[8-Ball]

myv65, your description of how work hardening takes place is a little off, though the result as you have described is the same, and the way it works is less important.

OK, I'm gonna run some stuff off the top of my head about how and why annealed copper might have a "slightly" higher thermal conductivity. (Please note that we don't really get taught about heat flow or thermal properties, this is just putting ideas together.) Also note, that if none of this makes sense, then I probably won't ave the time to take it any further.

Heat flow through most close packed solids, ie metals, is by phonons, lattice vibrations, which travel through the material. these vibrations are going to be largely affected by the bond strength/length between atoms.

Now this is where the hard/soft comes in.

No material is perfect!

The microstructure is not continuous. The copper we handle is polycrystalline, ie the material is split into grains, regions of crystalline material surrounded by other grains of material with a different orientation.

If that weren't bad enough, there are defects within the microstructure, whether they be missing atoms (vacancies), impurities, or dislocations. These are the main three we need to worry about.

Firstly, dislocations. Believe it or not, it is these defects which give high strength steels their strength. Drawn eutectoid steels have been produced specifically to have the highest dislocation density possible. These make up the cables on suspension bridges, tyre chord and so on.

When a material is subjected to a stress, a number of things begin to happen. Firstly elastic deformation. This is simply stretching of the bonds and is not permanent.

Once the elastic limit is reached, (yield strength), plastic deformation begins. This is when things get messed up.

A dislocation is a line defect. Imagine a stack of crystal planes, atoms packed together as tightly as possible. Now imagine half a plane wedged in there. The line at the edge of this half plane is a dislocation. Materials deform largely by various mechanisms of dislocation motion. Occasionally, dislocations can pass over one another and get pinned. Other dislocation formations can continuously generate new dislocations.

To cut a long story short, through deformation, a mass of dislocations are formed which tangle up with one another, grain boundaries, impurities, or anything else which will get in the way.

This is the process of work hardening.

Machining copper is essentially work hardening, and hard drawn copper, as discussed here has been deformed hugely at room temperatures to refine the microstructure.

All of these defects, grain boundaries included, have a stress field associated with them, distortions in the lattice surrounding them as the lattice attempts to accomodate this defect.

For a phonon trying to traverse a material, high defect concentrations is a nightmare.

So we need to aneal.

Dislocation/vacancy motion, essentially diffusion, are thermally activated. By heating the material dislocations move around, alingning themselves into "sheets" of dislocations forming low angle or tilt grain boundaries within grains. Vacancies diffuse to and from dislocations as they climb, and with time, the microstructure will coarsen, becoming large and fairly uniform.

A material in this state will allow the phonons to travel much more easily, thus increasing the thermal conductivity, at the expense of strength, hardness and so on.

It is for this reason that we have these different types of Cu, depending on the application requirements.

As a side note, diamond is such a good conductor because of it's near perfect crystalline structure high stiffness covalent bonds.

If this has gone over your head, please don't expect me to explain the whole thing in great detail.

To those who may have understood most of it, if there are small points which don't make sense, ask away, though I warn you, I will be busy, tired and a little grumpy over the next few weeks.

Again, let me emphasise that this is my attempt at putting together a reasonable explanation of what is happening, based on what I have learned. It was intended as an informative post aimed at those who want to learn more.

As for the annealing itself, I don't know off the top of my head at what temp the Cu should be annealed, or for how long, or indeed how it should be cooled. I will say that for near pure materials, the cooling rate is less of a critical factor as it is with high alloy steels.

Hope folks have benefitted from this.

8-ball