Nanotwin

Nanotwin refers to an ultrafine twin boundary within a crystalline material. In a nanotwinned crystal, the solid contains dense arrays of coherent twin boundaries that partition the crystal into nanoscale twin lamellae. The boundaries are symmetry-related mirror planes, typically coherent twin boundaries that are highly ordered. The thickness of individual lamellae is usually on the order of a few to a few tens of nanometers.

Nanotwins form under specific processing conditions and can arise during rapid solidification, severe plastic deformation, or controlled annealing in certain alloy systems. Common methods used to create nanotwinned metals include electrodeposition, often for copper or nickel, and deformation techniques such as high-pressure torsion or equal-channel angular pressing, sometimes combined with annealing. The twin spacing can be tuned by adjusting processing parameters to tailor properties.

The mechanical behavior of nanotwinned metals is characterized by a combination of high strength and enhanced ductility, attributed to the dense network of coherent twin boundaries that hinder dislocation motion while still allowing plastic deformation through twin boundary activity. This structure can also improve fatigue resistance and thermal stability by providing stable barriers to crack initiation and propagation. In many systems, reducing twin spacing strengthens the material, though the precise relationship depends on the material and temperature.

Examples and significance: nanotwinned copper has been extensively studied for its strong performance as a conductor with improved strength and maintained conductivity, making it of interest for microelectronic interconnects and MEMS. Other nanotwinned metals include nickel, aluminum, silver, and certain steel alloys. Limitations include the thermal stability of nanotwins at elevated temperatures, where boundaries can migrate and coarsen. Ongoing research aims to optimize processing routes and elucidate underlying mechanisms.