Grain size dependence of dissipation mechanisms in nanocrystalline aluminum using molecular dynamics simulations

The effect of grain size (GS) on modulus and dissipation determines its impact on damping. This study uncovers the phenomenon of dissipation valleys in nanocrystalline metals through molecular dynamics simulations. The dynamic analysis of the microstructure reveals that the primary dissipation mechanism in the aluminum matrix shifts from being dominated by grain boundary sliding only to being jointly dominated by grain boundary sliding and grain diffusion. Specifically, at smaller GSs (e.g., GS ​= ​10 ​nm), dissipation is primarily due to shear and thermal motion of atoms at grain boundaries, whereas larger GSs (e.g., GS ​> ​10 ​nm) lead to an increase in diffusion of atoms from grain boundaries to the interior of grains. The competition between these two dissipation mechanisms results in the appearance of dissipation valleys. Subsequently, the effect of GS on damping and loss modulus of nanograined aluminum is further validated and precisely characterized by combining spectrum analysis and the use of a Maxwell viscoelastic model that takes into account both GB sliding and intragranular diffusion. This study establishes a mapping relationship between GS, microstructural deformation mechanisms, and dynamic energy dissipation, providing guidance for the design of nanocrystalline metals.