Metals are composed of grains. Each grain is characterised by the orientation of its crystal lattice (referred to as the grain orientation). Before straining, each grain is a homogenous entity with one orientation.
During straining, the grain develops an orientation spread. The figure shows a typical microstructure after straining. The strain-induced orientation spread is illustrated by the colour shading of two grains.
Defects (called dislocations) are essential for both the formability and the strengthening of metals. They accumulate in boundaries inside each grain (thin lines in the figure). The morphology of the dislocation boundary structure varies substantially between grains; for example the morphology in Grain 2 is coarser than in Grain 1.
Initially weak grains may become strong after a strain path change and the boundaries inside each grain will gradually change. The small images within one of the grains in the figure illustrate this. As this happens to all of the grains, the interaction between grains will also change dramatically. The macroscopic mechanical properties are the combined result of all these processes.
