Material Science for E-TECH PDF

Materials Science for Engineering Technology (MET 161-3) Introduction Dr. Sandeepa Lakshad [email protected] 1  Course Content...

Materials Science for Engineering Technology (MET 161-3) Introduction Dr. Sandeepa Lakshad [email protected] 1  Course Content 1. Introduction (Evolution of engineering materials, Classification of materials.) 2. Structures of Materials (Atomic Structure, Bonding forces and energies, Primary interatomic bonds and secondary bonding) 3. Crystalline and non-crystalline materials 4. Phase diagram and microstructure 5. Electrical and Optical Properties of Materials (Conductors, semiconductors, and insulators.) 6. Mechanical Properties of Materials (Tensile, compression, impact energy, fracture toughness.) 7. Mechanical behavior of Materials (Stress-strain behavior, Elastic and plastic properties of materials, dislocations and strengthening mechanisms.) 8. Introduction to Failure Analysis and Prevention Fundamentals of fracture (Fractures Types (ductile, brittle, fatigue and creep), Corrosion, Nondestructive Testing and Techniques for failure analysis and Prevention.) 9. Selection of Engineering Materials 2 (Characterization of Materials, Design and safety factors)  Continuous Assessment : 40 % (Practices, 15 %; Mid-term,15 %; Assessments (or quizzes), 10 %) Final Assessment (3h) : 60 % Recommended Reading: Donald R. Rskeland, Pradeep P. Fulay, Wendelin J. Wright, The Science andEngineering of Materials, 6th Edition, ISBN-13: 978-0495296027, ISBN-10: 0495296023 William D. Callister, (2005), Materials Science and Engineering, (John Wiley & Sons publisher), ISBN 0471660817, 9780471660811 William D. Callister, David G. Rethwisch, (2013), Materials Science and Engineering, (John Wiley and Sons, Incorporated publisher), ISBN 1118476549,9781118476543 4 1. Introduction 5  What are Materials? "The matter refers to the substance from which an object is made or can be made." 6 What is Materials Science and Engineering (MSE) MSE is an interdisciplinary field of science and engineering that studies and manipulates the composition and structure of materials across length scales to control materials' properties through synthesis and processing. 7 Composition : Chemical make-up of a material. Structure : Description of the arrangement of its internal components. Synthesis : How materials are made from naturally occurring or man-made chemicals. Processing : How materials are shaped into useful components to cause changes in the properties of different materials 8 Structure Subatomic structure Involves electrons within the individual atoms, their energies, and interactions with the nuclei. 9 Atomic structure Relates to the organization of atoms to molecules or crystals. 10  Nanostructure Deals with aggregates of atoms that form particles that have nanoscale dimensions (less than about 100 nm). Cai et al., Nature (London) 466, 470 (2010) Microstructure Those structural elements are subject to direct observation using a microscope (structural features having dimensions between 100 nm and several millimeters). DOI:10.1016/j.jmrt.2019.11.036 12 Macrostructure Structural elements that may be viewed with the naked eye (with scale range between several millimeters and on the order of a meter). 13 Properties of Materials A property is a material characteristic that describes the kind and magnitude of response to a specific imposed stimulus. Structure-sensitive properties: yield strength, hardness, ductility, fracture toughness, fatigue strength, corrosion resistance, thermal conductivity, electrical conductivity. Structure-insensitive properties: elastic moduli, Poisson’s ratio, density, thermal expansion coefficient, specific heat. 14 Properties of Materials All important properties of solid materials may be grouped into six different categories. Mechanical properties (Ex: Strength) Electrical properties (Ex: Electrical conductivity) Thermal properties (Ex: Thermal expansion) Magnetic properties (Ex: magnetization) Optical properties (Ex: reflectivity) Deteriorative characteristics (Ex: corrosion resistance) 15 Why do technologists, engineers, and scientists study materials? “Simply, because things engineers design are made of materials.” Materials scientists and engineers are specialists who are totally involved in the investigation and design of materials. 16 There is No Engineering Without Materials 17 The final decision is normally based on several criteria. First, the properties required of the material (Ex: Strength) A second selection consideration is any deterioration of material properties that may occur during service operation. (Ex: significant reductions in mechanical strength may result from exposure to elevated temperatures or corrosive environments) Finally, economics: What will the finished product cost? 18 Application of the tetrahedron of MSE Performance/Cost Composition Synthesis and Processing Microstructure 19 20 What are Engineering Materials The materials used for the application of engineering works. Classification of Engineering Materials 1. Metal and Alloys 2. Ceramics and Glasses 3. Polymers 4. Composites 21 Metal and Alloys Metals are referred to as pure elements such as iron, aluminum, copper, titanium, gold, and nickel. An alloy is composed of one or more metallic elements and often also relatively small amounts of nonmetallic elements such as carbon, nitrogen, and oxygen. 22  Metals and alloys have relatively high strength, stiffness, ductility or formability, and shock resistance. They are particularly useful for structural or load- bearing applications. While pure metals can be utilized for certain applications, alloys are often preferred as they can enhance specific desirable properties. 23 Ceramics and Glasses Ceramics can be defined as inorganic crystalline materials. Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. 24  Glass is an amorphous material, often, but not always, derived from a molten liquid. The term “amorphous” refers to materials that do not have a regular, periodic arrangement of atoms. Glass-ceramics are formed by a specialized thermal process that involves creating small crystals within glasses. 25  Ceramics are strong and hard, but also very brittle. We normally prepare fine ceramic powders and convert them into different shapes. Ceramics have exceptional strength under compression. 