Grain boundaries

          Engineering materials may be either polycrystalline or single crystal type. A polycrystalline alloy contains an enormous quantity of fine grains. Grain boundary imperfections are those surface imperfections when ever crystals or grains of different orientation separate the general pattern of atoms and exhibits a boundary during nucleation or crystallization. This type of defect generally takes place during the solidification of the liquid metal.

Twin Boundary

          This is another planar surface imperfection.When the boundaries in which the atomic arrangement on one side of the boundary is a somewhat mirror image of the arrangement of atoms of the other side, as shown in Fig. The defect caused is known as Twin boundary. The region in which a twin boundary defect occurs is between the twinning panes as shown in Fig.  The region of material between these boundaries is approximately termed as "Twin". 

          Twins result from atomic displacements that are produced from applied mechanical shear forces, and also during annealing heat treatments following deformation. Annealing twins are typically found in crystals that have the F.C.C. crystal structure. While mechanical twins are observed in B.C.C. and H.C.P metals.

Tilt Boundary

          This is another type of surface defect called low-angle boundary as the orientation difference between two neighboring crystals is less than 10°. This is why the disruption in the boundary is not so drastic as in the high angle boundary. This type of boundary is associated with relatively little energy and is composed of edge dislocations lying one above the other. In general, one can described low-angle boundaries by suitable arrays of dislocation. The angle or tilt, 

Where b is the magnitude of Burgers vector, and

            D is the average vertical distance between dislocations.