Need some help with Eng Materials question - It's slipping me off!

Dorkman

For some reason I am having no luck researching this question for a weekly assignment. No amount of note trawling or textbook reading is informing me anymore, and one have any clue on what direction I should be headed?

The question that is giving me fits is "How do atoms move in ductile and brittle metal crystalline solid structures. What are the results of each."

Now from what I understand, Brittle metals tend to cleave at the atomic structure rather then dislocate. This also tells me that perhaps the atomic structure is more rigid thus unable to easily reassemble itself after the cleave occured. The atomic bonds break, unable to reassemble themselves and thus kablamo!

I am also thinking that ductile metals obviously have an easier time relocating should a dislocation occur, hence why they are ductile and and able to be formed into other shapes easily. But I don't know how to explain why.

Could it be because the bonds in a brittle material are stronger, thus when dislocation occurs its more jarring or requires more energy, thus not allowing the bonds to reestablish themselves?

Blake T

I have dealt with the atomic scale for ages but the difference between ductile and brittle materials is that ductile materials have a large values between their yield stress and their tensile stress and ductile have a small one. If you can look up what happens to a material once it passes yield that will be your answer.

Try this site www.matweb.com or search for a site called engineer's toolbox.

VeritasVR

Dorkman wrote: »

For some reason I am having no luck researching this question for a weekly assignment. No amount of note trawling or textbook reading is informing me anymore, and one have any clue on what direction I should be headed?

The question that is giving me fits is "How do atoms move in ductile and brittle metal crystalline solid structures. What are the results of each."

Now from what I understand, Brittle metals tend to cleave at the atomic structure rather then dislocate. This also tells me that perhaps the atomic structure is more rigid thus unable to easily reassemble itself after the cleave occured. The atomic bonds break, unable to reassemble themselves and thus kablamo!

I am also thinking that ductile metals obviously have an easier time relocating should a dislocation occur, hence why they are ductile and and able to be formed into other shapes easily. But I don't know how to explain why.

Could it be because the bonds in a brittle material are stronger, thus when dislocation occurs its more jarring or requires more energy, thus not allowing the bonds to reestablish themselves?

"Hello, I'm VeritasVR. Do you need help with materials-related science? I specialize in... etc etc"

Anyways, your first paragraph about brittle metals is correct. They break with catastrophic force because they have strong bonds/units cells.

I'm having trouble following your other statements about metals or materials though. A brittle ceramic acts differently from a brittle metal, considering ceramics are usually very strong ionic bonds and metals are typically weaker metallic bonds between two metal atoms. Brittle and ductile metals are actually pretty darn similar compared to the ionic bonds of ceramics.

I'm not sure if this is relevant, but ductile metals may be ductile due to plastic deformation, which is a whole bunch of dislocations in one area that don't cleave or anything. If you keep deforming the metal, a bunch of crystal planes stop aligning, so a flaw takes a lot more energy to make it through. The dislocations actually harden the material, making it brittle. Annealing (heating) the metal may restore original properties, giving atoms energy to realign themselves.

But yeah, different metals/alloys have different elements which affects

1) the geometry (and therefore strength) of the unit cell.
2) different bond strength.

Those are big indicators of atomic ductile/brittle behavior. There may be a third part to that, but those are two of the major factors that I can remember. This book was my bible for this sort of thing.

Jebus314

It's hard to tell what the question is getting at, as it is kind of vague. I think you and VeritasVR are on the same page with the bond strength's arguments, but I feel like the question may be getting at the idea of slip. If you you look up how dislocations can move in metals, you get some idea of how atoms might shift to accommodate stress. Thus making the materials ductile. Knowing how the dislocations/atoms move in ductile materials, you can then maybe think of why certain types of materials/atomic bonds would not allow this mobility, and thus would be more brittle.

Again it's confusing because I'm not sure if the question is referring to atomic movement on a grand scale (a la slip/dislocation movements) or on a small scale (a la diffusion of single atoms). I'm not sure diffusion would really have applicable affects on the ductility of a material though, so I would go with slip/dislocation movements.