26 Polymers Polymers are typically organic materials, including the familiar plastic and rubber materials. Although they have lower strength, polymers have a very good strength-to-weight ratio. They are typically not suitable for use at high temperatures. Many polymers have very good resistance to corrosive chemicals. 27 Thermoplastic polymers: Long molecular chains are not rigidly connected and have good ductility and formability. Thermosetting polymers: The molecular chains are tightly linked. They are stronger but more brittle. 28 Composite Materials The objective of composite materials is to combine the properties of different materials to generate properties that do not exist in any single material. Concrete, plywood, and fiberglass are examples of composite materials. Ex: Fiberglass is made by dispersing glass fibers in a polymer matrix. The glass fibers make the polymer stiffer, without significantly increasing its density. 29 Wood Wood is an engineering material that can be used in a variety of applications, from home construction to commercial buildings to industrial products. 30 Assignment 1: Discuss examples, properties, and applications of various engineering materials. Submit within 2 weeks. Ensure that the report does not exceed 2 A4 sheets or 4 pages. 31 Materials Science for Engineering Technology (MET 161-3) Structures of Materials Dr. Sandeepa Lakshad [email protected] 1  Course Content 1. Introduction (Evolution of engineering materials, Classification of materials.) 2. Structures of Materials (Atomic Structure, Bonding forces and energies, Primary interatomic bonds and secondary bonding) 3. Crystalline and non-crystalline materials 4. Phase diagram and microstructure 5. Electrical and Optical Properties of Materials (Conductors, semiconductors, and insulators.) 6. Mechanical Properties of Materials (Tensile, compression, impact energy, fracture toughness.) 7. Mechanical behavior of Materials (Stress-strain behavior, Elastic and plastic properties of materials, dislocations and strengthening mechanisms.) 8. Introduction to Failure Analysis and Prevention Fundamentals of fracture (Fractures Types (ductile, brittle, fatigue and creep), Corrosion, Nondestructive Testing and Techniques for failure analysis and Prevention.) 9. Selection of Engineering Materials 2 (Characterization of Materials, Design and safety factors) 2. The Structure of the Atom – A Brief Review 3  Atoms are composed of a nucleus (carries net positive charge) surrounded by electrons (carries net negative charge). The nucleus contains neutrons and positively charged protons. The electric charge carried by each electron and proton is 1.60 × 10!"# coulomb (C). The atomic number of an atom is equal to the number of protons in each atom. 4  Most of the mass of an atom is contained within the nuclei. The mass of each proton and neutron is 1.67 × 10!$% g, and the electron is 9.11 × 10!$& g. The atomic mass (M) is equal to the total mass of the average number of protons and neutrons in the atoms in the atomic mass unit (amu), and is also the mass of the Avogadro Constant 𝑁' of atoms. 𝑁' = 6.022 × 10$( atoms/mol Therefore, atomic mass has the unit of g/mol. 5 The Electronic Structure of the Atom Quantum Numbers The energy level to which each electron belongs is identified by four quantum numbers. principal quantum number n, secondary quantum number l, magnetic quantum number ml, and spin quantum number ms. 6  The principal quantum number reflects the grouping of electrons into sets of energy levels known as shells. Secondary quantum numbers describe the energy levels within each shell and reflect a further grouping of similar energy levels, usually called orbitals. The magnetic quantum number specifies the orbitals associated with a particular secondary quantum number within each shell. Finally, the spin quantum number (ms) is assigned values of 1/2 and -1/2, which reflect the two possible values of “spin” of an electron. 7 According to the Pauli Exclusion Principle, no two electrons may have the same four quantum numbers within each atom, and thus, each electron is designated by a unique set of four quantum numbers. The first three quantum numbers determine the number of possible energy levels. 8 The principal quantum number n is assigned integer values 1, 2, 3, 4, 5,... that refer to the quantum shell to which the electron belongs. Quantum shells are also assigned a letter; the shell for n = 1 is designated K, for n = 2 is L, for n = 3 is M, and so on. The secondary quantum numbers are assigned l = 0, 1, 2,... , n - 1. For example, when n = 2, there are three secondary quantum numbers, l = 0, l = 1, and l = 2. The secondary quantum numbers are designated by lowercase letters; s for l = 0, p for l = 1, d for l = 2, f for l = 3, and so on. 9 The total number of magnetic quantum numbers for each l is 2l+1. The combination of l and ml specifies a particular orbital in a shell. No more than two electrons with opposing electronic spins (1/2 and -1/2) may be present in each orbital. 10 The Aufbau Principle 11 Valence The valence of an atom is the number of electrons in an atom that participate in bonding or chemical reactions. Usually, the valence is the number of electrons in the outer s and p energy levels. The valence of an atom is related to its ability to combine chemically with other elements. 12  Atomic Bonding 1. Metallic bonds 2. Covalent bonds 3. Ionic bonds 4. Van der Waals bonds Metallic, Covalent, and Ionic bonds are called primary bonds (relatively strong bonds, resulting from the transfer or sharing of outer orbital electrons) Van der Waals bonds are secondary bonds (relatively weaker). 13 Metallic bonds The metallic elements have electropositive atoms that donate their valence electrons to form a “sea” of electrons surrounding the atoms. Valence electrons: electrons in the outermost shell of an atom. Because their valence electrons are not fixed in any one position, most pure metals are good electrical conductors of electricity 14 One reason for the good ductile nature of metals is that their metallic bonds are non-directional. Ductility refers to the ability of materials to be stretched or bent permanently without breaking. 