Recent work and teaching has caused me to reflect on several interesting properties of alloys. Alloying typically:
1. Lowers the melting temperature of the base metal (especially if the alloying metal melts at a lower temperature than base metal)
2. Increases the strength
3. Lowers the thermal conductivity
All of these properties occur for (at least partially) the same reason: the distortion of the crystal lattice by the alloying metal. As an example, consider copper. Pure copper melts at 1083C, by adding some tin, the copper crystal lattice is distorted. This distortion contributes to a lowering of the melting temperature. By adding about 15% tin to copper, the resulting bronze melts at about 960C. This reduction in melting temperature was important to the ancients, as achieving high melting temperatures was a challenge.
This distortion also hinders the movement of dislocations, hence increasing the alloy strength as shown in the diagram.
More interesting is that alloying almost always lowers thermal conductivity. Some folks may try to increase the thermal conductivity of a base metal, by adding a known good conductor like copper. The result is often that the thermal conductivity of the metal is decreased. To understand why this happens we must remember that thermal conductivity in metals is due to the flow of electrons and phonons. Phonons are quantized packets of acoustic energy. The distortion of the crystal lattice by the alloying metal causes the phonons to scatter more, thus decreasing thermal conductivity.
In general metals are very good conductors of heat, because of the combination of the electron and phonon mechanisms, however the best conductor of heat by far is diamond, which uses only the phonon mechanism......but very effectively.
Material Thermal conductivity
W/mK
Diamond 1000-2600
Silver 406
Copper 385
Gold 320
Aluminum 205
Brass 109
Steel 50.2
Lead 34.7
Ice 1.6
Glass 0.8
Water 0.6
Wood 0.04-0.12
Note that of common materials, copper is the best thermal conductor, almost four times that of brass (copper-zinc). It is interesting that ice conducts heat better than glass!
Cheers,
Dr. Ron