When the crystal structures of the two contiguous phases are similar and the lattice parameters are nearly equal, the boundary between the two crystals is called a coher­ent interface with one-to-one correspondence of atoms at the interface, as illustrated in Figure 2.40a. The surface energy of the coherent interface is 0.01-0.05 J m~2. When the phase partially loses the coherence with the matrix, the interface is called a semicoherent interface (Figure 2.40b). When there is no such similarity or no matching exists, the interface is called incoherent, which is quite similar in structure and energy to high-angle grain boundaries. An incoherent interface is shown in Figure 2.40c.

2.2.4

Volume Defects

Volume defects are three-dimensional in nature, and include precipitates, disper — soids, inclusions, voids, bubbles, and pores that can occur in materials under dif­ferent environmental conditions or processing conditions. They do have various important effects on the properties of materials. A TEM picture is shown in Figure 2.41 showing various types of precipitates present in a 2024 Al alloy. We will discuss these vital roles in Chapters 4 and 7 in detail.

2.2.5