Microstructure of Grain boundaries and Interphase boundaries in advanced crystalline materials
Many of the materials around us are polycrystalline and multi-phase materials composed of many crystals, “grains” and “phases”. Grain boundaries and interphase boundaries are interfaces between two grains of different orientations and two different phases, respectively. The grain and interphase boundaries play an important role to the mechanical and functional properties of many materials. Thus it is important to investigate the micro and atomic structure of grain boundaries and interphase boundaries. One example of the importance of the grain boundary is ultrafine grained (UFG) materials fabricated by severe plastic deformation (SPD). The yield stress of coarse grained materials increase with decreasing of their grain size. This phenomena is so-called the “Hall-Petch relationship”. In this relationship, the moving dislocations are pinned at the grain boundaries. However, the ductility is decreased with increasing of the strength. On the other hand, the UFG materials by SPD exhibit high strength and good ductility. In the UFG materials, there are much more grain boundaries than coarse grained materials as shown in Figure 1, and some researchers have suggested that grain boundaries in UFG materials must show different behavior compared with coarse grained materials. We have recently observed many grain boundaries in UFG materials [1], and investigated the mechanism of grain refinement and the relationship between grain boundaries and their mechanical properties. Based on the obtained results about the grain boundary atomic structure in UFG materials as well as grain boundary segregation and so on, we aim at establishing the concept of “grain boundary engineering” and “grain boundary science”.
[1] S. Ii et al., Mater. Sci. Forum. 584-586(2008), 716.
Fig. Volume fraction of Grain boundary in polycrystalline materials with various grain sizes.