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I'm interested in the effect of thermal conductivity on the choice of materials used for high temperature applications such as turbines.

For the particular case of Nickel-Chromium alloys (such as René 41 or Udimet 700), I have found that electrical conductivity is lower than the thermal conductivity of other metallic alloys.

Is this because it is enviable to have a low thermal conductivity in a high temperature application? In this case, why is it enviable?

Is the low thermal conductivity just the result of the alloy's composition and processing but not necessarily wanted?

Thanks!

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  • $\begingroup$ How can electrical conductivity be lower than thermal conductivity? They are different properties. $\endgroup$ – Eric S Dec 23 '20 at 2:46
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Short answer , no. They are pretty much at constant temperature other than a couple minutes at start-up or change of power setting. One exception is the few highest temperature turbine blades ( in some engines ) that have a few axial holes through them for cooling air to pass through ; but again they are at constant temperature during operation. Although whoever designed them use the thermal conductivity to design them. Each stage is at a different temperature but steady ; the compressor gets warmer at each stage, the turbine gets cooler at each stage.

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For most alloys, the electrical and thermal conductivities are proportional.

enter image description here

This relation is quantified by the Wiedemann–Franz law, which states that the ratio of the electronic contribution of the thermal conductivity (κ) to the electrical conductivity (σ) of a metal is proportional to the temperature (T).

There are some exceptions, however, where this relation does not hold (I recently read a relevant article for nanomaterials, which I'll try to find and post as a reference if you are interested).

So with respect to the thermal conductivity and whether it's low/high, there is usually not much freedom. It is mainly a property of the alloy.

Regarding whether low thermal conductivity is desirable, I would be surprised if that were true. However, I would expect that high temperature applications require more dimensional stability (corresponding to a lower thermal expansion coefficient). So it might be that while obtaining the lower coefficient of expansion, the thermal conductivity is also lowered.

For example, in some cases the thermal conductivity and expansion coefficient of alloys don't always follow one another (see below for a CuCrZr alloy - I couldn't find data on Ni-Cr),

enter image description here

Below is the thermal expansion coefficient for Ni-Cr alloys and you see that the behaviour can change quite a lot in some cases.

enter image description here

Finally, Electrical conductivity can be influenced by high level of cold deformation (e.g. work hardening) and small grain size. For example small grain size decrease the electrical conductivity moderately. However, work hardening and thermal processes that results in small grain size are commonly used to increase the strength of a material. That increase in strength is also welcomed for turbine applications.

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  • $\begingroup$ Thanks for the edits. It just before my bed time, and I am not sure, I think entirely straight. I will have a look at it again tomorrow. $\endgroup$ – NMech Dec 22 '20 at 20:41
  • $\begingroup$ The table left out Co Cr ( Vitallium ) the first high alloy in turbines , I think it is still used. $\endgroup$ – blacksmith37 Dec 23 '20 at 1:32
  • $\begingroup$ @NMech I did figure you had a cold or something! The same thing happens to me when I'm tired, ha ha. $\endgroup$ – Chemomechanics Dec 23 '20 at 1:35
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Low thermal conductivity is often a wanted property is high temperature applications like turbo machinery. Indeed in that case thermal barrier coatings (TBC) are often applied to nickel alloy(or single crystal) vanes. This layer has indeed a very low thermal conductivity in order to limit the temperature to which the base metal is subjected to. Therefore I would say that even if it is not a major point as the TBC layer is often required anyway it is desiderabile not to propagate heat inside the machinery to which the designed element is connected to

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