What determines malleability of metals?
In general, malleability is determined by how easily metal crystals can be moved around. Anything that gets in the way, such as another substance blocking crystal movement, makes the metal less malleable.

For example, pure iron is malleable, but add some carbon to molten iron, and the carbon precipitates out on cooling, blocking crystal movement and creating much harder and more brittle carbon steel. Think of it as a two-phase material.