15 Covalent bonds Covalent bonds are formed by sharing of valence electrons among two or more atoms. Covalent bonds have a fixed directional relationship with one another. A directional relationship is formed when the bonds between atoms in a covalently bonded material form specific angles, depending on the material. In the case of silicon, this arrangement produces a tetrahedron, with angles of 109.5°between the covalent bond. 16 Covalent bonds are very strong. As a result, covalently bonded materials are very strong and hard. 17 Ionic bonds When more than one type of atom is present in a material, one atom may donate its valence electrons to a different atom, filling the outer energy shell of the second atom. 18 Both atoms now have filled (or emptied) outer energy levels, but both have acquired an electrical charge and behave as ions. The atom that contributes the electrons is left with a net positive charge and is called a cation, while the atom that accepts the electrons acquires a net negative charge and is called an anion. The oppositely charged ions are then attracted to one another and produce the ionic bond. 19 Van der Waals bonds Molecules and atoms with induced or permanent dipole moments attract each other, creating van der Waals forces. 20 When a neutral atom is exposed to an internal or external electric field, the atom may become polarized. This creates or induces a dipole moment. Polarity in chemistry: the separation of an electric charge which leads a molecule to have a positive and negative end. In some molecules, the dipole moment can exist without being induced due to the atoms' nature and bond direction. These molecules are known as polarized molecules. 21 There are three types of Van der Waals interactions, London Forces: The interactions are between two dipoles that are induced in atoms or molecules. Debye interactions: When an induced dipole interacts with a molecule that has a permanent dipole moment. Keesom interactions (often referred to as Hydrogen bond) : The interactions are between molecules that are permanently polarized. 22 Although termed “secondary,” based on the bond energies, van der Waals forces play a very important role in many areas of engineering. Van der Waals forces between atoms and molecules play a vital role in determining the surface tension and boiling points of liquids. 23 Binding Energy and Interatomic Spacing 24 Interatomic Spacing The equilibrium distance between atoms is caused by a balance between repulsive and attractive forces. Equilibrium separation occurs when the total interatomic energy of the pair of atoms is at a minimum, or when no net force is acting to either attract or repel the atoms. 25 26 The minimum interatomic energy is the binding energy, or the energy required to create or break the bond. Consequently, materials with high binding energy also have high strength and melting temperatures. 27 An interesting point that needs to be made is that not all properties of engineered materials are microstructure-sensitive. Modulus of elasticity: A similar chemical composition of two aluminum samples with different grain sizes will result in an equal modulus of elasticity. Coefficient of thermal expansion 28  Materials Science for Engineering Technology (MET 161-3) Crystalline and non-crystalline materials Dr. Sandeepa Lakshad [email protected] 1  Course Content 1. Introduction (Evolution of engineering materials, Classification of materials.) 2. Structures of Materials (Atomic Structure, Bonding forces and energies, Primary interatomic bonds and secondary bonding) 3. Crystalline and non-crystalline materials 4. Phase diagram and microstructure 5. Electrical and Optical Properties of Materials (Conductors, semiconductors, and insulators.) 6. Mechanical Properties of Materials (Tensile, compression, impact energy, fracture toughness.) 7. Mechanical behavior of Materials (Stress-strain behavior, Elastic and plastic properties of materials, dislocations and strengthening mechanisms.) 8. Introduction to Failure Analysis and Prevention Fundamentals of fracture (Fractures Types (ductile, brittle, fatigue and creep), Corrosion, Nondestructive Testing and Techniques for failure analysis and Prevention.) 9. Selection of Engineering Materials 2 (Characterization of Materials, Design and safety factors) 3. Crystalline and non-crystalline materials 3 Short-Range order vs. Long-Range order No Order: In monoatomic gases or plasma, atoms or ions have no orderly arrangement. 4 Short-Range Order: A material displays short-range order if the special arrangement of the atoms extends only to the atom’s nearest neighbors. 5 Long-Range Order: The special atomic arrangement extends over much larger length scales, larger than 100 nm. 6 Classification of materials based on the type of atomic order: Liquid Crystals Amorphous Materials Crystalline Materials Monoatomic Gases Short and Long-range Only short-range Short and long-range No order order in small order order [Ex: Ar] volumes [Ex: Glass] [Ex: Metals] [Ex: LCD polymers] Single Crystal Materials Polycrystalline Materials 7 Crystalline Materials The atoms, ions, or molecules in crystalline materials form a regular, repeating pattern in three dimensions/ two dimensions/ one dimension. 8 Single Crystal Materials If a crystalline material consists of only one large crystal, we refer to it as a single crystal. Single crystals are useful in many electronic and optical applications. 9 Poly Crystalline Materials A polycrystalline material is composed of many small crystals with varying orientations in space. These smaller crystals are known as grains. The borders between crystals, where the crystals are in misalignment, are known as grain boundaries. 10 https://doi.org/10.1038/ncomms3811 HRTEM images 11 12 Liquid Crystals Liquid crystals are polymeric materials that have a special type of order. Liquid crystal polymers behave as amorphous materials (liquid-like) in one state. Upon exposure to external stimuli such as electric fields or temperature changes, certain polymer molecules align and form small crystalline regions. 13 Properties of Crystalline and amorphous materials Crystalline Materials Amorphous Materials Repetitive atomic Random atomic arrangement structure Melting point Glass transition temperature Definite geometrical Irregular shape shape 14 15 Lattice, basis, Unit cells, and Crystal Structures Crystalline Structure: Ordered arrangement of atoms, ions, or molecules. 