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Contents
Chapter One
Fundamentals of Engineering Materials
1.1. Introduction Materials are substance of which something is composed or made. Since civilization began, materials along with energy have been used by people to improve their standard living. Materials are everywhere about us since products are made of materials. Some of the commonly encountered materials are wood, concrete, steel, glass, rubber, iron etc. the production and processing of materials into finished products constitute a large part of our present economy. Engineers design most manufactured products and the processing systems required for their production. Since products are requiring materials, engineers should be knowledgeable about the internal structure and properties of materials so that they will able to select the most suitable ones for each application and should be able to develop the best processing methods.
1.2. Material science and engineering Material sciences is primarily concerned with the search of for basic knowledge about the internal structure, properties, and processing of materials. Whereas, Engineering material is concerned the use of the fundamental and applied knowledge of materials so that materials can be converted into products necessary or desired by society.
Figure 1.1. Materials knowledge Spectrum
Material sciences is at the basic knowledge end of the materials knowledge spectrum and engineering materials is at applied knowledge end, and there is no clear demarcation between then (Fig 1.1).
Applied Knowledge of materials
Material Sciences Engineering Materials
Materials Science and Engineering
Basic Knowledge
Resultant knowledge of structure, properties, processing, and performance of engineering materials
Figure 1.1. Shows three-ringed diagrams which indicates the relationship among basic sciences (and mathematics), material sciences, and engineering, and the other engineering disciplines. The basic sciences are located within the first ring or core of the diagrams, while the various engineering disciplines (Mechanical, electrical, civil, chemical, etc) are located in the out most third ring. The applied sciences, metallurgy, ceramics, and polymer sciences are located in the middle or the second ring. Material sciences and engineering is shown to form a bridge of material knowledge from the basic science to engineering disciplines.
1.3. Engineering materials classification For Convenience engineering materials are divided into three main classes: Metallic, polymeric (Plastics), and ceramics materials. In addation of three main classes of material, we shall consider two more types, composite materials and electronic materials because of recently fastly developed and great engineering materials.
Figure 1.2. Diagram representation of engineering material classification
Composite
Composite
Composite
Metals
Organic
Polymers
Ceramics
and Glass
conductors of electricity. Some of these materials are good insulators and are used for electrical insulative applications. In general, polymeric materials have low density and relatively low softening decomposition temperature.
1.3.3. Ceramic materials
Ceramic materials are inorganic materials which consist of metallic and non-metallic elements chemically bonded together. Ceramic materials can be crystalline, non- crystalline, or mixture of both. Most ceramic materials high hardness and high- temperature strength but tend to have mechanically brittleness. Advantage of ceramic materials are light weight, high strength, and hardness, good heat and wear resistance, reduced friction, insulative properties.
The insulative property along with the high heat and wear resistance of many ceramics make them useful for furnaces lining for high temperature of liquid metals such as steel.
1.3.4. Composite Materials
Composite materials are a mixture of two or more materials. Most composite materials consist of a selected filler or reinforced material and a compatible risen binder to obtain the specific characteristics and properties desired. Usually, the components do not dissolve in each other and can be physically identified by an interface between the components. Composites can be of many types. Some of the predominant types are fibrous (composed of fibers in a matrix) and particulate (composed of particle in a matrix. It can be used widely for transport equipments due to light weight and excellent corrosion resistant.
1.3.5. Electronic Materials
Electronic materials are not major type of materials by volume but are an extremely important type of materials for advanced engineering technology. The most important electronic materials is pure silicon which is modified in various ways to change its electrical characteristics.
1.3. 6. Smart Materials
Chapter Two
Atomic Structure and Interatomic Bonding
2.1. Atomic structure Atom is the basic and smallest unite of an element that can undergo chemical change. They are the basis for everything in the Universe. You should start by remembering that matter is composed of atoms. We're going to cover basics like atomic structure and bonding between atoms. Are there pieces of matter that are smaller than atoms? Sure there are. You'll soon be learning that atoms are composed of pieces like neutrons, electrons, and protons. The parts of atoms are the electrons, protons, and neutrons. What are electrons, protons, and neutrons?. There are three pieces to an atom. There are electrons, protons, and neutrons. That's all you have to remember. Three things! As you know, there are over 100 elements in the periodic table. The thing that makes each of those elements different is the number of electrons, protons, and neutrons. The protons and neutrons are always in the center of the atom. Scientists call the center of the atom the nucleus. The electrons are always found whizzing around the center in areas called orbital. You can also see that each piece has either a "+", "-", or a "0." That symbol refers to the charge of the particle. You know when you get a shock from a socket, static electricity, or lightning? Those are all different types of electric charges. There are even charges in tiny particles of matter like atoms. The electron always has a "-" or negative charge. The proton always has a "+" or positive charge. If the charge of an entire atom is "0", that means there are equal numbers of positive and negative pieces, equal numbers of electrons and protons. The third particle is the neutron. It has a neutral charge (a charge of zero).
will have one more electron than protons and become a negative ion with a - charge. When sodium and chlorine atoms are placed together, there is a transfer of electrons from the sodium to the chlorine atoms , resulting in a strong electrostatic attraction between the positive sodium ions and the negative chlorine ions. This explains the strong attraction between paired ions typical of the gas or liquid state. Ionic bond is non-direction bond because of ions attracted by cations in any direction.
Figure 4a. Formation of ionic bond in NaCl. Figure 4b. Na+ and Cl- ions formed by ionic bonding mechanism. Covalent Bonds : Some atoms like to share electrons to complete their outer shells.
