Taha Selman Cakir
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12th week

Deformation twinning:

Twinning is a deformation mechanism that represents reorientation or rotation of the crystal lattice. Figure 1.2.12 contains a schematic diagram of a crystal structure with a twin after the application of a shear stress. The twin is formed by the rotation of each atom about an axis through the center of the atom. The twin plane is the plane of symmetry. It is perpendicular to the plane of the figure and seperates the twinned and undeformed regions. Twinning occurs very rapidly, as in a 'snap through' mechanism and can produce a loud click. Twinning during a tensile test produces serrations or jumps in the tensile curve.

(Stouffer D. C., Dame T., Inelastic deformation of metals: models, mechanical properties, and metallurgy, p. 18, 19)

Spur gear reversal mechanism:

Spur rear reversal mechanism comprises five gears [Fig. 4.18a]. Gears 'A' and 'D' are keyed onto the driver shaft I.Gear 'B', an idler, can rotate freely on intermediate shaft II. The driven shaft III. is mounted with gears 'C' and 'E', which are free to rotate on the shaft, but cannot move axially. The central part of the special jaw clutch has teeth at both ends. Its internal splines mesh with the splines of the driven shaft. It can be slid axially. When the driving shaft I is rotating, all the gears rotate. Sliding the central part of the jaw clutch towards the left to mesh with the teeth of the half attached to gear 'C', rotates the driven shaft in the same direction as the driver. Meshing the central clutch half with the toothed half on gear 'E', rotates the driven shaft in the opposite direction. Thus, axial motion of the central clutch half reverses the direction of rotation of the driven shaft.


(Joshi P. H., Machine tools handbook: design and operation, p. 341)

Screw dislocation:

The motion of a screw dislocation is also a result of shear stress, but the defect line movement is perpendicular to direction of the stress and the atom displacement, rather than parallel. To visualize a screw dislocation, imagine a block of metal with a shear stress applied across one end so that the metal begins to rip. This is shown in the upper right image. The lower right image shows the plane of atoms just above the rip. The atoms represented by the blue circles have not yet moved from their original position. The atoms represented by the red circles have moved to their new position in the lattice and have reestablished metallic bonds. The atoms represented by the green circles are in the process of moving. It can be seen that only a portion of the bonds are broke at any given time. As was the case with the edge dislocation, movement in this manner in this manner requires a much smaller force than breaking all the bonds across the middle plane simultaneously.


(Alavdeen A., Venkateshwaran N., A textbook of engineering materials and mettalurgy, p. 42, 43)

Antiphase boundary :

Antiphase boundary occurs when wrong atoms are next to each other on the boundary plane. For example, with hexagonal close-packed (HCP) crystals, the sequence ...ABABAB... can be reserved at the boundary to ABABA I ABABA, where I represents the boundary plane. Antiphase boundaries and stacking faults are typically of very low energy, comparable to that of a coherent twin boundary.

(Lalena J. N., Cleary D. A., Principles of inorganic materials design, p. 67)