16  A lattice is a collection of points called lattice points, arranged in a periodic pattern. In other words, lattice means a three-dimensional array of points coinciding with atom positions. A lattice is a purely mathematical construct and is infinite in extent. A group of one or more atoms located in a particular way with respect to each other and associated with each lattice point is known as the basis. We obtain a crystal structure by placing the atoms of the basis on every lattice point. 17 (a) A one-dimensional lattice. The lattice points are separated by an equal distance. (b) A basis of one atom. (c) A crystal structure is formed by placing the basis of (b) on every lattice point in (a). (d) A crystal structure is formed by placing a basis of two atoms of different types on the lattice in (a). 18  When describing crystalline structures, atoms are often represented as solid spheres with well-defined diameters. This atomic hard-sphere model depicts spheres of nearest- neighbor atoms touching one another. 19 Unit Cells The atomic order in crystalline solids indicates that small groups of atoms form a repetitive pattern. Thus, it is often convenient to subdivide crystal structures into small repeat entities called unit cells when describing them. 20 21 22  Three-dimensional arrangements of lattice points are known as the Bravais lattices. There are 14 Bravais lattices. These 14 Bravais lattices are grouped into 7 crystal systems. 23 24 Simple Cubic (SC) Ex: Po 25 Body-Centered Cubic (BCC) Ex: Na, Li 26 27 Face Centered Cubic (FCC) Ex: Cu, Al 28 29 30 Number of atoms per unit cell Each unit cell contains a specific number of lattice points. When counting the number of lattice points belonging to each unit cell, we must recognize that, lattice points may be shared by more than one unit cell. 31 A lattice point at a corner of one unit cell is shared by seven adjacent unit cells (thus a total of eight cells); only one-eighth of each corner belongs to one particular cell. 32 Example: Determine the number of lattice points per unit cell in SC, BCC, and FCC crystal systems. If only one atom is located at each lattice point, calculate the number of atoms per unit cell. 33 34 Atomic Radius vs. Lattice Parameter (relationship between the apparent size of the atom and the size of the unit cell) By determining the length of the direction relative to the lattice parameters and adding the number of atomic radii along that direction, we can establish the desired relationship. 35 Example: Determining the Relationship between Atomic Radius and Lattice Parameters of SC, BCC, and FCC. 36 37 Calculate the atomic radius in cm for the following: (a) BCC metal with a = 0.3294 nm; and (b) FCC metal with a = 4.0862 Å. 38 Packing Factor (Atomic Packing Factor) Fraction of space occupied by atoms, assuming that the atoms are hard spheres. Volume of atoms in a unit cell Packing Factor = Volume of unit cell Number of atoms per unit cell ×Volume of an atom Packing Factor = Volume of unit cell 39 Example: Calculate the packing factor for FCC, SC unit cell. 40 Density The theoretical density of a material can be calculated using the properties of the crystal structure. Number of atoms per unit cell ×Atomic mass Denity = Volume of unit cell×Avogadro constant 41 Example: Determine the density of BCC iron which has lattice parameter of 0.2866 nm. 42 Crystallographic Points, Directions, and Planes When dealing with crystalline materials, specifying a particular point within a unit cell, a crystallographic direction, or some crystallographic plane of atoms often becomes necessary. 43 POINT COORDINATES (specifying a lattice position within a unit cell) The position of a lattice can be described using three coordinates, one for each axis (x, y, and z.) In this case, we have labeled them as Px, Py, and Pz. To specify the coordinates, we use three fractional indices: q, r, and s. These indices are expressed as multiples of the unit cell lengths a, b, and c. 44  𝑃! = 𝑞𝑎 𝑃" = 𝑟𝑏 𝑃# = 𝑠𝑐 Point P: 𝑞 𝑟 𝑠 45 Example: For the unit cell shown in the accompanying $ $ sketch, locate the point having indices 1 % % 46 Example: Specify indices for all lattice points of the unit cell. 47 CRYSTALLOGRAPHIC DIRECTIONS (line directed between two points or a vector) 1. First, a right-handed x-y-z coordinate system is constructed (its origin may be located at a unit cell corner for convenience). 2. The coordinates of two points that lie on the direction vector (referenced to the coordinate system) are determined. 3. Tail point coordinates are subtracted from head point components (that is, x2 - x1, y2 - y1, and z2 - z1). 4. These coordinate differences are then normalized in terms of (i.e., divided by) their respective a, b, and c lattice parameters, which yields a set of three numbers. 5. If necessary, these three numbers are multiplied or divided by a common factor to reduce them to the smallest integer values. 6. The three resulting indices, not separated by commas, are enclosed in square brackets. 48 In summary, the 𝑢, 𝑣, & 𝑤 indices are determined using the following equations: 𝑥& − 𝑥$ 𝑢=𝑛 𝑎 𝑦& − 𝑦$ 𝑣=𝑛 𝑏 𝑧& − 𝑧$ 𝑤=𝑛 𝑐 49 Determine the indices for the direction shown in the accompanying figure. 50 CRYSTALLOGRAPHIC PLANES (Miller indices) The orientations of planes for a crystal structure are represented in a similar manner. The procedure used to determine the h, k, and l index numbers is as follows: 51 1. When a plane passes through the chosen origin, we have two options. We can either construct another parallel plane within the unit cell through an appropriate translation or establish a new origin at the corner of another unit cell. 2. At this stage, the crystallographic plane either intersects or runs parallel to the three axes. We determine the coordinates for the intersection of the crystallographic plane with each of the axes, referenced to the origin of the coordinate system. These coordinates for the x, y, and z axes will be denoted as A, B, and C, respectively. 3. The reciprocals of these numbers are taken. A plane that parallels an axis is considered to have an infinite intercept and, therefore, a zero index. 4. The reciprocals of the intercepts are then normalized in terms of (i.e., multiplied by) their respective a, b, and c lattice parameters. That is, 5. If necessary, these three numbers are changed to the set of smallest integers by multiplication or by division by a common factor. 6. Finally, the integer indices, not separated by commas, are enclosed within parentheses, thus: (hkl). The h, k, and l integers correspond to the normalized intercept reciprocals referenced to the x, y, and z axes, respectively. 