(a) Briefly describe a twin and a twin boundary (b) Cite the difference between mechanical and annealing twins

A twin is a special type of defect that occurs in crystalline materials when two adjacent regions of a crystal have identical atomic structures but are mirror images of each other. This mirror arrangement results from a certain extent of the crystal lattice undergoing a deformation, leading to the formation of two sub-structures with the same lattice within the crystal. A twin boundary is the plane or line that separates these two mirrored regions of the crystal, also known as the twinning plane. At this boundary, the crystal lattice experiences a discontinuity as it transitions from one mirrored region to the other.

Mechanical twins are formed as a result of mechanical stress, such as an applied force, causing the crystal lattice to deform and create the mirrored sub-structures. This type of twinning often occurs in metals with low stacking fault energies and can lead to an increased capacity for deformation and hardening of the material. Annealing twins, on the other hand, are formed during the process of annealing (heating and slow cooling) of a material. This type of twinning occurs when the material slowly cools and new crystal grains with different orientations nucleate and grow. When two adjacent grains with mirrored orientations form, an annealing twin is created.

There are primarily two differences between mechanical and annealing twins: 1. Formation Process: Mechanical twins are formed due to mechanical stresses or applied forces, while annealing twins are formed during the annealing process of heating and cooling a material. 2. Microstructure Effect: Mechanical twins can lead to an increased capacity for deformation and hardening of the material by providing additional slip systems within the crystal structure. Annealing twins, in contrast, are usually formed during the recrystallization process and do not have as significant an impact on the mechanical properties of the material. In summary, mechanical and annealing twins are distinguished by the process of their formation and the effects they have on the microstructure and mechanical properties of the material.