This is an alloy of copper (Cu) with 30% zinc (Zn) by weight. The polycrystalline grain structure of the brass is clearly seen. The parallel sided bands across the grains are annealing twins.
These are regions of different crystallographic orientation within the brass grains. They show that the alloy has been shaped by cold working (deformation at room temperature) followed by annealing (high temperature heat treatment).
The cold work increases the strength of the alloy, but decreases its toughness and ductility. The anneal restores the toughness and ductility of the alloy, but there is a reducti0on in strength.
Brass is electrically conductive, corrosion resistant and easily machined.
You should compare this with the microstructure of steel.
You can see similar alloys and images of this microstructure at other magnifications in Categories: - Metals.