**3. A basic understanding of microstructure**

The morphology of materials can be defined through shape, size, and structure that plays an important role in both mechanical and corrosion resistance properties. It is well known that all materials are composed of atoms that are arranged in short/long-range order with regular/irregular patterns, those solids are familiar as crystalline and non-crystalline, respectively. The crystalline metals with different crystal structures, such as body-centered cubic, face-centered cubic, or hexagonally closed packed, are prorated into the single crystal and polycrystalline categories. Conversely, most polycrystalline metals are composed of a collection of many small single crystals named grains and are similar to pomegranate fruit, which is made up of many small seeds (see **Figure 3a**). The grains are separated from each other by grain boundaries while preserving the integrity of the metal. Similarly **Figure 3b** shows one grain (shown by yellow dotted line) that has a subgroup of several laths, and every lath having several crystal atoms.

#### **Figure 3.**

*The photographs of (a) pomegranate fruit, which compound of grains and separated by grain boundary like metal structure, (b) high strength steel structure consists of bainitic ferrite and martensite [27].*
