CRYSTAL STRUCTURES

CRYSTAL STRUCTURES
• Have you ever wondered how atoms assemble into solid structures? • How does the density of a material depend on its structures?

CRYSTAL STRUCTURES
• Solid materials can broadly be classified as crystalline and non crystalline (amorphous) solids. • In crystalline solid the arrangement of atoms is in a periodically repeating manner whereas no such patterns are found in a non-crystalline solid.

CRYSTAL STRUCTURES
• 2 types of crystalline solids: a) Single crystal : the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption. b) Polycrystalline solid : a collective aggregate of many crystals separated by well defined boundaries.

CRYSTAL STRUCTURES
• As a general rule, most metals are crystalline, while ceramics and polymers may be either crystalline or non-crystalline.

Differences between crystalline and non-crystalline solids
Characteristic
Atomic arrangements

Crystalline
Regular and orderly manner in all three dimensions

Non-crystalline
Irregular

Fracture mechanism Ductile manner. Solids Brittle manner. behave elastically up to Solids do not their yield points behave elastically

Tensile strength
Dislocation defects

High
Possible

Low
Not possible

Unit cell
• A crystalline solid consists of a number of crystals. A crystal structure can be considered as consisting of tiny blocks which are repeated in three dimensional pattern. • Each of the tiny block (called a unit cell) is actually made from the arrangement of a small group of atoms.

Crystal Structure
• Types of crystal structure: 1) Simple cubic 2) Body centered cubic (BCC) 3) Face centered cubic (FCC) 4) Hexagonal closed packed (HCP)

Simple Cubic

BODY-CENTERED CUBIC CRYSTAL STRUCTURE

FIGURE 3.2 For the body-centered cubic crystal structure, (a) a hard sphere unit cell representation, (b) a reduced-sphere unit cell, and (c) an aggregate of many atoms. (Figure (c) from W. G. Moffatt, G. W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. I, Structure, p. 51. Copyright  1964 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) FIGURE

FACE-CENTERED CUBIC CRYSTAL STRUCTURE

Hexagonal Closed Pack (HCP)

Recystallization terms
a) Crystal : may be defined as a small body having a regular symmetry form, bound by smooth surfaces which are acquired under the action of its inter-atomic forces. b) Space lattice: is composed of unit cells where the unit cells are stacked together endlessly to form the lattice. Each cell in the lattice is identical in size, shape and orientation with each other in the same crystal. c) Grain : An individual crystal in a polycrystalline metal. d) Grain Boundary: the boundary separating the two adjacent grains.

Interatomic Bonding
• Have you ever wondered why some materials behave differently from others, for example it is easy to stretch rubber but it is difficult to stretch metals? • Why metals are good electrical conductors while other non-metallic materials are poor conductors? • This is all due to the bonding of atoms.

Interatomic Bonding
• The bonds are developed between atoms due to forces of attraction and repulsion which keep nearby atoms in an equilibrium state. • The equilibrium means to have its electron configuration similar to that of inert gases.

Interatomic Bonding
• If the outermost shell is not complete with 8 electrons, atoms of most of the elements form bonds with one another to achieve this stable condition of 8 electrons at the outermost shell. • This can be achieved: i) Atoms sharing one or more electrons with other atoms ii) Atoms gaining one or more electrons with another atom iii) Atoms losing one or more electrons with another atom

Primary Bonds
• 3 types of primary bonds: a) Ionic bonding b) Covalent bonding c) Metallic bonding

Ionic bonding
• It is always found in compounds that are composed of both metallic and nonmetallic elements. • Atoms of a metallic element easily give up their valence electrons to the nonmetallic atoms. • The attractive bonding forces are coulombic; that is, positive and negative ions, by virtue of their net electrical charge, attract one another.

Ionic bonding
• Ionic bonding is termed nondirectional, that is, the magnitude of the bond is equal in all directions around an ion.

Covalent Bonding
• In covalent bonding stable electron configurations are assumed by the sharing of electrons between adjacent atoms. • Two atoms that are covalently bonded will each contribute at least one electron to the bond, and the shared electrons may be to belong to both atoms.

Covalent Bonding
• Covalent bonding is schematically illustrated in Figure 2.10 for a molecule of methane (CH4). The carbon atom has four valence electrons, whereas each of the four hydrogen atoms has a single valence electron.

Metallic bonding
• Metallic bonding, the final primary bonding type, is found in metals and their alloys. • Metallic materials have one, two, or at most, three valence electrons. • With this model, these valence electrons are not bound to any particular atom in the solid and are more or less free to drift throughout the entire metal. • They may be thought of as belonging to the metal as a whole, or forming a ‘‘sea of electrons’’ or an ‘‘electron cloud.’’ • The remaining nonvalence electrons and atomic nuclei form what are called ion cores, which possess a net positive charge equal in magnitude to the total valence electron charge per atom.

Metallic bonding
• These free electrons act as a ‘‘glue’’ to hold the ion cores together.

SOLIDIFICATION
• Solidification is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. • The melting point of a solid is the temperature at which it changes state from solid to liquid. • When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point or crystallization point.

SOLIDIFICATION
• Various stages in the solidification of a polycrystalline specimen are represented schematically in Figure 3.33.

Cont.
1) Initially, small crystals or nuclei form at various positions. These have random crystallographic orientations, as indicated by the square grids. 2) As the nucleus grows, the spherical morphology becomes unstable and its shape becomes perturbed. The solid shape begins to express the preferred growth directions of the crystal. 3) The small grains grow by the successive addition from the surrounding liquid of atoms to the structure of each. 4) The solid then attempts to minimize the area of those surfaces with the highest surface energy. The dendrite thus exhibits a sharper and sharper tip as it grows.

Cont.

5) As solidification proceeds, an increasing number of atoms lose their kinetic energy, making the process exothermic. 6)The extremities adjacent grains impinge on one another as the solidification process approaches completion.

Cont.

7) As indicated in Figure 3.33, the crystallographic orientation varies from grain to grain. Also, there exists some atomic mismatch within the region where two grains meet; this area, called a grain boundary.

Cont.

Solidification phases term
1) Nucleation: The initial stage in a phase transformation. 2) Dendrite : is a characteristic tree-like structure of crystals growing as molten metal freezes. 3) Grain : An individual crystal in polycrystalline metal.

Figure showing dendrite structure

Dendritic growth

Question
• 1) List the significant differences between ionic, covalent and metallic bonds • 2) List the common crystal structures in metallic solids. • 3) What is a unit cell?