Malleability and bending of metal at cryo temp

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Discussion Overview

The discussion centers on the behavior of a bent copper wire when subjected to cryogenic temperatures, specifically whether it would experience any unbending or changes in malleability as it cools to approximately 30-80 Kelvin. The context includes practical applications related to superconducting circuitry and the physical properties of metals at low temperatures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if a bent copper wire would unbend when cooled to cryogenic temperatures, seeking clarification on the material's behavior under such conditions.
  • Another participant asserts that the copper wire will remain in its bent position due to the stresses created during bending, which do not relieve at low temperatures.
  • A different viewpoint mentions "smart materials" that can return to their original shape with temperature changes, contrasting this with copper's properties as a pure element that does not exhibit such behavior.
  • One participant acknowledges the potential for shrinkage of the copper wire due to cooling, suggesting that this might affect contact but does not imply unbending.
  • The thermal expansion coefficient of copper is noted, with calculations provided for expected shrinkage when cooling from room temperature to cryogenic temperatures.

Areas of Agreement / Disagreement

Participants generally disagree on the possibility of unbending at cryogenic temperatures, with some asserting that the bent shape will remain due to internal stresses, while others explore the implications of thermal shrinkage on contact rather than shape change.

Contextual Notes

The discussion includes assumptions about material properties at low temperatures and the specific conditions under which the copper wire is used, which may influence the interpretations of behavior but remain unresolved.

uWave_Matt
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I think the broadest way to frame this question would be as follows:

"If a metal conductor (copper wire) were bent at room temperature, then placed in a cryo-cooler and cooled to ~30-80 kelvin, would it experience any 'unbending?'"

More specific to the situation, I've bent down the center conductor of a coaxial cable (very slightly) to act as a probe to a piece of superconducting circuitry secured in a jig, and pumped the chamber down. Is it possible that, at superconducting temperatures, that the probe loses malleability causing it to become more rigid and thus, straighten?

I apologize for my limited understanding on physical properties of metals. My only real exposure to them is from an introductory MEMS course.

Thanks for any feedback that you can give.
 
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Hi Matt,
The copper wire will remain bent in the position you bend it. There are stresses imposed in the material that will remain in the material regardless of how low a temperature you go. They don't relieve themselves and cause the material to change shape. The copper will shrink as I'm sure you're aware, and the modulus of elasticity will increase slightly, but it won't change shape because of stresses that are created in the material due to bending.

If you need any information on material properties at low temperature, post here or in Materials & Chemical Engineering forum.
 
You might have half-remembered something about "smart materials" which do "straighten out" and then return to their bent shape as the temperature rises and falls.

That only happens to specially "designed" alloys with a particular type of internal atomic structure, not to materials like copper wire which is almost a pure single chemical element.
 
Q_Goest said:
Hi Matt,
The copper will shrink as I'm sure you're aware, and the modulus of elasticity will increase slightly, but it won't change shape because of stresses that are created in the material due to bending.

Perhaps the shrinkage is significant enough to cause loss of contact and I was trying to over-complicate it. It'll go down in the lab notebook and thesis as "possible explanation."

Thanks to both of you for expelling this unbending business from my mind.
 
uWave_Matt said:
Perhaps the shrinkage is significant enough to cause loss of contact and I was trying to over-complicate it.

Thermal expansion coefficient of copper = about 17 x 10^-6 / K

So if you are cooling from 300K to say 50K, the "shrinkage" will be about 0.4% of the original length.

Of course the interesting number is the difference between this and the shrinkage of the other parts of your test rig, not the absolute value.
 

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