Alternative Approaches to Measure a Dislocation Density

A dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials. The theory was originally developed by Vito Volterra in 1905, but the term 'dislocation' was not coined until later by the Professor Sir Frederick Charles Frank of the Physics Department at the University of Bristol. In 1934, Egon Orowan, Michael Polanyi and G. I. Taylor, roughly simultaneously, realized that plastic deformation could be explained in terms of the theory of dislocations. Dislocations can move if the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge. In effect, a half plane of atoms is moved in response to shear stress by breaking and reforming a line of bonds, one (or a few) at a time. The energy required to break a single bond is far less than that required to break all the bonds on an entire plane of atoms at once. Even this simple model of the force required to move a dislocation shows that plasticity is possible at much lower stresses than in a perfect crystal. In many materials, particularly ductile materials, dislocations are the "carrier" of plastic deformation, and the energy required to move them is less than the energy required to fracture the material. Dislocations give rise to the characteristic malleability of metals [2]. It is important to study dislocations to explain different phenomena, which cannot be explained in another way. For example, dislocations are responsible for hardening effect: In FCC metals the yield stress and the strain hardening are governed by the forest interactions, i.e., the interactions with the dislocations intersecting the glide planes of moving dislocations. Dislocations also affect the electrical and optical properties of polycrystals. The studies of dislocations are mostly focused on their nucleation, geometry, on their role in plastic deformation and on their kinetics. In this paper I am aimed to investigate different ways of measuring dislocation density. In particular, I am interested to analyze the recently developed method of measuring dislocation density using an ultrasound [1]. Most of the experimental dislocation observation methods are not based on the identification of the dislocation line itself, which is practically difficult to implement, but on the registration of stresses or distortions of the lattice due to dislocation. In essence, dislocations were first directly observed by transmission electron microscopy (TEM) by the group of Peter B. Hirsch at Oxford [3]. The main methods of observing dislocations today are: