Why do aluminium sheets wrinkle easily but are hard to un-wrinkle? I notice when I buy aluminium sheets (the kind used for wrapping food) they come in straight, smooth rolls. Once I use them though, they become wrinkled, and they are impossible to un-wrinkle. Image of a wrinkled sheet:

Why is this the case? Only thing I can think of is the Second Law of Thermodynamics, but I can't see why that would be applicable. It's still the same sheet of aluminium, after all, and it's not like it's getting mixed with something else (like when making coffee). Furthermore, the manufacturers are able to make it unwrinkled, and it doesn't become wrinkled until it's perturbed. I'm tagging the question with thermodynamics anyway, because I don't know what else it could be.
 A: When aluminum is plastically deformed as during the wrinkling process, the aluminum right in the wrinkles gets stronger than the undeformed aluminum. This process is called strain hardening and is most easily demonstrated with a piece of aluminum wire. If you bend the wire into an angle and then try to unbend it again, the bent zone resists unbending more strongly than the wire near the bend, and so the originally bent portion stays bent and the  adjacent portions of the wire bend instead. If you then try to unbend them, the process just propagates outwards along the wire and instead of getting a straight piece of wire, you get a wire with a bunch of wavy bends in it.
You can unbend the wire if you squeeze it between two flat surfaces really hard and roll the wire between them, and at least in principle it is also possible to "iron out" the wrinkles in a piece of aluminum sheet too.
A: When the Aluminium sheet has been deformed beyond the elastic limit the bonds between the Aluminium atoms have been broken/deformed irreversibly.
This is called plastic deformation.
So a small amount of deformation (elastic) will allow the Aluminium sheet to revert to its original condition when the deforming force is removed but large deformations (plastic) do not.
Aluminium sheet is produced by passing heated ingots of aluminium through rollers numerous times and then processing them as necessary to obtain the required properties.
The rolling process is irreversible but controlled to produce a smooth surface.
Aluminium is ductile ie has ability to be permanently deformed without fracturing and you will notice this property when you crease Aluminium sheet.
