Material Properties Lecture Notes PDF

Material Properties Dr. Orhan S. Abdullah Since the earliest days of evolution of mankind, the main special features between human and the other mammals has been the ability to use and develop materials to satisfy our human requirements. Nowadays we use many t...

Material Properties Dr. Orhan S. Abdullah Since the earliest days of evolution of mankind, the main special features between human and the other mammals has been the ability to use and develop materials to satisfy our human requirements. Nowadays we use many types of materials, fashioned in many different ways, to satisfy our requirement for housing, heating, clothes, transportations, medical care and defense. In the recent years studying the metallurgy science gave to humanity an ever growing range of useful alloys and made them more understanding of the materials resources and nature enable the engineers to select the most appropriate materials and to use them with greatest efficiency in minimum quantities whilst causing minimum pollution in their extraction, refinement and manufacture. Selection of materials: Let's now start by looking at the basic requirements for selecting materials that are suitable for a particular application. For example figure1 shows electrical cable. Plastic is used for the outer casing it is a good electrical insulator and prevents electric shock if a person touches it. As well as being a good insulator the plastic is cheap, tough and easily moulded to shape. It has been selected for the casing of these properties. The cable core was made from brass; this metal has been chosen because of its special properties. These properties are good electrical conductivity, ease of extruding to shape, ease of machining, adequate strength and 1 Material Properties Dr. Orhan S. Abdullah corrosion resistance. The precious metal silver is an even better conductor, but it would be far too expensive for this application and it would also be weak and soft. Figure 1: The electrical cable. Thus the reasons for selecting the materials in the above example can be summarized as: Commercial factors such as: Cost, availability, ease of manufacture. Engineering properties of materials such as: Electrical conductivity, strength, toughness, ease of forming by extrusion and corrosion resistance. Engineering materials: Almost every substance known to man has found its way to into the engineering workshop at some time or other. The most convenient way to study the properties and uses of engineering materials is to classify them into families as shown in the figure below. 2  Material Properties Dr. Orhan S. Abdullah Engineering materials Metals Non-metallic Composite Nano materials materials Ferrous metal Non-ferrous metal Cast iron Aluminum Glass Steel brass Wood Silver Ceramic Gold Rubber Diamonds Plastic Figure 2: Classification of engineering materials. 1. Metals: 1.1 ferrous metals:  These are metals and alloy containing a high proportion of the element iron.  They are the strongest materials available and are used for applications where high strength is required at relatively low cost and where weight is not for primary importance.  As an example of ferrous metals such as: bridge building, the structure of large building and the bodies and highly stressed engine parts of road vehicles. 3 Material Properties Dr. Orhan S. Abdullah 1.2 Non-ferrous metals:  These materials refer to the remaining metals known to mankind.  The pure materials are rarely used as structural materials as they lack mechanical strength.  They are used where their special properties such as corrosion resistance, electrical conductivity and thermal conductivity required.  There are mainly used with other metals to improve their strength. 2. Non-metallic materials: Such materials are so diverse that only a few can be listed here to give a basic introduction to some typical applications.  Wood: This is naturally occurring fibrous composite material used for the manufacture of casting patterns.  Rubber: This is used for hydraulic and compressed air hoses and oil seals. It is used widely for vehicle tyres when it is compounded with carbon black.  Glass: It is made by melting together silica, calcium carbonate and sodium carbonate. And it is used for electrical insulators, laboratory equipment and to reinforce plastics.  