52 7. In summary, the h, k, and l indices may be determined using the following equations: In summary, the ℎ, 𝑘, & 𝑙 indices are determined using the following equations: 𝑛𝑎 ℎ= 𝐴 𝑛𝑏 𝑘= 𝐵 𝑛𝑐 𝑙= 𝐶 53 Determine the Miller indices for the plane shown in the accompanying sketch. 54 Thank You! 55  Materials Science for Engineering Technology (MET 161-3) Electrical and Optical Properties of Materials 1 Electrical Properties of Materials 2 Electronic And Ionic Conduction An electric current is created when electrically charged particles move in response to an externally applied electric field. In most solid materials, electronic conduction, or the flow of electrons, is responsible for generating the current. In addition, for ionic materials, a net motion of charged ions that produces a current is possible; this is termed ionic conduction. 3 Electrical Conduction OHM’S law 𝑉 = 𝐼𝑅 Voltage J/C = Current C/s × Resistance V/A The value of 𝑅 is influenced by specimen configuration and, for many materials, is independent of current. 4 Electrical Resistivity (𝝆) The electrical resistivity (𝜌) is independent of specimen geometry but related to 𝑅 through the expression 𝑅𝐴 𝜌= 𝑙 where 𝑙 is the distance between the two points at which the voltage is measured and 𝐴 is the cross-sectional area perpendicular to the direction of the current. The units for 𝜌 are ohm-meters (Ω·m). 5 Electrical Conductivity (𝛔) 1 𝜎= 𝜌 The SI units for electrical conductivity are siemens per meter (S/m), in which 1 S/m = 1 (Ω·m)−1. Electrical conductivity indicates the ease of conducting electric current. 6 Current Density (𝑱) 𝐽 = 𝜎𝐸 The current density is the current per unit of specimen area 𝐼/𝐴 , and E is the electric field intensity, or the voltage difference between two points divided by the distance separating them. i.e. 𝑉 𝐸= 𝑙 7 Based on how easily they conduct electric current, solid materials are classified as conductors, semiconductors, and insulators. Conductors typically have conductivities on the order of 107 (Ω·m)−1. Semiconductors have intermediate conductivities, generally from 10−6 to 104 (Ω·m)−1. Insulators have very low conductivities, ranging between 10−10 and 10−20 (Ω·m)−1 8 Energy Band Structure in Solids In all conductors, semiconductors, and many insulating materials, only electronic conduction exists, and the magnitude of the electrical conductivity is strongly dependent on the number of electrons available to participate in the conduction process. 9  Let's consider N number of atoms. At relatively large separation distances, each atom is independent of all the others and has the atomic energy levels and electron configuration as if isolated. When atoms come close to each other, their electrons and nuclei start to influence each other. This influence causes each unique atomic state to split into a series of closely spaced electron states in the solid, forming what is known as an electron energy band. 10 11 The conventional representation of the electron energy band structure for a solid material at the equilibrium interatomic separation. 12 Valence Band: The band of electron orbitals from which electrons can transition to the conduction band when excited. Conduction Band: The band of electron orbitals that electrons can transition to from the valence band when excited. 13 Metals, Semiconductors, and Insulators 14 Fermi Energy The energy corresponding to the highest filled state at 0 K is called the Fermi energy Ef, as indicated. 15 Conduction in terms of band and atomic bonding models In the presence of an electric field, only electrons with energies greater than the Fermi energy can be accelerated. These electrons are the ones that take part in the conduction process and are referred to as free electrons. Another type of charged electronic entity found in semiconductors and insulators is called a hole. Holes have energies less than Ef and also participate in electronic conduction. 16 17 Semiconductivity 18 n-Type Extrinsic Semiconduction 19 20 p-Type Extrinsic Semiconduction 21 22 Electron Mobility When an electric field is applied, a force is exerted on the free electrons. As a result, they all undergo acceleration in the opposite direction of the field due to their negative charge. According to quantum mechanics, an accelerating electron does not interact with atoms in a perfect crystal lattice. 23  The imperfections of the lattice, including impurity atoms, vacancies, interstitial atoms, dislocations, and even the thermal vibrations of the atoms themselves, cause electron scattering. Each time an electron scatters, it loses kinetic energy and changes direction, resulting in resistance to electric current. The electron mobility indicates the frequency of scattering events; its units are square meters per volt-second (m2/V·s). 24 The conductivity of most materials can be expressed as 𝜎 = 𝑛 𝑒 𝜇! 𝑛 is the number of free electrons per unit volume 𝑒 is the absolute magnitude of the electrical charge 𝜇! is the mobility 25 Optical Properties of Materials 26 Electromagnetic Radiation In classical mechanics, electromagnetic radiation is considered to be wave-like, consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of propagation. 27 28 Light interaction with solids Light can be reflected, absorbed, and transmitted when it interacts with solids. 29  Materials that can transmit light with relatively little absorption and reflection are called transparent materials (one can see through them). Translucent materials are substances through which light is diffusely transmitted; this means that light is scattered within the material to the extent that objects are not clearly distinguishable when viewed through it. Materials that do not allow the transmission of visible light are called opaque. 30 31 Atomic and Electronic interaction All electromagnetic radiation traverses a vacuum at the same velocity (𝑐), that of light—namely, 3 × 108 m/s. 