I have currently used one day old or freshly prepared modified Poulton's reagent for etching pure aluminium and Al 6061 alloy and also varying the time between 8 to 15 seconds. But the microstructures in Scanning Electron Microscope do not show clear grain boundaries. I want to know about standard reagent and the procedure to get microstructures with clear grain boundaries.
Keller's etching reagent is best suitable for most of the aluminum alloys and its composites. Distilled water (95%) + Nitric acid (2.5%)+ Hydrochloric acid (1.5%)+ Hydrofluoric acid (1%).
You can try the following solution also,
Methanol (25 ml)+ Hydrochloric acid(25ml) + Nitric acid(25ml)+ Hydrofluoric acid (2-3 drops).
Dear Dr. Akash Mukhopadhyay,
for Al and Al Alloys, especially for microstructure analysis, I have almost always successfully used Keller's solution, as Dr. Priyaranjan Samal said.
Keller’s Etching solution:
Distilled water 190 ml, Nitric acid 5 ml, Hydrochloric acid 3 ml and Hydrofluoric acid 2 ml
Conditions: 10-30 second immersion – use fresh.
Comments: For most aluminum and aluminum alloys
You can consult other alternatives to the website:
http://www.metallographic.com/Etchants/Aluminum%20etchants.htm
Best regards, Pierluigi Traverso
As Pierluigi Traverso and Priyaranjan Samal said, Kellers solution is the best reagent for clear microstructure and grains boundaries reveal in 6061 alloy. However, if your goal is to achieve SEM and comply with future EDX or chemical analysis, this reagent will remove lots of precipitates and therefore give false results.
For a clear view of grain boundaries without chemical reaction, I suggest you to improve your polishing protocol by adding a final step using a high pH (10 or so) colloidal silica solution.
This step should reveal grain boundaries without any chemical etching of the precipitates. However, this step should be done few hours before examination in SEM or sample should be stored in a sufficient vacuum chamber.
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