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human
ELI5: Carbon makes steel strong. But why does too much carbon makes the steel brittle?

Carbon is added to steel to increase its strength and hardness but why does too much carbon makes it brittle? Can some explain in terms of the iron crystal lattices ?
gpt
Metals have crystals. The atoms in the metal form these grids with space in between. Regions inside metals all with the same pattern and orientation are called grains. A single metal can have many small grains or it can be few large grains

Defects in metals form inside these structure at grain boundaries. Where the crystal faces one way and then suddenly a different crystal faces another, cracks form easily. 

Imagine a metal sword made of two grains. The boundary runs across the sword the short way, through the middle. If the sword gets hit, the crack only has to start there to make it all the way across the sword through the shortest path possible. 


Compared to an iron atom, a carbon atom is very small. These atoms can get in between iron atoms and force new grain boundary patterns to form around them. So if there are a lot of carbon atoms, the grain boundaries are very small. Like the lines of a jig-saw puzzle, cracks have to jog around all kinds of grain boundaries to make it all the way through the metal. No single crack can follow one grain boundary all the way through. 

If there are too many grains because they are super small, the metal becomes weak in a new way. It becomes soft and more malleable because of all these small slip interfaces.