𝑐 = 𝜆𝜈 𝜆 is the wavelength and 𝜈 is the frequency 32 According to quantum mechanics, electromagnetic radiation is viewed as a group of packets of energy called photons. ℎ𝑐 𝐸 = ℎ𝜈 = 𝜆 ℎ is the Plank’s constant (6.63 × 10-34 J.s or 4.13 × 10-15 eV.s) 33 Electron Transition The absorption and emission of electromagnetic radiation may involve electron transitions from one energy state to another. Consider isolating one atom, and the change in energy experienced by an electron Δ𝐸 = ℎ𝜈 34 35 Absorption Metals Non-Metals 36  When a photon is absorbed, an electron can be excited from the almost filled valence band, across the band gap, and into an empty state in the conduction band. This creates a free electron in the conduction band and a hole in the valence band. This process only occurs if the energy of the photon is greater than the band gap energy (𝐸𝑔). "# ℎ𝜈 > 𝐸𝑔 or > 𝐸𝑔 $ 37 When impurities are present 38 Transperency in vissible range Minimum band gap ℎ𝑐 𝐸𝑔(𝑚𝑖𝑛) = 𝜆 (𝑚𝑎𝑥) Maximun band gap ℎ𝑐 𝐸𝑔(𝑚𝑎𝑥) = 𝜆 (𝑚𝑖𝑛) 39 How we see color 40 When light passes through transparent materials, it can appear colored because certain wavelengths of light are absorbed while others are transmitted. The material will appear colorless if all visible wavelengths are absorbed uniformly (or without absorption). 41 For example, cadmium sulfide (CdS) has a band gap of about 2.4 eV. Hence, it absorbs photons with energies greater than about 2.4 eV, corresponding to the visible spectrum's blue and violet portions). Some of this energy is reradiated as light having other wavelengths. Non-absorbed visible light consists of photons with energies between 1.8 and 2.4 eV. Because of the composition of the transmitted beam, CdS takes on a yellow-orange color. 42 Transmission The phenomenon of transparency refers to the property of a solid, allowing light to pass through it. ' ()* 𝐼% = 𝐼& 1 − 𝑅 𝑒 Here, 𝐼! is the incident light beam intensity at the front face, 𝐼" is the intensity at the back face, 𝑅 is the reflectance, 𝛽 is the absorption coefficient, and 𝑙 is the thickness of the sample. 43 Reflection When light radiation passes from one medium into another with a different refraction index, some of the light is scattered at the interface between the two media, even if both are transparent. ' ' 𝐼+ 𝑛' − 𝑛, 𝑛- − 1 𝑅= = = 𝐼& 𝑛' + 𝑛, 𝑛- + 1 44 Thank You! 45 Materials Science for Engineering Technology 1 6. Mechanical Properties of Materials 2 The Tensile Test 3 Terminology for Mechanical Properties Stress: Force applied per unit area. Normal Stress: The applied force acts perpendicular to the area of interest. Shear Stress: The applied force acts in a direction parallel to the area of interest. 4  Tensile and Compressive forces (or stresses) are normal forces (stresses). 5 The Tensile Test: Use of the Stress-Strain Diagram. Engineering Stress and Strain Metals Elastomers Ceramics 6 Engineering Stress and Engineering Strain are defined by, F Engineering Stress = S = A! ∆l Engineering Strain = e = l! 7 8 Strain: Change in dimension per unit length. Elastic Strain: Fully recoverable strain resulting from an applied stress. A material subjected to an elastic strain does not show any permanent deformation. In many materials, elastic stress and elastic strain are linearly related. 9 Plastic Strain: Unrecoverable strain resulting from an applied stress. When the stress is released, the material does not go back to its original shape. 10 Strain rate: The rate at which strain is developed. Impact loading: A type of loading that causes a high strain rate. 11 Properties obtained from the Tensile Test: Elastic Limit: The critical stress value needed to initiate plastic deformation. Proportional Limit: Maximum level of stress in which the stress and strain relationship is linear. 12  In most materials, the elastic limit and proportional limit are quite close; however, neither the elastic limit nor the proportional limit values can be determined precisely. Offset Strain Value: 0.002 or 0.2% engineering strain (Typically, but not always) For engineering calculations, draw a line parallel to the linear portion of the engineering stress-strain curve starting at this offset value of strain. 13 14 Offset Yield Strength (Yield Strength): The stress value corresponding to the intersection of the offset line and the engineering stress-strain curve. This is actually the stress at which a material ceases elastic deformation and begins plastic deformation. For materials that do not exhibit well-defined yield points of 0.2%, the offset method is typically used. 15 The transition from elastic deformation to plastic deformation is referred to as Yielding. In some materials, this transition is rather abrupt. Such transition is known as the Yield Point Phenomenon. 16  For these materials, the yield strength is defined from 0.2% offset. Stress-strain curve of mild steel 17 When we design parts for load-bearing applications, we prefer no plastic deformation. As a result, we must select a material with a design stress considerably lower than the yield strength at the temperature at which the material will be used. On the other hand, when we want to shape materials into components, we need to apply stresses that are well above the yield strength. 18 Properties obtained from the Tensile Test: Elastic Properties The modulus of elasticity, or Young’s modulus (E): The slope of the stress-strain curve in the elastic region (precisely during the proportional limit). This relationship between stress and strain in the elastic region is known as Hooke’s Law. S E= e 19 20 Modulus of Resilience (𝐄𝐫 ): The elastic energy that a material absorbs during loading or subsequently releases when the load is removed. Represented by the area under the elastic portion of a stress-strain curve. For linear elastic behavior, $ E# = yield strength strain at yielding (Nm/) % 21 Properties obtained from the Tensile Test: Tensile Strength Tensile Strength (ultimate tensile strength): The stress obtained at the highest applied force. 22 Tensile Toughness (work of fracture): The energy absorbed by a material prior to fracture. Sometimes Tensile Toughness is measured as the area under the true stress–strain curve. Since it is easier to measure engineering stress– strain, engineers often equate tensile toughness to the area under the engineering stress–strain curve. 23  In many ductile materials, deformation does not remain uniform. At some point, one region deforms more than others, and a large local decrease occurs in the cross-sectional area. This locally deformed region is called a “neck.” This phenomenon is known as necking. Because the cross-sectional area becomes smaller at this point, a lower force is required to continue its deformation. 24 25 True Stress and True Strain F True Stress = 𝜎 = 𝐴 𝑙 True Strain = 𝜀 = ln 𝑙! 𝐴 is the instantaneous area over which the force is applied. 𝑙 is the instantaneous sample length. 