Ceramic: these are produced by baking naturally occurring clays at high temperatures after moulding to shape. They are used for high voltage insulators and high temperature resistant cutting tool.  Diamonds: these can be used for cutting tools for operation at high speeds for metals finishing where surface is greater importance. 4 Material Properties Dr. Orhan S. Abdullah 3. Composite materials: Many of the modern technologies require materials with unusual combinations of properties that cannot be met by the conventional metal, alloys, ceramics, and polymeric materials. This is especially true for materials that are needed for aerospace, underwater, and transportation applications. The composite material results from binding of two phases, the first is the matrix, and the second is the reinforcements phase. The distinguishing characteristics for composite materials result from the result of binding the two phases and organizing the distribution of reinforcement phases inside the matrix phases. The composite material can be classified according to the type of matrix into: 1. Metal matrix composite ( MMCS), 2. Ceramic matrix composite (CMCS), 3. Polymer matrix composite (PMCS), 4. Carbon – carbon composite. 4. Nano materials: A nanometer is one millionth of a millimeter approximately 100,000 times smaller than the diameter of a human hair. Nano materials are of interest because at this scale unique optical, magnetic, electrical, and other properties emerge. These emergent properties have the potential for great impacts in electronics, medicine, and other fields. 5 Material Properties Dr. Orhan S. Abdullah The Space Lattice and Unit Cells: Atoms or ions of a solid are arranged in a pattern that repeats itself in three dimensions; they form a solid that is said to have a crystal structure.  Atoms, arranged in repetitive 3-Dimensional pattern, in long range order give rise to crystal structure.  Properties of solids depend upon crystal structure and bonding force.  An imaginary network of lines, with atoms at intersection of lines, representing the arrangement of atoms is called space lattice.  Unit cell is that block of atoms which repeats itself to form space lattice. 6 Material Properties Dr. Orhan S. Abdullah There are several types of pattern in which metallic atoms can arrange themselves on solidification, but the most common is as follows:- 1. Body-Centered Cubic (BCC): As shown in figure 3 (a), as an example of the materials for this type: Chromium, Molybdenum, Niobium, Tungsten, and Iron. 2. Face-Centered Cubic (FCC): As shown in figure 3 (b), as an example of the materials for this type: Aluminum, Copper, Nickel, Iron, Gold, and Silver. 3. Hexagonal- closed Packed (HCP): As shown in figure 3 (c), as an example of the materials for this type: Beryllium, Cadmium, Magnesium, and Zinc. Figure 3: The three principle types of structural in which metallic elements crystallize: (a) Body-Centered-Cubic (b) Face-Centered-Cubic (c) Hexagonal-Closed-Packed. There are some metals that are undergo a change of structure at different temperatures. Iron metal for example is arranged in a body centered- cubic (BCC) at room temperature, when the metal is heated and reaches a temperature of 910˚C, the atoms rearrange themselves into Face-Centered-Cubic (FCC) crystals. If the metal is heated to the still higher temperature of 1400˚C the atoms again rearrange themselves, this time back into Body-Centered-Cubic form. 7 Material Properties Dr. Orhan S. Abdullah The unit cell is the smallest parallel surfaces of the crystalline structure that can be removed or repeated in different directions. It is also differ from each other in shape or size in the crystalline lattice from one material to another. The atoms that belong to the unit cell are called the basic atoms, its number is different from one shape of arrangement to another, this number can be found from the following equation:- N = NC + NI + NF Where N: is the number of the basic atoms in the unit cell. NC: is the number of the atoms in the corner. NI: is the number of the atoms inside the cube. NF: is the number of the atoms in the center of the face. Principal Metallic Crystal Structures:  90% of the metals have Body Centered Cubic (BCC), Face Centered Cubic (FCC) and Hexagonal Close Packed (HCP) crystal structure.  