Micrograph 1 shows annealing twins in an a brass which has an fcc atomic arrangement. Such twins can form by growth faults when a crystal grows at the expense of others during recrystallization or grain growth. These twin boundaries are straight and parallel to 111 planes in both twins. Often there are several parallel twin planes indicating that growth faults have occurred repeatedly, roughly perpendicular to the growth direction. This phenomenon occurs frequently in fcc metals with a low stacking fault energy but not in Al which is fcc but has a high stacking fault energy.

Micrograph 2 is taken at a higher magnification and practically all grains show twins.

Micrograph 3 shows mechanical twins (deformation twins) in an fcc alloy of Cu with Cd. Mechanical twins are not common in such materials.

Micrograph 4 shows plate-like martensite in an Fe-30%Ni alloy. Micrograph 4b is taken at a higher magnification in an electron microscope and shows that at least some regions of the martensite are composed of very thin parallel twins. They are probably formed by the action of transformation stresses.

Micrograph 5 shows mechanical twins in a steel with a very low carbon content, formed by a mechanical shock at low temperature. In this connection they are called Neumann bands. These twins are primarily formed as thin lenses or plates with smooth sides but the matrix grain may later on grow back and make their sides very jagged. The many black spots in the picture are oxide particles caused by a high oxygen content in the steel melt, used to decrease the carbon content.

Micrograph 6 shows at a very high magnification that the sides of a twin can be very straight.

Micrograph 7 is taken on a piece of pure Fe which has been alloyed with Zn by diffusion from the surface, shown at the bottom of the picture. The specimen was deformed slowly at room temperature and the picture shows that the high Zn layer has deformed by the formation of mechanical twins but the pure Fe has not. The whole material is ferritic (bcc). Evidently, the addition of Zn promotes mechanical twinning of ferrite.

Micrograph 8 shows mechanical twins in pure Sn which has a tetragonal atomic arrangement. Mechanical twinning is an important deformation mechanism in several non-cubic materials.