram. The diagram should be labeled: Mg-Li phase diagram. This error is regretted.
Best regards,
Malur
Simply to confirm the assumed segregation of Li and Mg: if you simply take an BSE image, can you clearly differentiate the two phases? If Li would deliver a lower signal you already need to see this very clearly in your BSE image (calculate the backscatter coefficient). It does not need to prepared for EBSD. 1µm diamond paste and perhaps NO water for cleaning should give you at least for this problem an answer.
What do you mean by "can not analyze by EBSD"? Do you get any pattern at all on the beta phase? or you get pattern but the software can not index it well?
If you do not get pattern at all --> sample preparation. I agree with Prof. Raabe's suggestion, but it could be even better if you do all the procedure using ethanol instead of water. Do not use H2O even for grinding! Only ethanol.
If you get pattern but not successful indexing --> Are you sure you have the beta phase available in the database of your EBSD software? Maybe the crystal structure is super saturated or meta-stable that could not be indexed properly.
Regarding Ehsan Ghassemali's comments about the crystal structure of beta phase, it appears that it is neither supersturated nor meta-stable but it is an ordered BCC structure, stable in the range of 200K-500K. Thus it is implied that at 300 K ambient temperature beta phase has an ordered BCC structure of lithium-rich solid solution. I have attached an article describing this possibility. The article however, is based on MC predictions and no experimental confirmation is provided. Perhaps the corresponding author of this article should consulted if any experimental confirmation has been provided after this article was published in 2010.
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