HCP is denser version of simple hexagonal crystal structure.  Most metals crystallize in these dense-packed structures because energy is released as the atoms come closer together and bond more tightly with each other.  The distance between the atoms (interatomic distance) in crystal structures can be determined experimentally by x-ray diffraction analysis. 8 Material Properties Dr. Orhan S. Abdullah Body Centered Cubic (BCC) Crystal Structure:  Represented as one atom at each corner of cube and one at the center of cube.  Each central atom has 8 nearest neighbors.  Therefore, coordination number is 8. The coordination number, CN = the number of closest neighbors to which an atom is bonded = number of touching atoms. Examples:- Chromium (a=0.289 nm) Iron (a=0.287 nm) Sodium (a=0.429 nm) , where a is the lattice constant.  Each of these cells has the equivalent of two atoms per unit cell.  One complete atom is located at the center of the unit cell.  An eighth of a sphere is located at each corner of the cell, making the equivalent of another atom.  Therefore each unit cell has (8x1/8) + 1 = 2 atoms. 9 Material Properties Dr. Orhan S. Abdullah Face Centered Cubic (FCC) Crystal Structure:  FCC structure is represented as one atom each at the corner of cube and at the center of each cube face.  The coordination number according to the spherical model it can recognize : 1- Four atoms in the front corners 2- Four atoms in the back face centers of the same cell. 3- Four atoms in front face centers of the next unit cell. Therefore, the coordination number CN= 12. Examples:- Aluminum (a = 0.405 nm) Gold (a = 0.408 nm).  Each unit cell has eight octants (8x1/8) atom at corners and six half –atoms at the center of six faces.  Therefore each unit cell has basic number =4 (8 x 1/8)+ (6 x ½) = 4 atoms. 10 Material Properties Dr. Orhan S. Abdullah Hexagonal Close-Packed Crystal Structure (HCP):  The HCP structure is represented as an atom at each of 12 corners of a hexagonal prism, 2 atoms at top and bottom face and 3 atoms in between top and bottom face.  APF = 0.74the same as that for the FCC crystal structure since in both structures the atoms are packed as tightly as possible.  In both the HCP and FCC crystal structures each atom is surrounded by 12 other atoms and thus both structures have a coordination number of 12.  Each atom has six 1/6 Atoms at each of top and bottom layer.  Two half atoms at top and bottom layer and 3 full atoms at the middle layer.  Therefore each HCP unit cell has (2 x 6 x 1/6) + (2 x ½) + 3 = 6 atoms.  The ratio of the height c of the hexagonal prism of the HCP crystal structure to its basal side a is called the c/a ratio.  The c/a ratio for an ideal HCP crystal structure consisting of uniform spheres packed as tightly together as possible is 1.633.  Cadmium and zinc have c/a ratios higher than ideality (elongated).  Magnesium, cobalt, zirconium, titanium, and beryllium have c/a ratios less than the ideal ratio (compressed). 11 Material Properties Dr. Orhan S. Abdullah The atomic packing factor (APF): 1- Body Centered Cubic (BCC): APF = APF = M2 = a2 +a2 thin M2 = 2a2 (4R)2 = M2 +a2 (4R)2 = 2a2 +a2 [(4R)2 = 3a2 ] (Square root) √ 4R = √ a thin sub in eq (1) √ APF = √ √ √ ( ) APF = √ APF = 𝝅 APF = 0.68 for B.C.C 12 Material Properties Dr. Orhan S. Abdullah 2- Face Centered Cubic (FCC): APF = APF = …..(1) (4R)2 = a2 +a2 [(4R)2 = 2a2 ] (Square root) √ 4R= √ a thin sub in eq. (1) √ √ √ ( ) APF = √ APF = 𝝅 APF = 0.74 for F.C.C 13 Material Properties Dr. Orhan S. Abdullah 3- Hexagonal Close-Packed (HCP): ( ) APF = Area = 1/2 * a * a sin 60 = 1/2 a2 sin 60 Area = 6 * area = 6* 1/2 a2 sin 60 = 3 a2 sin 60 Hexagonal volume = hexagonal area* hexagonal height (C) Where: C = 1.633 a Volume = 3 a2 sin 60 * 1.633 a = 4.24 a3 APF = sub a= 2R APF = APF = 0.74 for H.C.P 14 Material Properties Dr. Orhan S. Abdullah The table below show the coordination number CN, the basic number N, the relationship between Rand a, atom volume, cell volume and the atomic packing factor (APF): BCC FCC HCP Coordination 8 12 12 number CN basic number N 2 4 6 the relationship √ √ a= 2R between Rand a atom volume cell volume APF 0.68 0.74 0.74 EX1/ If the atomic radius for Pb = 0.175 nm, find the volume of the unit cell? Ans: Pb is F.C.C R= 0.175 nm √ Or √ a= 0.495 nm. √ 15