Examining Ductility of Al-Cu Alloy: Brittle vs Copper & Aluminum

[tanky322]

Homework Statement

Examine the ductility of the Al-Cu alloy. Note how brittle the mixture is- most unlike copper or aluminum-Why?


2. Relevant Information
The alloy was a 50-50 mix of Al and Cu heated to 700C and then poured into a mold, before the eutectic temp was reached the remaining molten metal was poured from the mold.

The Attempt at a Solution

I understand that the material is hypoeutectic(Eutectic is about 67%Al). I believe that at the eutectic point the material would be very brittle, I am under the impression that the material instantly solidifies. This in my opinion, might be wrong, would cause a minimum amount of grain boundaries which would not impede dislocation movement, thus making the material very brittle.

Now I am not really sure if that's right?


Thanks!

Andrew

 

[Mapes]

But easy dislocation movement is a characteristic of ductile, not brittle, materials. If strain energy can be easily stored in the form of plastic deformation, then it's difficult to fracture the material in a brittle way. So this argument isn't too convincing.

One thing an Al-Cu alloy contains that pure Cu and Al don't is precipitates of a different phase. How will these precipitates affect dislocation motion?

 

[tanky322]

Hi mapes, thanks for responding.

I ended up figuring it out I think. This is what I ended up putting in my report for an answer. I got it from a DOE paper http://www1.eere.energy.gov/industry/imf/pdfs/intermetallics.pdf"

The brittle nature of the Al-Cu alloy can be attributed to the fact that it is an intermetallic compound. Intermetallics have unique characteristics of ceramics and metals, this allows them to have mechanical properties that place them in between the ductile nature of a metal and the harder and more brittle ceramics. At the atomic level intermetallics contain a mixed bonding type. They are both metallic and covalent/ionic, this causes them to have a non-ordered atomic structure. The abundance of grain boundaries created causes the material to have a very short plastic zone, this is due to the fact that the dislocations are impeded by the grain boundaries.

Thanks!

 

[Mapes]

Any time. Yes, the flip side of strengthening a metal is increased brittleness.