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Aiden CC

Shibuichi, Up Close

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This semester I've been working with some of the MatSci faculty at my school to do a metallurgy focused independent study (half a class worth of science credit to learn about metals processing and analysis). The first part of that has been learning about phase diagrams, and for that I decided to use a copper-silver system since its a binary eutectic system and has some interesting micro-structures. I chose a composition the base shibuichi composition of 75% copper, 25% silver, and also decided to try a pour into cloth stretched over a bowl in a bucket of water (a traditional Japanese technique) in addition to a graphite ingot mold.

 

No Red 20x.png

This is an optical micrograph of the ingot sample. Focus isn't great, and the polish could be better (though this stuff is so soft I also think I scratched it with a cotton ball). The copper colored phase consists of the copper dendrites which formed above the eutectic temperature while the silver colored phase is the metal which solidified at he eutectic temperature. I'm still not sure what all of the dark grey inclusions are. I looked at he composition of one in the SEM and it consisted of copper and silver oxides, though a professor suggested some of them may also be inter-metallic inclusions, so it warrants more looking into.

20x Etched.png

This is the sample cooled in water at the same magnification. The main difference I notice is that the phase formed in the eutectic transformation looks slightly different. Also, I think the dark interiors of some of the dendritic copper grains may be a result of coring, with the copper solidified at higher temperatures containing less dissolved silver (the sample has been etched in a solution of ammonium hydroxide and hydrogen peroxide to preferentially attack the copper).

No Red 100x.png

A closer look at the ingot sample. You can see the copper and silver in the phase formed in the eutectic solidification fairly well. I definitely enjoy looking at these micro-structures!

100x Etched.png

Here is the water cooled sample at the same magnification. You can see more of the possible coring, as well as what looks like more copper in the eutectic phase. I want to look at it more quantitatively, but it may be that the faster cooling has something to do with it? 

0.066 1.60k Top View.png

I also recently got trained on using the SEM, which is really cool. This is a sampled of the ingot which I rolled, taken at roughly 20% reduction (starting thickness was 0.083", this was 0.066"). I believe the dark discoloration is from some problems with my etch. Deformation was along the vertical axis of the photo, this sample was polished parallel to the rolled surface.

I have a few more rolled samples (50% reduction and 80% reduction) that I want to examine to look at deformation and possibly re-crystallization. I guess this is only tangentially knife related (though I'm going to switch over to steels after a couple of weeks on brasses/bronzes!), but if figured it was worth posting. 

Thanks for looking!

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I am glad you posted the SEM image, because I think it confirms my suspicion that the dark (black) spots are not inclusions, but rather tear-out.  The tiny remnant of a grain that catches on the polishing pad and tears out at the grain boundary.  You can see that there is actually a pit in the one vertically centered, far left on the SEM image.  

Unless you were referring to the gray spots, which could just be uneveness in the etch, but could be something else, too.  

Can you also show macro pics of the samples?  Fun stuff, thanks for sharing!

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On 10/11/2018 at 7:49 PM, Jerrod Miller said:

I am glad you posted the SEM image, because I think it confirms my suspicion that the dark (black) spots are not inclusions, but rather tear-out.  The tiny remnant of a grain that catches on the polishing pad and tears out at the grain boundary.  You can see that there is actually a pit in the one vertically centered, far left on the SEM image.  

Unless you were referring to the gray spots, which could just be uneveness in the etch, but could be something else, too.  

Can you also show macro pics of the samples?  Fun stuff, thanks for sharing!

Tear out could make sense, this stuff is pretty soft. Here are the bulk samples/one of the mounted sections:BA9666E1-78DA-4D9D-925D-F4F1B13A2B6B.jpeg0BC6F983-27AD-45FF-84A1-B80827F168B3.jpeg

This stuff is really pretty, especially out of a water pour. I polished/etched some more rolled samples this morning, I’ll get some pictures up when I have more time. 

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