Kinetics and energetics of room-temperature microstructure in nanocrystalline Cu films: The grain-size dependent intragrain strain energy

Abstract

The fast kinetics of the low-temperature microstructure evolution in nanocrystalline metals requires an additional driving force from the excess intragrain energy in addition to the driving forces from the grain boundary energy, surface or interface energy, and thermal strain energy. If the excess volume of the grain boundary induces lattice distortions in grain interiors, the intragrain energy is the elastic-strain energy and can be determined from a grain-size-dependent strain model. Considering the available intragrain strain energy, we use transmission electron microscopy, x-ray diffraction line-broadening analysis, and theoretical models to investigate the kinetics and energetics of room-temperature nanostructure relaxation and abnormal grain growth in electroplated nanocrystalline Cu films devoid of thermal strains and high-density dislocations. The experimental data of grain sizes and microstrains are consistent with the theoretical size-dependent strain model. The limited nanostructure relaxation of Cu occurs with the grain boundary width reduction and intragrain strain release, which cannot alter the structural anisotropy and intrinsic high-energy state of nanograins. Based on quantitative descriptions of the variations in grain size, microstrain, and transformed fraction during abnormal grain growth, the possible driving forces and grain boundary mobility were systematically evaluated. The results indicate that the size-dependent intragrain strain energy provides a crucial driving force for rapid nanograin growth and texture transition, whereas the low nanograin boundary mobility in Cu films is probably correlated with the strained-lattice migration and faceted-boundary migration.