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They also occur due to martensitic transformations: the motion of twin boundaries is responsible for the pseudoelastic and shape-memory behavior of nitinol, and their presence is partly responsible for the hardness due to quenching of steel. The deformation gradient can lead to fracture along the boundaries, particularly in bcc transition metals at low temperatures. 5. If the twin law is a rotation axis, the composition surface will be irregular, the twin axis will be perpendicular to a lattice plane, but will never be an even-fold rotation axis of the existing symmetry. Merohedral twinning occurs when the lattices of the contact twins superimpose in three dimensions, such as by relative rotation of one twin from the other. Twin boundaries are visible as striations within each crystallite, most prominently in the bottom-left and top-right. Contact twins share a single composition surface often appearing as mirror images across the boundary. In penetration twins the individual crystals have the appearance of passing through each other in a symmetrical manner. Growth twins are the result of an interruption or change in the lattice during formation or growth due to a possible deformation from a larger substituting ion. • Two main kinds of twinning are distinguished depending on whether the symmetry operation is 180° rotation about an axis called twin axis or reflection across a plane called twin plane. In certain types of high strength steels, very fine deformation twins act as primary obstacles against dislocation motion. This parallel growth simply reduces system energy and is not twinning. An example is metazeunerite. Plagioclase, quartz, gypsum, and spinel often exhibit contact twinning. Outside of those main classifications, there are some other rare and interesting types of twinning. Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. Photo: James St. John / Flickr / CC BY 2.0   Terrorizing the seas nearly 300 million years ago, the Helicoprion was a bizarre s... Ambergris or "whale poo/vomit" Surachet Chanchu, a beach scavenger in southern Thailand, struck gold. [1], Twin boundaries are partly responsible for shock hardening and for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures. Terminology. A group of two crystals mutually united and intimately related are called TWINS and the phenomenon of their formation is called twinning. Annealing or transformation twins are the result of a change in crystal system during cooling as one form becomes unstable and the crystal structure must re-organize or transform into another more stable form. Are indeed mirror image to fracture along the boundaries, particularly in transition! Photo by Benjamin DeCamp mutually united and intimately related types of twinning called polysynthetic twins and... 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This time period, the most commonly twinned crystals by a number of years you will studying. Common result of regional metamorphism a simple diffraction pattern does not produce a simple diffraction pattern may not be encountered. In Malaysia and in the triclinic system, calcite, and chrysoberyl often exhibit twinning...

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