Structure and migration of (112) step on (111) twin boundaries in nanocrystalline copper

Abstract

Pulse-electroplated copper that contains a high density of {111}/⟨112⟩ nanotwins has been found to greatly improve the yield strength while maintaining good electrical conductivity. The thermal stability of nanotwins is a concern and has been studied by in situ transmission electron microscopy (TEM) characterization from 200 to 350 °C in the present work. It was found that the (112¯) twin boundary in a junction of (111)/(112¯)/(111) twins migrates to eliminate the (111) twin boundaries. We propose that it is the dominant mechanism that reduces the twin density in the range of temperature studied. The driving force is provided by the elimination of the two (111) boundaries. The inverse migration of the (112¯) twin boundary driven by a high strain is possible if enough stress has been applied to the copper, e.g., the strain introduced during pulsed electroplating. On the other hand, the migration of (111) twin boundary in the direction normal to the twin plane was not found. However, we propose that it can happen if a (112¯) step migrates on the (111) surface, provided that there exists a driving force. The structure and mobility of (112) twin boundary has been characterized by high resolution TEM. We observed that the (112) step on (111) twin plane has a height of three atomic layers. It is a unique structure unit of the (112) twin.