I am trying to make optical microscopy analysis of samples of aluminum alloy (AA) 6082, AA 6082-T6 and pure Al. I want to measure the grain size/grain boundary before starting a DMA test and right after reaching 300ºC (at a heating rate of 1ºC/min) in the DMA. Quantifying and comparing the grain boundary in both cases may help ascertain the relationship between the quantity of grain boundary and the level of internal friction.
I was able to obtain micrographs of AA 6082 and AA 6082-T6 right after reaching 300ºC in the DMA (see attached images; average grain size of around 300 and 175 microns, respectively), but I have not been able to obtain micrographs for as-received AA 6082, AA 6082-T6 and pure Al, and for pure Al heated up to 300ºC (see multiple attempts in the google.drive links). I have tried up to four different metallographic etchants suitable to aluminum (Keller's etch, Krolls reagent, etc., as described in this link: http://www.metallographic.com/Etchants/Aluminum%20etchants.htm).
These etchants worked well for AA 6082 and AA 6082-T6 right after reaching 300ºC in the DMA, but have not worked well for as-received AA 6082, AA 6082-T6 and pure Al, and for pure Al heated up to 300ºC. This problem is dragging part of my research since a year ago. Can somebody please help me figure out what is wrong?
Thanks!
http://www.metallographic.com/Etchants/Aluminum%20etchants.htm
https://drive.google.com/open?id=0B9ZUHgkvA9-rSHVBUl9RT0NQdHc
https://drive.google.com/open?id=0B9ZUHgkvA9-rYVF0U05CeTVHbnc
https://drive.google.com/open?id=0B9ZUHgkvA9-rUkYyd2pfQ1pOa0k
https://drive.google.com/open?id=0B9ZUHgkvA9-rZ2JENDBmMGRLdmc
Hi
did you look in standard ASTM E 407?
Did you try repeat polishing and etching?
Good luck from Bosnia
From my experience, Keller's reagent will be Ok for all of them, the heated alloys, as-received and pure Al, but you should try to expose both the as-received and Pure Al for less time, may be around 15 sec. Of course, we suppose that you followed the Standard method in sample preparation and cleaning.
Pure aluminum (especially if it is much more pure than 99.99%) can have extremely large grain sizes. You may be using too high magnification and it appears that you are etching to the point of pitting rather than getting grain boundary delineation. Try a macro-etch first to check for unusually big grains. A strong alkaline solution may work better for you than acid solutions. If your aluminum has Cu in the mix it might form a black smut, which can be removed with HNO3. On Research Gate there is a reference on etching pure aluminum by George Vander Voort et al., that has some specific formulations. He recommends the use of polarized light to really bring out the grain contrast. If you haven't done it yet, try playing around with your polarizer settings on samples you've already etched (maybe the less deeply etched ones). Good luck.
https://www.researchgate.net/file.PostFileLoader.html?id=56b494b57c1920b66d8b4587&assetKey=AS%3A325745235775488%401454675124664
it seems that your sample surfaces are not very flat, Anyway, you can use imageJ software to threshold your image and from that you will be able to measure the pixel areas that lie on the grain boundaries.
Thank you very much to all of you for your replies and valuable advice:
1) Dear Dr. Wang, while it is possible that one or two samples were contaminated at some point of the process, I have made many different tests on many different samples and I think most samples should not be contaminated. I'll try anodizing
2) Dear Dr. Fakic, yes, I have repeated polishing and etching the same samples; at least for 6 of them I have polished and etched again, with a different etchant, though. I will check the ASTM standard
3) Dear Dr. El-Nasr, yes, I think I followed the standard method for sample preparation and cleaning, but I will double-check. In addition, I have etched samples with Kroll's reagent on one side and Keller's reagent on the other, for 15, 30 and 60 seconds, also for 2min, and then I tried more diluted, customary etchants for 60min, 120min, etc. Some of these etchants worked for AA 6082-T6 and AA 6082 both heated up to 300ºC, but not for AA 6082-T6 or AA 6082 as-received, or pure Al.
4) Dear Dr. Adam, it is true that some surfaces may not be very flat, as seen in some of the highest magnification images, but most images (maybe not most of the ones I attached or posted in google drive) taken in very flat samples or very flat regions of certain samples show the same problems.
Again, thank you all for your valuable advice
The answer lies in the Time factor. Your etchants are pretty good etchants. But you have to try different time periods like more than 5 minutes, find out the time for surface burn and then try to decrease it. I had the same problem once and I tried with 5 minute and got the answer.
Dear Dr Kirby, effectively, the etchant and the samples when etched are both at room temperature. I will also follow your advice. Thank you and thanks also to Dr. Zhong and Dr. Mathew.
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