RENE N5 microstructure (etched with Kallings 2) why are some regions darkened?

[Kim_W]

Dear engineers,
This is my first post and i am new to this very interesting forum.
Please excuse me if i did skip the introduction part, i'll try to introduce myself in the appropriate section if this is necessary!

Well, this is my metallurgical finding/question:
As i work in the aviation industry (but quite new to the materials they use) i had to cut out a High Pressure Turbine (HPT) blade for analysis. I did it to learn more about the material (RENE N5) and get more in depth knowledge.
The specimen was cut with water cooling, polished and finally swap etched with Kallings 2 reagent for 5 seconds:

close up:

my question is as following: why are some regions darkened by the etchant? They quickly appeared during etching. My own hypothesis is a phase change, caused by local overheating of this particular area (like grinding burns when you etch with Nital).

In order to answer my question and develop above mentionned hypothesis i tried to look into the available literature but i couldn't find anything that clearly answers my question.
But... There are gamma and gamma prime phases.. So i examined the microstructure at high magnification and i found out that the darkened regions do show a coalescence and coarsening of the gamma prime matrix (please correct me if i am wrong). I also examined the microstructure of a section which was cetainly NOT overheated (dovetail) and i observed a cubical matrix of gamma prime (please correct me if i am wrong).
Well, are the above mentioned observations the cause of darkened regions after etching? And if yes, what is exactly happening to cause some regions to be ''more'' susceptible for kalling 2?

High-mag picture of darkened microstructure:

High-mag picture of certainly non overheated microstructure (cool area such as the dovetail):

Thank you very much for your time!
Best regards,
Kim

 

[EdStainless]

Finding ref photos for these high temp alloys is tough. Keep searching, they are out there. There have been hundreds of papers published on this alloy alone. Don't be afraid to look at papers from the '60s and '70s, that is likely where you will need to go.
It looks like you have mounted in a caseable resin. I looks like you have some adhesion issues with etchant being trapped between the mount and the sample. It also look like an aggressive etch, it might be good to back off a bit.
Most of the superalloy work that I did involved electrolytic etchants. Is there a special reason that you chose this etch? What are you specifically looking for in the structure? In many cases you end up looking at unetched samples, and then ones etched 2 or 3 different ways in order to see all of the features.

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P.E. Metallurgy, consulting work welcomed

 

[Compositepro]

Those spots look to me like the etchant went into cracks in the potting compound at the edges of the part and then wicked out after the excess etchant was wiped off. Like a dye penetrant test.

 

[Kim_W]

Thanks for the reply. It is indeed tough to find reference material/pictures for this kind of superalloy.
It was cold mounted with transparent clarocit and there is indeed a very very small uniform debonding gap between specimen and mounting material (only visible at high mag). However, the dark spots appeared immediately when etchant was applied. Breaking off the mounting material, polishing and etching it again did not change anything. So my guess is that the base material at that location is really susceptible for discoloration.

I will try another etchant (electrolytic) to see if something changes. In any case, with kalling 2 the microstructure can be nicely observed (dedritic segregation and direction of solidification). This is pretty nice to see.
Another thing i would like to see, is a phase change due to prolonged overheating. I think this is something we can see in above pictures but i am not really sure.

From a logical approach, what is your opinion about a possible phase change that leads to locally dark etched regions? Kalling2 darkens martensite and austenite appears light brown (from literature).

Regards,
Kim

 

[EdStainless]

There are homogeneity issues that can result in black or white spots when you etch. I don't recall these forming over time, but they can become more pronounced. I might be worth marking locations and then repolishing so that you can look at those places unetched (polarized and DIC lighting) and again lightly etched (maybe with a different etchant).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Kim_W]

Thanks for the answer!
This week i'll cut some more specimens and do it in that sequence, first DIC, mark suspicious areas and after that electrolytic etchant.