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I was playing with a key chain loop in a (very boring) chemistry class and then I straightened the loop into a wire keeping two end of the loop (now wire) curved so as to easily twist it. It was more or less a S shaped structure of metal with a longer straight part in the middle of S.

On twisting a lot, it started getting hotter. Why did that happen?

It was a circular cross section wire and I do not exactly know which metal, if it helps.

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By twisting and untwisting the wire, you did work (in the physics-specific sense of the word) on the wire. Effectively, by exerting a force on the wire, you transfer energy into it. That energy has to go somewhere, and in this case it ended up as heat.

Some of your body heat was probably transferred into the wire in the process as well; but if the wire felt warm to the touch after this process, its temperature was probably above your skin temperature, which would imply that the heating wasn't just due to your own transferred body heat.

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I believe the heat is due to plastic deformation, as you repeatedly twist the wire. So what is plastic deformation?

Stress strain

When the metal is strained no further than $A$, its response is purely elastic and the wire will return fully to $O$. But if the deformation exceeds $A$, the deformation will be plastic and the wire will only restore itself to $C$.

As Michael Seifert noted in his answer, the work done by these repeated plastic deformations is then converted to heat and the wire heats up slightly.

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    $\begingroup$ Or a bit more than just "slightly". Bend a metal paperclip, hold it to your lips. Warning: might cause a burn. $\endgroup$
    – user137289
    Nov 13, 2017 at 23:14

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