Simulation of Transmission Electron Microscope Images of Dislocations Pinned by Obstacles

Abstract

From a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron microscope (TEM), the obstacle strength factor could be evaluated from pinning angles of dislocation cusps. We simulated this process: we produced a dislocation cusp by molecular dynamics simulation of interaction between an edge dislocation and a void or a hard precipitate in copper, and calculated the TEM image by multislice method. In two-beam conditions, cusp images showed inside-outside contrast depending on the sign of the diffracting vector and other variations with the specimen geometry. The pinning angles measured on TEM images ranged up to a few tens of degrees and were between the true angles for the two partial dislocations. Characteristics and contrast mechanisms of cusp images were discussed based on those of dislocation dipoles. (doi:10.2320/matertrans.MD201312) (Received September 3, 2013; Accepted October 2, 2013; Published November 15, 2013) Various crystal lattice defects induced by neutron irradi- ation, such as precipitates, voids, dislocation lines and loops are responsible for degradation in mechanical properties: increase in yield stress, loss of ductility, and increase in ductile-brittle transition temperature. A number of researches have been devoted for defect structural evolution and mechanical property changes under neutron irradiation for nuclear materials development. Understanding the mecha- nisms involved is necessary to construct models for estimating the lifetime of components of nuclear power plant. In an elementary process of dislocation-obstacle interac- tion, a gliding dislocation is pinned by obstacles and bows out to form arcs between the neighboring pinning points, which induces cusps on the dislocation at obstacles. The apex angle of the dislocation cusp is referred to as the pinning angle o. The dislocation breaks away by bypassing or cutting through the obstacle when the pinning angle reaches a critical value oc. Stronger obstacles have smaller critical angles. The obstacle strength factor i ¼ cosðoc=2Þ and the distance between the neighboring pinning points are the key parame- ters that relate the defect microstructure to the change in macroscopic mechanical properties. There have been re- ported a few attempts to evaluate the factor from a direct observation of dislocation-obstacle interaction utilizing in situ straining experiments in transmission electron micro- scope (TEM). 1­3) The method has not been widely applied so far, due to high technical levels required for in situ experiments. Another difficulty comes from TEM images with a limited resolution both in time and space for measuring pinning angles of radiation-induced obstacles, typically less than a few nanometers in size, at a moment of the breakaway. Alternatively, molecular dynamics (MD) simulations have been applied extensively for the dislocation- obstacle interaction. 4­7) In the present study, we performed TEM image simulation of dislocation cusps to examine whether TEM images reveal the cusp structure in the scale suitable for evaluating the obstacle strength factor. For this purpose, we stopped MD simulation of interaction between an obstacle and an edge dislocation just before the breakaway. We then calculated TEM images of dislocation cusps under various conditions using the multislice method. We compared apex angles between the cusp structure and on the calculated images, which would support the experimental evaluation of the obstacle strength factor.