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Twins and twin boundaries are defects of interest for enhancing technological materials because they may influence, positively or negatively, different properties. Twinning has remarkable effects on the thermal stability, as well as mechanical and electrical properties, of FCC metals. Since their discovery, nanotwinned materials have opened new avenues of research and applications. The nanotwin structure consists of a high density of nano-scaled lamellae with twin spacing below 100 nm in a columnar grain structure. As a result of nanotwinning, numerous coherent twin boundaries are stacked within the grain. They separate the internal grain into distinct twin and matrix lamellae regions. The orientation of twin lamellae mirrors its corresponding matrix lamellae across the twin boundary. The twin boundaries are specifically structured to follow the ordered arrangement of nano-scale twin lamellae in the grains. Therefore, nanotwinned materials have greatly improved material properties as compared with the bulk material.
The mechanism of the formation of nanotwin structures has become a research topic of great importance. Numerous theories have been introduced to explain the occurrence of twinning. Although the cause of twinning formation has been discussed from various viewpoints, its fundamental origin can be simply addressed via an analysis of energy. A crystal structure with a continuous lattice exhibits the lowest free energy state of that collection of atoms. Thus, variation from such a structure could result in an increase in the energy of the bound atoms. The formation of twins creates a distinct lattice compared to the matrix. The twin boundary is established in the region where twin and matrix meet to fulfil the minimum interfacial energy requirement. The twin boundary is a single plane of atoms connecting two non-mixing phases. Such an arrangement ensures that the structural change across the twin boundary is less abrupt because there is no significant lattice discontinuity. Therefore, whilst the energy level at a twin boundary is higher than in the rest of the crystal, the difference in energy levels is negligible. A similar phenomenon of close-to-minimum energy can also be observed within all types of intergrowths, where the crystal orientation across the intergrowth boundaries behaves similarly to the crystal orientation across twin boundaries.
Nanotwin structures offer promising characteristics in broad applications like the development of nano-devices in optical systems, nano-electromechanical materials, high-performance mechanical structures, and biological sensing (Sun, 2018). To date, comprehensive coverage of nanotwinned metals is available based on the FCC materials. Efforts on other nanotwinned structures, such as hexagonal closed-packed (HCP) or body-centred cubic (BCC) metals, alloys, and synthetic materials, continue to be an essential part of exploration. Furthermore, the nanotwinned structures are known to be beneficial alternatives in numerous material areas like mechanical structures (Anderoglu et al., 2008; Yu et al., 2013) and interconnection materials in the integrated circuit industry (Cheng et al., 2017; Zheng et al., 2021). Clearly, many research options for focusing on nanotwinned hierarchical materials are available for the near future. Advanced technological fields require materials that can meet the relevant field-specific requirements. Therefore, future research on nanotwinned structures will not be restricted to one area of application. Instead, their potential will expand into new areas of work.