Materials Science Lecture Notes PDF

1  Classification of Materials Metals This group of materials includes metallic elements such as iron, aluminum, copper, titanium, gold, and nickel, as well as small amounts of nonmetallic elements like carbon, nitrogen, and oxygen. Metals are generally denser than ceramics and polymers....

1  Classification of Materials Metals This group of materials includes metallic elements such as iron, aluminum, copper, titanium, gold, and nickel, as well as small amounts of nonmetallic elements like carbon, nitrogen, and oxygen. Metals are generally denser than ceramics and polymers. Stiff and strong. Ductile (capable of a large amount of deformation without fracture). Resistance to fracture (fracture toughness). Good conductors of electricity and heat Not transparent to visible light. Some of the metals Fe, Co, and Ni have desirable magnetic properties. 2  Ceramics Ceramics are composed of both metallic and non-metallic elements, commonly found in the form of oxides, nitrides, and carbides. aluminum oxide (or alumina, Al2O3), silicon dioxide (or silica, SiO2), silicon carbide (SiC), silicon nitride (Si3N4). The traditional ceramics—those composed of clay minerals (i.e., porcelain), as well as cement and glass. ceramic materials are relatively stiff and strong—stiffnesses and strengths are comparable to those of the metals Very hard. Ceramics have shown extreme brittleness and are prone to fracturing. Newer ceramics are now designed with enhanced resistance to breaking. These upgraded materials are utilized in cookware, cutlery, and automotive engine components. Furthermore, ceramic materials are typically insulative to the passage of heat and electricity (i.e., have low electrical conductivities), and are more resistant to high temperatures and harsh environments than metals and polymers. Some of the oxide ceramics (e.g. Fe3O4) exhibit magnetic behavior. 3  Polymers Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (i.e., O, N, and Si). Furthermore, they have very large molecular structures, often chainlike in nature, that often have a backbone of carbon atoms. Some of the common and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. Polymers have low density. Stiffness and strength of polymers are lower than metal and ceramics. Many of the polymers are extremely ductile and pliable (i.e., plastic), which means they are easily formed into complex shapes. In general, they are relatively inert chemically and unreactive in a large number of environments. One major drawback to the polymers is their tendency to soften and/or decompose at modest temperatures, which, in some instances, limits their use. they have low electrical conductivities and are nonmagnetic. 4 5 Bohr atomic model Electrons are assumed to revolve around the atomic nucleus in discrete orbitals, and the position of any particular electron is more or less well-defined in terms of its orbital. Atomic Bonding in Solids An understanding of many of the physical properties of materials is enhanced by a knowledge of the interatomic forces that bind the atoms together. Valence Electron The valence electrons are those that occupy the outermost shell. Electropositive Elements They are capable of giving up their few valence electrons to become positively charged ions. Electronegative Elements They readily accept electrons to form negatively charged ions, or sometimes they share electrons with other atoms. PRIMARY INTERATOMIC BONDS ( CHEMICAL BONDS) (VALENCE ELECTRONS) Ionic Bonding Covalent Bonding Metallic Bonding 6 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. Sodium chloride (NaCl) is the classic ionic material. In sodium chloride, all the sodium and chlorine exist as ions. Ionic bonding is termed nondirectional; that is, the magnitude of the bond is equal in all directions around an ion. It follows that for ionic materials to be stable, all positive ions must have as nearest neighbors negatively charged ions in a three-dimensional scheme, and vice versa. ionic bonding in sodium chloride (NaCl) Ionic materials are characteristically hard and brittle and, furthermore, electrically and thermally insulative.(Ionic bonding) 7 Covalent Bonding In covalent bonding, stable electron configurations are assumed by sharing electrons between adjacent atoms. Two covalently bonded atoms will each contribute at least one electron to the bond, and the shared electrons may be considered to belong to both atoms. 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. The number of covalent bonds that is possible for a particular atom is determined by the number of valence electrons. (2, 8, 7) (2, 8, 7) CH4 Cl2 molecule 8 Metallic Bonding With metallic bonding, the valence electrons form a “sea of electrons” that is uniformly dispersed around the metal ion cores and acts as a form of glue for them. Metallic materials exhibit this type of bonding The remaining nonvalenced 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. Note Metals are good conductors of both electricity and heat, as a consequence of their free electrons. By way of contrast, ionically and covalently bonded materials are typically electrical and thermal insulators because of the absence of large numbers of free electrons. 