Single crystal and poly crystal metal

[helloy]

What difference would you observe about the yield stress, young modulus,etc if your material were a single crystal? Explain the difference.

 

[Mapes]

Please show your own work first. Also, it looks like this and your other questions would be a better fit in the homework forum.

 

[helloy]

my ans is:If the material were a single crystal, the yield stress would be lower. The dislocation could move easily because there is only 1 crystal. In addition, single crystal metals have directional stiffness. But I'm not sure about that

 

[Mapes]

Sounds good. To fill this in further, I'd mention the relative difference between the yield stress and Young's modulus for the two types of samples (i.e., is \sigma_y half as much for a single crystal? A thousandth as much? Similarly, does the Young's modulus in different directions vary by a factor of two? A factor of a thousand?).

 

[helloy]

Thax, I'll think about it

 

[The Wister]

I came to this forum to learn more about single crystal casting/properties/etc. Sooooo, instead of starting a new thread I will just continue this one.

Sounds good. To fill this in further, I'd mention the relative difference between the yield stress and Young's modulus for the two types of samples (i.e., is \sigma_y half as much for a single crystal? A thousandth as much? Similarly, does the Young's modulus in different directions vary by a factor of two? A factor of a thousand?).

And the answer to this barrage was? I've taken my fair share of materials courses and the only time single crystals are brought up, it is just to discuss that they are used for fan blades. Or that processing is """expensive""" with no comparative examples.

Can single crystal structures be heat treated to different phases? If they have dislocations, this implies that they can be forged/annealed/work hardened/etc, no? How would the UTS and modulus in the preferred (maximum value) direction compare to a polycrystalline sample of the same dimensions?

I know cost is a big prohibitor, but why have we not seen single crystal casting methods used to create parts in applications like MotoGP, F1, and others? It seems that their only purpose is to make fan blades and computer chips.

 

[The Wister]

Any help here? Seems like I couldn't find anything on my own for a reason...

 

[Mapes]

The advantage of single crystals (in mechanics) is that grain boundary creep is removed as a creep mechanism. If grain boundary creep isn't the dominant cause of failure, then nobody's going to spend money to grow a single metal crystal part.

 

[The Wister]

The advantage of single crystals (in mechanics) is that grain boundary creep is removed as a creep mechanism. If grain boundary creep isn't the dominant cause of failure, then nobody's going to spend money to grow a single metal crystal part.

Seems to me like they could find a place in IC engines...if anyone knew more about them.