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I have measured the resistivity of a nichrome ($80$ Nickel $20$ Chrome) wire as a function of temperature. enter image description here

Because my setup is quite crude (just a power supply and a couple of multimeters to measure voltage and current), I do not know if the bump that I get around $0.6A$ is physical or an artifact of the measuring setup.

However, I stumbled across this plot online (https://super-metals.com/wp-content/uploads/2015/04/Nichrome-Alloys-for-Heating.pdf) enter image description here I can see that the same bump is present more or less at the same temperature. It is not very clear if this temperature factor is the temperature coefficient of resistance or what.

So my question is:

What is the physical explanation for that bump?

I am a particle Physicist so you can assume I understand the basics of thermodynamics and quantum mechanics.

If you happen to know the real source of the figure it would help immensely.

EDIT$1$: I have substituted the Resistance vs Current plot with the more explicative Resistivity vs Temperature plot.

EDIT$2$: I found another site online where the same behavior was observer: http://www.brysonics.com/heating-a-nichrome-wire-with-math/

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  • $\begingroup$ The temperature coefficient of resistance has little to do with your plot. Essentially, in your case you did the experiment near ambient temperature, and Joule heat probably hasn't increase that much the resistance of your wire. The temperature coefficient of resistance you have plotted is completely flat in the region of interest. $\endgroup$ Apr 9, 2021 at 7:59
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    $\begingroup$ That is definitely not the case. I have modeled the wire temperature quite accurately and the in the range of 0A to 1.3A the wire temperature varies from ambient to 1400 C. Besides that, the wire was glowing yellow at 1.2A. In any case, even if we do not consider the first plot, what is the meaning of the bump in the second plot? $\endgroup$ Apr 9, 2021 at 8:04
  • $\begingroup$ Alright, I am quite confused. I preferred your resistance (instead of resistivity, which you did not measure but calculate), or voltage, vs I. If the temperature coefficient is worth 1/R dR/dT, the bump in R(T), or R(I) for that matter, should yield a negative temperature coefficient. But the 2nd plot has positive values in the whole range. $\endgroup$ Apr 9, 2021 at 9:16
  • $\begingroup$ Nevermind, I see what they mean now, it;s a relative change in %, not the absolute temperature coefficient. $\endgroup$ Apr 9, 2021 at 9:24
  • $\begingroup$ I don't know the answer to your question, I think it's a good one (hence my upvote). It would be interesting to see Ni and Cr's curves, too, at least to see whether this comes from either of these metals. $\endgroup$ Apr 9, 2021 at 9:27

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The nichrome wire undergoes a phase transition at that mysterious temperature which changes its unit cell structure. The two different unit cells have slightly different bulk resistivities. Superimposed on that shift is the usual increase in resistivity with temperature for metallic solids.

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  • $\begingroup$ Do you have a reference for that phase transition ? Thanks. $\endgroup$ Apr 9, 2021 at 20:20
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    $\begingroup$ @StephenG, a google search will bring up the Cr-Ni phase equilibrium diagram versus temperature. There's a lot of possible activity there depending on the exact percentages of the alloy constituents. $\endgroup$ Apr 9, 2021 at 21:28
  • $\begingroup$ It seems that this is the correct answer. I found this article that supports this answer. The article is from 2002 and at that time it was still not clear which kind of structural phase transition was happening. $\endgroup$ Apr 11, 2021 at 6:40

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