a twin boundary

A twin boundary is a special, very tidy kind of interface where the crystal on one side is a perfect mirror image of the crystal on the other. Hold a page up to a mirror: the reflection is ordered and related to the original, not random. At a twin boundary the atomic pattern reflects across the boundary plane, so the two regions (the two 'twins') are like a shape and its mirror reflection sharing a common plane. Because the mirror relationship keeps most atoms in sensible positions, a twin boundary costs far less energy than an ordinary grain boundary.

Twins form in two ways. Annealing twins appear during heat treatment and grain growth, especially in FCC metals with low stacking-fault energy — brass and austenitic stainless steel famously show broad, straight-sided twin bands right across their grains when you look at them under a microscope. Deformation (mechanical) twins form instead by sudden shear, common in BCC and HCP metals that have few slip systems: when dislocation slip cannot accommodate the shape change fast enough, a whole region flips into its mirror orientation in one coordinated shear. That is what makes the audible 'tin cry' when you bend a bar of tin.

Twinning matters because it is an alternative way for a crystal to change shape when ordinary slip is hard, and it reorients grains so that new slip systems can line up favorably — extending ductility in metals like titanium and magnesium that would otherwise be brittle. In the microscope, straight parallel-sided twin bands are also a handy fingerprint for identifying a material and reading its processing history. Like a grain boundary, a twin boundary can still act as a mild obstacle to dislocations and add a little strength.