Each pair of shared atoms is called a covalent bond. Covalent bonds are called directional because the atoms tend to remain in fixed positions with respect to each other. Covalent bonds are also very strong. Examples include diamond, and the O- O and N-N bonds in oxygen and nitrogen gases.
Metallic Bonds In metals, the metal atoms lose their outer electrons to form metal cations. The electrons from all the metal atoms form a "sea" of electrons that can flow around these metal cations. These electrons are often described as delocalized electrons - delocalized means "not fixed in one place" or "free to move".
Figure 1. Metallic bond of Siliver
AS THE METAL CATIONS AND THE ELECTRONS ARE OPPOSITELY CHARGED, THEY
WILL BE ATTRACTED TO EACH OTHER, AND ALSO TO OTHER METAL CATIONS.
THESE ELECTROSTATIC FORCES ARE CALLED METALLIC BONDS, AND THESE ARE
WHAT HOLD THE PARTICLES TOGETHER IN METALS.
The lack of oppositely charged ions in the metallic structure and lack of sufficient valence electrons to form a true covalent bond necessitate the sharing of valence electrons by more than two atoms. Each of the atoms of the metal contributes its valence electrons to the formation of the negative “electron cloud". These electrons are not associated with a particular ion but are free to move among the positive metallic ions in definite energy levels. The metallic ions are held together by virtue of their mutual attraction for the negative electron cloud. A result of the sharing of electrons is the cations arrange themselves in a regular pattern. This regular pattern of atoms is the crystalline structure of metals. In the crystal lattice, atoms are packed closely together to maximize the strength of the bonds. An actual piece of metal consists of many tiny crystals called grains that touch at grain boundaries. The metallic bond yields three physical characteristics typical of solid metals: Metals are good conductors of electricity. Metals are good conductors of heat.
Metals have a lustrous appearance. In addition, most metals are malleable, ductile, and generally denser than other elemental substance.
Chapter Three
Crystal structure of Metals
3.1. Introduction 3.2. Crystal structure Crystal: a solid composed of atoms, ions, or molecules arranged in a pattern that is repeated in three dimensions. The unit cell: is a structural unit or building block that can describe the crystal structure. Repetition of the unit cell generates the entire crystal. Crystalline structure : a regular three dimensional patterns of atoms or ions in space. Space lattice : a three dimensional array of points each of which has identical surroundings. Lattice point : one point in an array in which all the points of each has identical surroundings. When discussing crystal structure, it is usually assumed that the space lattice continues to infinity in all directions. The intersections of the lines, called lattice points , represent locations in space with the same kind of atom or group of atoms of identical composition, arrangement, and orientation. The geometry of a space lattice is completely specified by the lattice constants a , b , and c and the interaxial angles ơ, Ƣ, and ƣ. The unit cell of a crystal is the smallest pattern of arrangement that can be contained in a parallelepiped, the edges of which form a, b , and c axes of the crystal.
All crystal systems can be grouped into one of seven basic systems, which can be arranged in 14 different ways, called Brava’s. It should be noted that the unit cell edge lengths and axial angles are unique for each crystalline substance. The unique edge lengths are called lattice parameters_._ Axial angles other than 90° or 120° can
also change slightly with changes in composition. When the edges of the unit cell are not equal in all three directions, all unequal lengths must be stated to completely define the crystal. The same is true if all axial angles are not equal.
3.3. Major Metallic Crystalline Structures Almost all structural metals crystallize into one of three crystalline patterns: face
centered cubic, hexagonal close-packed or body-centered cubic (Fig.1.7.1)
Primary Metallic Crystalline Structures (BCC, FCC, HCP) As pointed out on the previous page, there are 14 different types of crystal unit cell structures or lattices are found in nature. However most metals and many other solids have unit cell structures described as body center cubic (BCC), face centered cubic (FCC) or Hexagonal Close Packed (HCP). Since these structures are most common, they will be discussed in more detail.
The BCC arrangement does not allow the atoms to pack together as closely as the FCC or HCP arrangements. The BCC structure is often the high temperature form of metals that are close-packed at lower temperatures. The volume of atoms in a cell per the total volume of a cell is called the packing factor. The BCC unit cell has a packing factor of 0.68. Some of the materials that have a BCC structure include lithium, sodium, potassium, chromium, barium, vanadium, alpha-iron and tungsten. Metals which
have a BCC structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the BCC structure. It should be noted that there are other important mechanisms for hardening materials, such as introducing impurities or defects which make slipping more difficult.
3.3.2. Face Centered Cubic (FCC) Structure
The face centered cubic structure has atoms located at each of the corners and the centers of all the cubic faces (left image below). Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. Additionally, each of its six face centered atoms is shared with an adjacent atom. Since 12 of its atoms are shared, it is said to have a coordination number of 12. The FCC unit cell consists of a net total of four atoms; eight eighths from corners atoms and six halves of the face atoms as shown in the middle image below. The image below highlights a unit cell in a larger section of the lattice.
In the FCC structure (and the HCP structure) the atoms can pack closer together than they can in the BCC structure. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer. To picture packing arrangement, imagine a box filled with a layer of balls that are aligned in columns and rows. When a few additional balls are tossed in the box, they will not balance directly on top of the balls in the first layer but instead will come to rest in the pocket created between four balls of the bottom layer. As more balls are added they will pack together to fill up all the pockets. The packing factor (the volume of atoms in a cell per the total volume of a cell) is 0.74 for FCC crystals. Some of the metals that have the FCC structure include aluminum, copper, gold, iridium, lead, nickel, platinum and silver.
