On the role of hierarchical twins for achieving maximum yield strength in nanotwinned metals

The dependence of the strength of polycrystalline metals with hierarchical nanotwins on twin spacing and grain size is theoretically studied with a dislocation-based theoretical model, which describes the twin spacing-dependent flow stress. The analysis indicates that the flow stress first decreases with the decrease of twin spacing in primary twin lamellae, and then rises up to an ultimate value, while it is reinforced directly when reducing the size of the secondary twin lamellae. It is found that there exist optimal twin spacings in primary and secondary twin lamellae, as the function of the grain size, which yields a maximal strength in this hierarchical nanostructure. The present results provide a new perspective to design the microstructures for reinforcing the mechanical properties in nanostructural materials. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747333]