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kik0kik0
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Join Date: Sep 2006
Posts: 31

Aluminum Crystal Size

04/09/2009 11:01 AM

Dear All

I am dealing wtih Aluminum powder, and I asked for making an XRD for the powder to know its composition. I received with the Xray chart something called crystallite size of Al. I know that Al has FCC crystal structure.

I also read that by mechanical milling crystallite size is reduced.

The question is what is crystallite size and how it changes during mechanical milling of powder?

Thanks is advance
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Schroeder
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Join Date: May 2006
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Good Answers: 1
#1

Re: Aluminum crystal size

04/09/2009 1:33 PM

crystallite size is essentially your grain size. The Hall-Petch relation predicts that as the grain size or crystallite decreases the yield strength increases

By various heat treatments you can affect a materials grain size. Materials that are brittle could be anneal by heating up. This heat causes the grains (crystals) to grow. By quenching hot parts in air, oil, water , LN causes a material's grains to rapidly solidify, which results in a fine grain structure.

I do a lot of milling of aluminum and other metals (Ti, Mg, Ni, Co, WC etc) in liquid Nitrogen. Inside the mill you have ball bearings and powder metal (Al) being spun around rapidly in a liquid nitrogen slurry. When 2 balls collide the Al powder caught between is crushed, folded and mechanically micro-welded. Well you do this over 5, 8 , 12 , 24_ _ hours the grains keep getting refined smaller and smaller. Eventually the grains get so small we need to use our scanning electron microscope to image them. Also Al cryomilling is a great process because you add 10nm nitride particles into the Al, these nitrides keep the grains from growing during heating. That means the cryomilled Al can go to a higher temperature than the virgin Aluminum before yielding.

If you would like some more info or want something cryomilled and consolidated into a billet or near net-shape part please email me

steven.schroeder@calnanocorp.com
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Campbell Lighting
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United States - Member - Donald here, Campbell Lighting Co. Engineering Fields - Retired Engineers / Mentors - New Member

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#7
In reply to #1

Re: Aluminum crystal size

04/10/2009 1:04 AM

Hello Steven..

Off topic here.. but I have a question for you..

Years ago, we bought a product that was a potting compound made with filled alumina..

it was a very good heat sink for a transformer, and I just wondered the formula, and where I could purchase this type product?

Donald

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lyn
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#8
In reply to #7

Re: Aluminum crystal size

04/10/2009 8:04 PM

Google: Military Specification: MMM-A-132 TYPE 1 CLASS 3

We used to get it from Emerson & Cuming.

Shell Chemical, Loctite, Lord Chemical, National Starch should all offer this line of product.

My experience is 20 years old.
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Milo
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#2

Re: Aluminum Crystal Size

04/09/2009 1:57 PM

Greetings kik0kik0!

You have to know what a crystallite is to understand this task. A crystallite is a single grain in a poly crystaline substance.

So The question that remains is "How does the act of milling somehow cause the transformation of these subparticles of the substance?"

I think that as the size of these grains becomes smaller they may require more or less energy to "transform" from one crystallographic phase to another, based on the crystallite size. Formally this would be "grain size dependent phase transformation temperature shift." Whew! I'll let the mystics and the math wonks figure out the exact hows and whys of this, I'm not equipped to take us there.

This is more a big issue with the ceramic boys, I think, than any metallurgy I've had to do. I haven't worked in this area, but as an escaped lab rat, I think I'm pretty close.

I'm sure that some one with about 20 years of p-chem could probably explain the thermodynamics. At which point I will excuse myself to go catch a mouse for Del to shoot with his crossbow...

The pharma boys (and girls) use Rietveld analysis with Xray data for this, as that information is importnat for uptake and thus dosage. Specialized equipment and image analysis software are available to do this

You could also try to Google "Rietveld analysis."

The immediate question for you though, remains, how does the act of milling somehow transform the grainsize of crystallites in the polycrystal?

