Atomistic visualization of deformation in gold

Atomistic visualization of deformation in gold

A mechanical force acting on a solid causes deformation and fracture. These two processes are closely related to several fields of technology and have been studied for a long time by scientists and engineers. In particular, the elucidation of the deformation process and its mechanism has been a fundamental subject in solid-state physics and metallurgy. Various types of deformation mechanisms have been proposed from mechanical tests and structure analyses, for example, dislocation mechanisms,1–3 twinning,4–6 and grain boundary sliding.7,8 All of these mechanisms have been explained using atomistic models. According to these models, deformation proceeds through generation, multiplication, growth, and annihilation of line- and plane-type internal lattice defects, such as dislocations, stacking faults, and twins. Detailed studies have been performed for the elucidation of the relations between the deformation processes and the mechanisms. Several types of transmission electron microscopy ~TEM! have played a significant role in this elucidation. For example, the structures of dislocations were investigated by conventional static TEM ~Ref. 9! and it is known that their behaviors can be analyzed by conventional dynamic TEM using cinema photography.10,11 In particular, the
behaviors were directly observed by in situ deformation and conventional dynamic TEM.12,13 The electron-irradiated induced motions of twins in gold thin films14 and gold clusters,15 and surfaces and stacking faults in cadmium telluride16,17 were also observed at an atomic level by dynamic high-resolution TEM ~DHRTEM! using television camera and video tape recording systems. The deformation process, however, has not been directly observed in real space on an atomic scale and the elemental atomic processes in deformation have still not been clarified. A different type of microscopy is required in order to investigate the atomic processes: DHRTEM using a television camera and video tape recording system with a piezodriving specimen holder is expected to be the optimum method to analyze the atomic process of deformation.18,19 The purpose of the present study is to elucidate the atomic processes of mechanical deformation in gold by direct atomistic visualization by DHRTEM. EXPERIMENTAL PROCEDURES A piezodriving specimen holder for a transmission electron microscope was developed for subnanometer scale mechanical deformation and in situ observations. Figure 1 is an illustration of the specimen holder. The mobile side is connected with a pipe-type piezoelectric device for fine displacement and a microscrew motor for coarse displacement. The specimen on the mobile side is mounted on the tip of a lever connected with the piezodevice. The mobile side are displaced along the x direction from 0 to 1 mm by the motor. The fine displacement along the x direction is controlled by homogeneous elongation and shrinkage of the piezodevice. The fine displacements along the y and z directions are controlled by elongation and shrinkage on one side of the pipe. Resolution of the fine displacement by the piezodevice is less than 0.16 nm along the x direction and 0.22 nm along the y and z directions. Piezodriving methods are used for the displacement of scanning needles in several combination-type microscopes of reflection electron microscopy and scanning tunneling microscopy ~STM!, or TEM and STM.

PHYSICAL REVIEW B VOLUME 57, NUMBER 18 1 MAY 1998-II 0163