Their particles have a random formation and such as a result such substances

Their particles have a random formation and such as a

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Their particles have a random formation, and such as a result, such substances are said to be amorphous (without shape). Many important materials are noncrystalline: liquids and gases, for example. Water and air have noncrystalline structures. A metal loses its crystalline structure when it is melt. Such as glass, plastics and rubber are materials that fall into this category. While many important plastics are mixture of crystalline and noncrystalline forms. Two closely related features differentiate noncrystalline from crystalline materials: 1. Absence of long range order in the molecular structure of a noncrystalline. It can be visualized with reference to Fig. 1. They closely packed and repeating pattern of the crystal structure and random arrangement of atoms in the noncrystalline materials. Fig. 1. Difference in structure between (a) crystalline and (b) noncrystalline materials. 2. Differences in melting and thermal expansion characteristics. It could be demonstrated by a metal when it is melts. When the metal is molten an increase in volume compared to the material’s solid crystalline state. This effect is characteristic most materials when melted (a noble exception is ice; liquid water is denser than ice).
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~ Page 53 of 79 ~ EXERCISE 1. Assuming that within metal crystals the atoms can be considered as hard spheres which are in contact, indicate co-ordination number for each of the following structures: (a) Body-centred cubic structure (BCC); (b) Face-centred cubic structure (FCC); (c) Close packed hexagonal structure (CPH)/(HCP). 2. Draw neat diagrams of the following crystal structures: (a) Body-centred cubic structure (BCC); (b) Face-centred cubic structure (FCC); (c) Close packed hexagonal structure (CPH)/(HCP). 3. Calculate the number of atoms contained in each of the following crystal types: (a) Body-centred cubic structure (BCC); (b) Face-centred cubic structure (FCC); (c) Close packed hexagonal structure (CPH)/(HCP). 4. State the name given to non-crystalline solids. 5. With the aid of sketches, briefly explain the difference between unit cell and space lattice. 6. Name the three (3) most common crystal structures found in metals and, for each structure, name two metals which possess that structure at room temperature.
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~ Page 54 of 79 ~ SOLIDIFICATION OF METALS When metals are in liquid state there is no orderly arrangement of the atoms which are free to move about with respect to each other, thus in the liquid state, metals possess mobility. As the temperature of a molten metal falls a point is reached where the metal starts to solidify. At this point the atoms change from a disordered or amorphous state to an ordered or crystalline state. Like all pure crystalline substances, pure metals solidify at a fixed temperature as shown in Fig. 1 (a) below. However, under industrial conditions, such purity will not be obtained and crystal nucleus will form around an impurity particle such as a particle of slag.
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