Dorkman

Yeah, I have a nasty habit of using Metals and Materials meaning the same thing. Whenever I type materials in that paragraph, I am referring to metals.

Well good, I think I have brittle taken care of.

Would it be wise to assume that because a material is ductile, it is considered more pure? This would also lead me to believe that since it is in a more pure state, that has a more uniform and orderly lattice, thus meaning to can experiece dislocations more frequently?

I am reaching for answers here :S

**edit** Sorry, we are on the topic of slipping and dislocations. Should have specified

Jebus314

Dorkman wrote: »

Yeah, I have a nasty habit of using Metals and Materials meaning the same thing. Whenever I type materials in that paragraph, I am referring to metals.

Well good, I think I have brittle taken care of.

Would it be wise to assume that because a material is ductile, it is considered more pure? This would also lead me to believe that since it is in a more pure state, that has a more uniform and orderly lattice, thus meaning to can experiece dislocations more frequently?

I am reaching for answers here :S

I'm not exactly a material science guy, but I think you may have taken a wrong turn somewhere. To start I don't think being more pure would allow more dislocations. In fact I would think it would be the other way around, where impurities were causing the dislocations. Second, I don't think it is necessarily the formation/number of dislocations that implies ductility, but rather the mobility of the dislocations. The movement of the dislocations to accommodate high stress areas is what gives the ductility. If the dislocations are stuck, then they aren't really helping anything.

EDIT: do you have the materials science and engineering: an introduction book by Callister? Look at pages 26-29 and then pages 175-177. It talks about the different bond types and whether they are usually ductile or brittle. As well as dislocation movements.

Dunadan019

I'm gonna guess this is something simple like:

In ductile metals a metalic bond forms. that is when the electrons from the atoms are free to move around and no longer attached to any individual atoms. The atoms are effectivly in a sea of electrons. This gives them greater movement abilities so the metal can be bent.

Dorkman

I am working out of Engineering Materials: Properties and Selection - 9th edition.

I don't know Jebus, we had just been taught that impurities in metal are what gives it it's strength. (Iron by itself is soft and ductile, but when alloyed with Carbon, created harder Steel) This Strength is due to dislocations being blocked by the ununiformed make up of the atoms in their lattices.

I guess if worse comes to worse, I seem to be on the right track with brittle metals, but just can't seem to describe why ductile moves as it does. 50% isn't too bad!

VeritasVR

Dunadan019 wrote: »

I'm gonna guess this is something simple like:

In ductile metals a metalic bond forms. that is when the electrons from the atoms are free to move around and no longer attached to any individual atoms. The atoms are effectivly in a sea of electrons. This gives them greater movement abilities so the metal can be bent.

Yes, that's the explanation I was looking for to supplement my second point about bond strength. This is why metallic bonds are weaker than ionic (and covalent?) bonds in ceramics. However, if the question asks more about slip, that's on a slightly larger scale.

Pure metals are typically softer than mixed metals (alloys) or metals with non-metallic impurities.

Indeed, a single element crystal matrix forms bonds with itself and may align itself into nice, ordered unit cells depending on the properties of the element. Gold, iron, aluminum, and many other metals are softer than mixing them with other metals. The geometry of the crystal unit cell changes due to the mass and ratio of several types of elements combining together. The bond strength here is still metallic, so AFAIK it's not a huge indicator of mechanical brittle/ductile behavior.

However, once you start including stuff like carbon and gaseous elements, both the unit cell and bond type are altered. Carbon atoms in iron act like road blocks against oncoming dislocations, so they prevent slip and ultimately make the alloy stronger and therefore more brittle. They're also stronger than metallic bonds (again, I think covalent). Steel is much stronger, harder, and brittle than iron. All of those properties are proportional in the majority of materials. The inverse is true as well; iron is weaker, softer, and more ductile than steel.

I hope I'm throwing enough stuff at you in order to maybe answer that vague question. It is vague.

Also, in later MatSci classes, I found that 50% was an A.

Dorkman

I am sure I will come up with something. Thanks for the helps peeps!

If worse comes to worst I will just demand a proper solution post-assignment. I don't mind duffing assignments as long as I can use them to figure out this garbage for tests/exams