Defects and boundaries are often created intentionally within materials to provide extra strength. However, this process comes at a price. Although the material is now stronger, it is also more brittle and its ability to stretch and deform is drastically reduced. Researchers from the Massachusetts Institute of Technology in the US and the Chinese Academy of Sciences in China have devised strategies to overcome this loss of ductility and the answer comes in the form of nanoscale twin boundaries (TBs) [Lu, et al., Science (2009) 324, 349].
The team has identified three structural characteristics of boundaries that are essential for improving strength and ductility, namely TBs that are coherent with their surrounding matrix, are thermally and mechanically stable, and have feature sizes less than 100 nm.
The problem with traditional methods used to create boundaries at crystallographic planes or in atomic vacancies is that a mismatch is created between two regions such that the arrangements of atoms do not mirror each other on each side of the boundary. This is why the material becomes brittle. However, nanoscale TBs can be engineered to ensure coherent internal interfaces. These TBs have high thermal and mechanical stability and act as slip planes at which internal stress is released. Introducing nanoscale TBs into pure Cu increases the metal's mechanical strength by an order of magnitude but only marginally affects its conductivity. If the TB is 15 nm thick, it gives 14% elongation to failure which can be reduced further with finer TBs.
Making coherent nanoscale TBs is of course, a technical challenge. They can be fabricated through physical and chemical processes such as pulsed electrodeposition, sputter deposition, phase deformation, phase transformation, and recrystallization.
Electrodeposition can create a high density of nanoscale TBs, up to 100 nm in thickness, which nucleate at the material's grain boundaries, decreasing the total interfacial energy through orientation differences. Their formation is kinetically driven and can be engineered by changing deposition conditions. On the other hand, nanoscale TBs can be fabricated at a high deposition rate by sputter deposition. In this case, thin films can be grown that have coherent TBs parallel to the surface.
Whereas both these deposition methods are ideal for creating thin foils, plastic deformation is a process that is better adapted to bulk metals and alloys as it gives rise to very thin TBs inside materials that have low stacking fault energies, such as steels.