How Fast do Grain Boundaries Move when You Recrystallize Aluminum?
When aluminum is processed in rolling mills from “direct chill ingots”, each of which is several feet across, down to sheet that is a millimeter thick (or less), many steps are involved. In addition to the mechanical action (rolling) to decrease the thickness, several annealing (heating) steps are also needed. What do these annealing steps do? They primarily restore the workability of the material: rolling tends to harden the material whereas annealing restores the initial softness. Internally, the grain boundaries move through the material, sweeping up dislocations as they go. Moving a boundary means that one type of crystal (orientation) increases and another type decreases; thus, the texture (crystallographic preferred orientation) changes. Controlling texture is critical to the successful use of the material in, e.g. beverage cans, aircraft parts. Because a grain boundary divides two different orientations, it has a particular character in its own right. Since it takes 5 variables
