Single crystal and poly crystal metal

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Discussion Overview

The discussion revolves around the differences in mechanical properties, such as yield stress and Young's modulus, between single crystal and polycrystalline metals. Participants explore theoretical implications, practical applications, and the challenges associated with single crystal materials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that single crystal metals may exhibit lower yield stress due to easier dislocation movement, while others express uncertainty about this claim.
  • There is a proposal to quantify the differences in yield stress and Young's modulus between single crystal and polycrystalline samples, questioning whether these differences could be significant.
  • One participant raises questions about the heat treatment of single crystal structures and their ability to undergo processes like forging and annealing, while also inquiring about the comparison of ultimate tensile strength (UTS) and modulus with polycrystalline samples.
  • Concerns are expressed regarding the limited applications of single crystal casting methods, with examples like MotoGP and F1 being mentioned, alongside a discussion of the high costs associated with single crystal production.
  • Another participant notes that the removal of grain boundary creep in single crystals could be an advantage, but questions the economic viability of producing single crystal parts if grain boundary creep is not a significant failure mechanism.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the mechanical properties of single crystal versus polycrystalline metals, with multiple competing views and uncertainties remaining regarding their applications and advantages.

Contextual Notes

Participants acknowledge limitations in their understanding and the need for more comparative examples and data regarding single crystal materials.

helloy
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What difference would you observe about the yield stress, young modulus,etc if your material were a single crystal? Explain the difference.
 
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Please show your own work first. Also, it looks like this and your other questions would be a better fit in the homework forum.
 
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
 
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?).
 
Thax, I'll think about it
 
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.

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

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