Can twins enhance the elastic stiffness of face-centered-cubic metals?

It has usually been reported that the elastic stiffness of polycrystals is lower than that of the corresponding monocrystals. Recent experimental results made by Tanigaki et al. (2013) indicate that twin boundaries can improve the elastic stiffness of synthesized nano-polycrystalline diamonds. These researches imply that it may be a universal law for the twin boundary enhancing elastic stiffness. To verify this hypothesis, the elastic properties of ten face-centered-cubic (FCC) structure metals' perfect crystals and twin crystals have been studied by using first-principles calculations. Our research findings indicated that twins cannot always enhance the elastic stiffness of FCC structure metals. These results further clarify the fact that twin boundary enhancing elastic stiffness is not a universal law.