26 Ductility: The ability of a material to be permanently deformed without breaking when a force is applied. There are two measures of ductility: percentage elongation and percentage reduction in area. 27 Percentage elongation 𝑙& − 𝑙' % Elongation = × 100 𝑙' Percentage reduction in area 𝐴' − 𝐴& % Reduction in area = × 100 𝐴' 𝐴& is the final cross-sectional area at the fracture surface. 28 Effect of Temperature: The mechanical properties of materials depend on temperature. The tensile properties of an Aluminum alloy. 29 Stiffness: Measure of a material's resistance to deformation under an applied load. 30 The Bend Test for Brittle Materials In ductile metallic materials, the stress–strain curve typically exhibits a maximum before failure occurs at a lower stress due to necking. In more brittle materials, failure occurs at the maximum load, where the tensile strength and breaking strength are the same 31 In some brittle materials, the tensile test cannot be performed without causing cracking due to surface flaws. These materials may be tested using the bend test. By applying the load causing bending, a tensile force acts on the material opposite the midpoint. Fracture begins at this location. The flexural strength, or modulus of rupture, describes the material’s strength: 32 3-point bend test. Flexural strength for three-point bend test 3𝐹𝐿 𝜎()*+ = 2𝑤ℎ% 33 The modulus of elasticity in bending, or the flexural modulus. , 𝐿 𝐹 𝐸()*+ = 4𝑤ℎ, 𝛿 34 Stress-deflection curve for an MgO ceramic obtained from a bend test. 35 6.7. Strain Rate Effects and Impact Behavior When a material is subjected to a strain rate is extremely rapid, it may behave in much more brittle a manner than is observed in the tensile test. Ex: If you stretch a plastic very slowly, the polymer molecules have time to disentangle or the chains to slide past each other and cause large plastic deformations. If, however, we apply an impact loading, there is insufficient time for these mechanisms to play a role and the materials break in a brittle manner. 36 Impact test is often used to evaluate the brittleness of a material under these conditions. In contrast to the tensile test, in this test, the strain rates are much higher (103 s-1). 37 Many test procedures are available: the Charpy test and the Izod test. The Izod test is often used for plastic materials. The test specimen may be either notched or unnotched. V-notched specimens better measure the resistance of the material to crack propagation. 38 39  In the test, a heavy pendulum, swings through its arc, strikes and breaks the specimen, and reaches a lower final elevation. If we know the initial and final elevations of the pendulum, we can calculate the difference in potential energy. This difference is the impact energy absorbed by the specimen during failure. For the Charpy test, the energy is usually expressed joules (J). The results of the Izod test are expressed in units of J/m. 40 The ability of a material to withstand an impact blow is often referred to as the impact toughness of the material. Fracture toughness of a material is defined as the ability of a material containing flaws to withstand an applied load. 41 Now you should be able to explain Tensile toughness | Fracture toughness | Impact toughness 42 Properties Obtained from the Impact Test 1. Ductile to Brittle Transition Temperature (DBTT) This temperature may be defined by the average energy between the ductile and brittle regions Results from a series of Izod impact tests for a tough nylon thermoplastic polymer. 43 2. Notch Sensitivity The notch sensitivity of a material can be evaluated by comparing the absorbed energies of notched versus unnotched specimens. The absorbed energies are much lower in notched specimens if the material is notch- sensitive. Notches caused by poor machining, fabrication, or design concentrate stresses and reduce the toughness of materials. 44 3. Relationship to the Stress-Strain Diagram The energy required to break a material during impact testing (impact toughness) is not always related to the tensile toughness. In general, metals with both high strength and high ductility have good tensile toughness; however, this is not always the case when the strain rates are high. For example, metals that show excellent tensile toughness may show brittle behavior under high strain rates (they may show poor impact toughness). Thus, the imposed strain rate can shift the ductile to brittle transition. 45 4. Use of Impact Properties The energy required to break a material during impact testing (impact toughness) is not always related to the tensile toughness. In general, metals with both high strength and high ductility have good tensile toughness; however, this is not always the case when the strain rates are high. For example, metals that show excellent tensile toughness may show brittle behavior under high strain rates (they may show poor impact toughness). Thus, the imposed strain rate can shift the ductile to brittle transition. 46 47 Fracture Mechanics Fracture Toughness (work of fracture): Fracture toughness measures the ability of a material containing a flaw to withstand an applied load. Note that this does not require a high strain rate 48 Viscous material: the strain develops over a period of time, and the material does not return to its original shape after the stress has been removed. The development of strain takes time and is not in phase with the applied stress. Viscoelastic material (anelastic): the development of a permanent strain is similar to that in a viscous material. Unlike a viscous material, when the applied stress is removed, part of the strain in a viscoelastic material will recover over a period of time. 49  The term “anelastic” is typically used for metals, while “viscoelastic” is usually used for polymeric materials. Stress relaxation: In viscoelastic materials held under constant strain, the stress level decreases over time. Recovery of strain and stress relaxation are different terms and should not be confused. The nylon strings in a tennis racket provide a common example of stress relaxation. We know that the level of stress, or the “tension,” as the tennis players call it, decreases with time. Recovery of strain refers to a return to the original shape once the stress is removed. 50 51 6.3. Various types of strain response to an imposed stress. Ideal Elastic Solids: Ex: Metals (below yield stress), ceramics and glasses. 