What determines malleability of metals? I'm designing an aircraft, and certain sections need to hold under a lot of stress. I know that it is the ways the metal crystallizes that effects the mallebility, but does the distance between the atoms/ crystal arrangement also effect malleability?
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.