Particular deformation twinning

Twinned crystals of many materials are produced during both growth and deformation. Many mineral crystals are found in a twinned form in nature and deformation twins are often obtained when metals are deformed, especially at low temperatures. The most commonly obtained types of twinned crystals are those in which one part of the crystal is a mirror image of another part. The boundary between the two regions is called the twinning plane. The particular types of twins that form in a crystal depend upon the structure of the crystal lattice and deformation twins can be explained in terms of a simple shear of the crystal lattice. 

In addition to the importance that attaches to rigid body motions, shearing deformations occupy a central position in the mechanics of solids. In particular, permanent deformation by either dislocation motion or twinning can be thought of as a shearing motion that can be captured kinematically in terms of a shear in a direction s on a plane with normal n. 

The entropy forces are weak. They are strong enough, however, to cause the retraction of the specimen when the applied load is removed. This is because (we emphasize again) the polymer chains are in the liquid state. If the tie points did not exist—if the molecules were not pinned together at particular points—the assembly would flow in a liquid-like manner. It is this combination of long-range, weak entropy forces and liquid molecules which confers on rubbers their twin properties of high deformability and, paradoxically, complete retraction (3.N.4). 

The plastic deformation of crystalline solids proceeds by a process of slip and/or twinning on certain crystal planes and in certain crystal directions. In metals the slip planes (denoted by ) are usually those having the highest atomic density and they are the most widely spaced. The slip directions (denoted by < >) in the plane are those having the highest linear atomic density. A particular combination of slip plane and slip direction is referred to as a slip system. 

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