**1. Introduction**

A grain boundary (GB) in the homophase polycrystalline metal is an interface between two crystals of the same crystal structure [1, 2]. According to the dimensional scale of defects in polycrystalline materials, the GB is classified as a planar defect. GBs play an important role in the mechanical properties of polycrystalline metals [3, 4]. For bulk polycrystalline metals, the GB is considered to be stable during the plastic deformation processes and acts as a sink, source or obstacle to the dislocations [1]. With the decreasing microstructural length scale of polycrystalline materials, the volume fraction of atoms residing in or near the grain boundaries

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

increases. Experimental results indicate that GB width (*δ*) is approximately 0.5 nm for facecentered cubic (fcc) NC alloys and slightly larger than 1.0 nm for body-centered cubic (bcc) NC alloys [3, 5, 6]. Assuming the grains have the shape of spheres, the volume fractions of intercrystal regions and GB as a function of grain size (*d*) are shown in **Figure 1**. It can be seen from **Figure 1** that for nanostructured materials with grain size of 5 nm, nearly 50% of atoms will reside in or near the GB [7]. Therefore, nanostructured materials can be considered to compose of two parts: the core crystallites and a network of intercrystal regions (grain boundaries, triple junctions, etc.) [8]. In NC materials, grain boundary mediated processes, such as emission and absorption of dislocations by grain boundaries, grain rotation, and GB sliding will dominate the plastic deformation as the grain size is smaller than a certain critical value [9]. The properties of the nanostructure materials are thus determined not only by their reduced microstructural length scale, but also by the nature of their GB structures [10, 11]. Due to high volume fraction of the GB in NC materials, thermodynamic driving force exists to drive GB migration which results in the low stability of NC materials. The main objective of this chapter is to provide a comprehensive review of the experimental and simulation results on the microstructure instability of NC materials system under various deformation conditions.

This chapter is structured as follows: Section 2 describes the basic methods established in the field of GB structure to describe the GB structure. The section that follows addresses the GB

**Figure 1.** The contribution of different microstructural elements to the volume fraction as a function of grain size (*d*), assuming a grain-boundary thickness (*δ*) of 1 nm.

stability phenomena in NC materials, the GB effect on the monotonic and cyclic deformation processes of NC materials. Finally, the application method to enhance the microstructural stability of NC and potential investigations in the future are discussed.
