it's a technical term used to describe where stresses of whatever source actually alter the grain structure of the metal, it forms a crystalline pattern when viewed under magnification.Exactly what do you mean by "crystallization"?
Such a crystallization due to stress alters the grain structure of the metal and hence it's physical strength properties at the crystallization location.
take the push tab on the top of a soda can, it's somewhat shiny as-is.
Work the tab till it breaks off, you'll notice it's no longer shiny at the point of breakage but a duller appearance or color, that'd be a simple example of metal crystallization.