Mechanics of Superplastic Deformations at Atomic Scale

Abstract

The behavior of materials can be modeled at different hierarchical levels with varying spatial scales as shown in Figure 1. A structure or a specimen with a scale greater than 10 -3 m represents the macroscopic scale where the principle of continuum mechanics is generally used. At this level, only materials macrostructure and global properties are considered; At the other extreme. atomic scale spans the lengths of a few nanometers (10 -9 - 10 -6 m), which is compatible with the length scale of crystalline defects (e.g. vacancy, impurity, dislocation, grain boundary and interface) Though at this level the structure cannot be directly related to the macro-level property, it provides very useful information for an understanding of mesoscopic (10 -6 - 10 -3 m) behavior. In order to understand the origin of superplasticity, it is important to understand the mechanics of grain boundary sliding, considere the primary source of the large strain. An appropriate scale to study grain boundary sliding is the atomic scale and is the focus of this paper. In the atomistic simulation. interatomic potentials using Embedded Atom Method (EAM) are used in conj unet ou wit h molecular statics calculations. Atomistic simulations are performed on a series of grain boundary structures in aluminum, and the energies associated with each of their equilibrium configurations are computed. The propensity for grain boundary sliding GBS) is also evaluated by computing the energy associated with incremental equilibrium configurations during the sliding process, and GBS is compared with GB cleavage based on the energy consideration. It is also shown that in certain types of grain boundaries, GBS is always accompanied by GB migration. Also the amount of sliding and migration is proportional to the applied force levels, grain boundary energy and time.