The amorphous borders between grains in a metal

[rabbitweed]

I am taking Engineering 101, and trying to get the hang of metal structure on a 'grains' level.

Ok, so my understanding is this;

Metals are polycrystalline materials, that can be divided up into 'grains'. These grains are basically crystals/lattices with a repeating pattern, separated by amorphous solids. The place where two grains are separated by an amorphous solid is known as a grain boundary.

All correct so far?!

Ok, my question is; say in a lump of Fe ore, would these amorphous solids separating the grains be made of Fe, or some impurity?

Does pure metal have grain boundaries? It would seem to me they would likely form one giant crystal...but my understanding is hazy.

 

[Astronuc]

Polycrystalline materials (metals and ceramics) imply 'grains' which are the subunits in a solid. Each grain has a given lattice orientation, and the grain boundaries are more or less amorphous due to the mismatch between the adjacent lattices.

Ores are metal oxides, sulfides, carbonates, . . . , but not pure metal.

One could conceiveably have a single crystal of pure metal or alloy, but more often pure metals are polycrystalline due to the nature of the solidification process. Impurities are usually present in the ppm range.

 

[Mapes]

These grains are basically crystals/lattices with a repeating pattern, separated by amorphous solids.

This isn't the best way to put it. There are regions of interrupted crystal arrangements between grains, but these regions are hardly "solids." If two grains are nearly aligned, the grain boundary can be as mild as a periodic dislocation, and nobody would say that this interface is amorphous.

Does pure metal have grain boundaries? It would seem to me they would likely form one giant crystal...but my understanding is hazy.

Yes, essentially every metal you ever see will be polycrystalline. Although there's a driving force to form a single crystal (since the grain boundaries have a higher energy), the kinetic limitations are prohibitive.