I must have missed lab that day.

milo

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Schroeder
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#3

Re: Aluminum Crystal Size

04/09/2009 2:54 PM

how it changes during mechanical milling of powder?

there is a lot of literature out there on this

Again you are causing repeated welding, fracturing and re-welding of a mixture of powder particles. This causes intense deformation that can force atoms into positions of non equilibrium state. You're deforming the crystal lattice on the atomic level and in the micro level you have metallic powders agglomerating (getting Bigger).

Phase transformation has little to do with the crystallite reduction in milling, It's all about the high intensity metal forming breaking the atomic bonds. That's why these materials have such high dislocation grain boundaries.

I think this answers why the crystalite (grain) size is getting smaller.

Mechanical Alloying and Milling by C. Suryanarayana

http://books.google.com/books?id=HBYqEgsordMC&dq=mechanical+alloying&printsec=frontcover&source=bl&ots=xTZtUqdUWk&sig=Z4aDmYJ6uNfM5nOIGIqB4smCY5s&hl=en&ei=q0DeSeXlHNq9twezhMmRAQ&sa=X&oi=book_result&ct=result&resnum=7#PPA24,M1

The formation of metastable phases by mechanical alloyinghttp://resources.metapress.com/pdf-preview.axd?code=c615j88xg1851311&size=largest
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Milo
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#4
In reply to #3

Re: Aluminum Crystal Size

04/09/2009 3:04 PM

Thanks. You clearly have the answers on this one!

Nothing like actual experience.

Thanks for the links.

milo

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kik0kik0
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Join Date: Sep 2006
Posts: 31
#5

Re: Aluminum Crystal Size

04/09/2009 5:17 PM

Thanks alot my friend, your answer is really of a great help for me.

So as part of your experience, what would happen if the milling process was performed without nitrogen or in usual cases in Argon. i.e. in normal atmosphere?

Would this affect the final particle size?!!
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Schroeder
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Join Date: May 2006
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#6
In reply to #5

Re: Aluminum Crystal Size

04/09/2009 7:56 PM

FIRST THING

If you are trying to mill Aluminum yourself be very Very VERY CAREFUL. You are working with a pipe bomb that is pyrophoric and could combust by mere moisture in the air. The right concentration of particles in the air + H2O=dead scientists. Remember milling breaks off the materials oxide layer causing Al to react with air. I've personaly taken a sample (thimble) of a certain material I've cryomilled put it ouside on the concrete kicked it with my shoe and watched a fire shower go into the air. This is how pyrotechnics make the powder for their fireworks.

BE SAFE!!! PLEASE

Few things will affect final particle size.

-first point metals agglomerate into large particles and ceramics mill down in particle size

-second soft metals (Al, Mg, Ti) really agglomerate (weld) into larger particles. So much the machine will stall out or break. It becomes like a metal taffy gumball, this material is not good and really #*@$ things up. The solution for that is a lubrication to keep the agglomerating down. Stearic acid helps with this, I will tell you only that nothing about quantity there of. Also important now with stearic you have just added H2 and O2 to your material this must be vacuum degassed out after milling and before consolidation or material won't be very good.

-gas atms. have way more heat involved compared to cryomilling and this increases reaction rate, and may reduce req. milling time. However with heat means agglomeration. And this brings up explosion issues possibly. What's nice about our cryomilling the powder comes out in a liquid nitrogen slurry because it's in liquid the powder won't react with air until the LN has boiled off.

- So Ar gas atm. will likely have larger final particle size with no lube added

-there can also be a difference between liquid Nitrogen & Liquid Argon

-Milling certain materials in alcohol has also been done. SCARY STUFF right there

-Engineering of powder metals for bulk consolidation is a fascinating topic. Start from an application figure out what properties and materials you want, then how do you make materials with those properties. Grain Size? for formability and strength, Nitrides? for increased thermal stability, Alloys (Cu, Ag, Si) can be also added so easily to a PM, _ _ _ _
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Campbell Lighting (1); kik0kik0 (1); lyn (1); Milo (2); Schroeder (3)

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