<P> Crystals that grow adjacent to each other may be aligned to resemble twinning . This parallel growth simply reduces system energy and is not twinning . </P> <P> Twin boundaries occur when two crystals of the same type intergrow, so that only a slight misorientation exists between them . It is a highly symmetrical interface, often with one crystal the mirror image of the other; also, atoms are shared by the two crystals at regular intervals . This is also a much lower - energy interface than the grain boundaries that form when crystals of arbitrary orientation grow together . </P> <P> 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 . 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 . In certain types of high strength steels, very fine deformation twins act as primary obstacles against dislocation motion . These steels are referred to as' TWIP' steels, where TWIP stands for TWinning Induced Plasticity . </P> <P> Of the three common crystalline structures BCC, FCC, and HCP, the HCP structure is the most likely to form deformation twins when strained, because they rarely have a sufficient number of slip systems for an arbitrary shape change . High strain rates, low stacking - fault energy and low temperatures facilitate deformation twinning . </P>

Define a twin boundary associated with a martensite structure
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