9 Fundamental Concepts of Crystal Structures A crystalline material is one in which the atoms are situated in a repeating or periodic array. All metals, many ceramic materials, and certain polymers form crystalline structures under normal solidification conditions. Crystal Structure representation Atomic hard sphere model (spheres having well-defined diameters) Lattice (three-dimensional array of points coinciding with atom positions (or sphere centers). Unit Cells (smallest repeat unit). METALLIC CRYSTAL STRUCTURES The Simple Cubic Crystal Structure (SC) 𝟏 Atom is associated with each SC unit cell. 10  𝑵𝒐. 𝒐𝒇 𝒂𝒕𝒐𝒎𝒔 = 𝟏 × 𝟖 = 𝟏 𝒂𝒕𝒐𝒎. 𝟖 Cubic unit cell length = a, and atomic sphere radius = R The relationship between the cube edge length (a) and the atomic radius (R): a = 2R The Face-Centered Cubic Crystal Structure (FCC) Atomic hard sphere Unit Cell Lattice model Atoms located at each of the corners and the centers of all the cube faces. Some of the familiar metals having this crystal structure are copper, aluminum, silver, nickel, lead, and gold. 𝟒 Atoms are associated with each FCC unit cell. 𝟏 𝑵𝒐. 𝒐𝒇 𝒂𝒕𝒐𝒎𝒔 = 𝟏 × 𝟔 + × 𝟖 = 𝟒 𝒂𝒕𝒐𝒎𝒔 𝟐 𝟖 Atoms (spheres) touch one another across a face diagonal. Cubic unit cell length = a, and atomic sphere radius = R 11 The relationship between the cube edge length (a) and the atomic radius (R) Face Diagonal Atomic Packing Factor (APF) The APF is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic hard-sphere model) divided by the unit cell volume. 𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒂𝒕𝒐𝒎𝒔 𝒊𝒏 𝒂 𝒖𝒏𝒊𝒕 𝒄𝒆𝒍𝒍 APF = 𝒕𝒐𝒕𝒂𝒍 𝒖𝒏𝒊𝒕 𝒄𝒆𝒍𝒍 𝒗𝒐𝒍𝒖𝒎𝒆 12 The Body-Centered Cubic Crystal Structure (BCC) Unit Cell Lattice Atomic hard sphere model 13  Atoms located at all eight corners and a single atom at the cube center. Chromium, tungsten, Iron (α), Tungsten, Tantalum, Molybdenum exhibit a BCC structure. 𝟐 Atoms are associated with each BCC unit cell. 𝑁 𝑁. 𝑁 𝑁 𝑁 𝑁 𝑁 𝑁 𝑁=𝟏+ 𝟖 𝟏 × 𝟖 = 𝟐 𝒂𝒕𝒐𝒎𝒔. Center and corner atoms touch one another along cube diagonals. 14 15 The Hexagonal Close-Packed Crystal Structure (HCP) The top and bottom faces of the unit cell consist of six atoms that form regular hexagons and surround a single atom in the center. Another plane that provides three additional atoms to the unit cell is situated between the top and bottom planes. Cadmium, Magnesium, Zinc, and Titanium have this crystal structure. Unit cell has two lattice parameters a and c. Ideal ratio c/a = 1.633. 3 mid-plane atoms shared by no other cells: 3×1 = 3 atoms. 𝟏 12 hexagonal corner atoms shared by 6 cells: 𝟏𝟐 × = 𝟐 𝒂𝒕𝒐𝒎𝒔. 𝟔 𝟏 𝟐 Top/bottom plane center atoms shared by 2 cells: 𝟐 × = 𝟏 𝒂𝒕𝒐𝒎𝒔. 𝟐 𝑁 𝑁. 𝑁 𝑁 𝑁 𝑁 𝑁 𝑁 𝑁 =3+2+1= 6 atoms. Atomic packing factor, APF = 0.74 (same as in FCC). 16 𝟑. 𝟔 Show that the atomic packing factor for HCP is 0.74. For HCP, there are the equivalent of six spheres per unit cell, and thus Now, the unit cell volume is just the product of the base area times the cell height, c. This base area is just three times the area of the parallelepiped ACDE shown below. The area of ACDE is just the length of CD times the height BC. But CD is just a or 2R, and 17 Theoretical density for metals A knowledge of the crystal structure of a metallic solid permits computation of its theoretical density ρ through the relationship 𝒏𝑨 𝝆= 𝑽 𝑪𝑵𝑨 Where 𝑵𝑨 = Avogadro’s number 16.022 1023 atoms/mol2 𝒏 = number of atoms associated with each unit cell 𝑨 = atomic weight (g/mol) 𝑽𝑪 = volume of the unit cell Example Copper has an atomic radius of 0.128 nm, an FCC crystal structure, and an atomic weight of 63.5 g/mol. Compute its theoretical density. 18 𝟑. 𝟐 If the atomic radius of aluminum is 0.143 nm, calculate the volume of its unit cell in cubic meters. Aluminum has an FCC crystal structure. The FCC unit cell volume may be computed from 𝟑. 𝟕 Iron has a BCC crystal structure, an atomic radius of 0.124 nm, and an atomic weight of 55.85 g/mol. Compute and compare its theoretical density. 19 𝟑. 𝟖 Calculate the radius of an iridium atom, given that Ir has an FCC crystal structure, a density of 22.4 g/𝐜𝐦𝟑, and an atomic weight of 192.2 g/mol. 20 𝟑. 𝟗 Calculate the radius of a vanadium atom, given that V has a BCC crystal structure, a density of 5.96 g/𝐜𝐦𝟑, and an atomic weight of 50.9 g/mol. For BCC, n = 2 atoms/unit cell, and 21 𝟑. 𝟏𝟎 Some hypothetical metal has the simple cubic crystal structure shown in Figure. If its atomic weight is 70.4 g/mol and the atomic radius is 0.126 nm, compute its density. 22 𝟑. 𝟐𝟑 Below is a unit cell for a hypothetical metal. (a) To which crystal system does this unit cell belong? (b) What would this crystal structure be called? (c) Calculate the density of the material, given that its atomic weight is 141 g/mol. 23  Polymorphism The ability of a solid material to exist in more crystal structure. Pure iron has a BCC crystal structure at room temperature, which change to FCC iron at 912oC. Allotropy the property of some chemical elements to exist in two or more different forms. Crystal system Lattice parameters Axial lengths (a, b, & c) Inter axial angles (𝛼, 𝛽, & 𝛾) 24 25 26