52 Elastic + Plastic deformation: Ex: Ductile metals above yield stress. 53 Viscoelastic: Ex: Thermoplastics at a temperature around Tg 54 Creep: Ex: Metals and ceramics above 0.4Tm 55 6.4. Newtonian and non-Newtonian materials A description of the resistance to flow under applied stress is required when dealing with molten materials, liquids, and dispersions. 56 Newtonian Materials The relationship between applied shear stress (𝜏) and shear strain rate (𝛾)̇ is linear. 𝜏 = 𝜂 𝛾̇ The slope of the shear stress versus the steady-state shear strain rate curve is defined as the viscosity (𝜂) of the material. The units of 𝜂 is Pa.s (in SI system) 57 Non-Newtonian Materials The relationship between applied shear stress (𝜏) and shear strain rate (𝛾)̇ is nonlinear. - 𝜏 = 𝜂 𝛾̇ , here 𝑚 ≠ 1 Non-Newtonian materials are classified as shear thinning or shear thickening. 58 Materials Science for Engineering Technology 1 Course Content: Introduction Evolution of engineering materials, Classification of materials. Structures of Materials Atomic Structure, Bonding forces and energies, Primary interatomic bonds and secondary bonding. Crystalline and non-crystalline materials Phase diagram and microstructure Electrical and Optical Properties of Materials Conductors, semiconductors, and insulators. Mechanical Properties of Materials Tensile, compression, impact energy, fracture toughness. Mechanical behavior of Materials Stress-strain behavior, Elastic and plastic properties of materials, dislocations and strengthening mechanisms. Introduction to Failure Analysis and Prevention Fundamentals of Fracture Fractures Types (ductile, brittle, fatigue and creep), Corrosion, Nondestructive Testing and Techniques for failure analysis and Prevention. Selection of Engineering Materials 2 Characterization of Materials, Design and safety factors Various types of strain response to an imposed stress. 3 Ideal Elastic Solids: Ex: Metals (below yield stress), ceramics and glasses. 4 Elastic + Plastic deformation: Ex: Ductile metals above yield stress. 5 Viscoelastic: Ex: Thermoplastics at a temperature around Tg 6 Creep: (Time-dependent deformation at elevated temperature and constant stress.) Ex: Metals and ceramics above 0.4Tm 7 Dislocations and strengthening mechanisms 8 Defects of Crystals Point Defects: Vacancy, Impurity atoms Line defects: Dislocations Surface defects: Grain boundaries Volume defects: Void, cracks 9 Dislocations Type of linear crystalline defect known as a dislocation. 10 Slip Dislocation motion is termed slip. The crystalographic plane among which the dislocation line travels is the slip plane. 11 Edge and Screw Dislocations Edge and screw are the two fundamental dislocation types. An edge dislocation moves in a direction perpendicular to the dislocation line. The motion of a screw dislocation takes the direction of movement perpendicular to the stress direction. 12 Dislocation Density The units of dislocation density are millimeters of dislocation per cubic millimeter or just per square millimeter. 13 Characteristics of Dislocations Strain fields surrounding dislocations significantly influence dislocation mobility and multiplication. During plastic deformation, the number of dislocations increases dramatically. One key cause of these new dislocations is the multiplication of existing dislocations. Additionally, grain boundaries, internal defects, and surface irregularities like scratches, which create stress concentrations, can also act as sites for the formation of new dislocations during deformation. 14 15 Slip System Dislocations do not move with the same degree of ease on all crystallographic planes of atoms and in all crystallographic directions. The preferred plane of dislocation motion that occurs is called the slip plane; it follows that the direction of movement is called the slip direction. This slip plane and slip direction combination is termed the slip system. 16 For a particular crystal structure, the slip plane is the plane that has the densest atomic packing—that is, has the greatest planar density. 17 18 19 Mechanisms of Strengthening in Metals The ability of a metal to deform plastically depends on the ability of dislocations to move. Virtually all strengthening techniques rely on this simple principle: Restricting or hindering dislocation motion renders a material harder and stronger. 20 1. Strengthening by grain size reduction 2. Solid-solution strengthening 3. Strain Hardening 21 1. Strengthening by grain size reduction During plastic deformation, slip or dislocation motion must take place across this common boundary—say, from grain A to grain B 22 The grain boundary acts as a barrier to dislocation motion for two reasons: a. Because the two grains have different orientations, a dislocation passing into grain B must change its direction of motion. This becomes more difficult as the crystallographic misorientation increases. b. The atomic disorder within a grain boundary region leads to a discontinuity of slip planes between adjacent grains. 23 In high-angle grain boundaries, dislocations do not travel through the grain boundaries during deformation. Instead, they tend to accumulate at grain boundaries, creating stress concentrations ahead of their slip planes. These stress concentrations generate new dislocations in adjacent grains. 24 A finer-grained material is tougher and more stthan a coarse-grained one, as it has a larger total grain boundary area to hinder dislocation movement. 25 2. Solid solution strengthening Alloys are stronger than pure metals because impurity atoms that enter solid solutions generally impose lattice strains on the surrounding host atoms. As a result, interactions between lattice strain fields and dislocations occur, restricting dislocation movement. 26 27 3. Strain hardening Strain hardening, also known as work hardening or cold working, is the process by which a ductile metal becomes harder and stronger through plastic deformation. The strain-hardening phenomenon is explained based on interactions of dislocation–dislocation strain fields, similar to those discussed. 28 29 Recovery, Recrystallization, and Grain Growth 30 During recovery, some of the stored internal strain energy is relieved by dislocation motion due to enhanced atomic diffusion at the elevated temperature. Even after recovery is complete, the grains are still in a relatively high strain energy state. 31 Recovery During recovery, some of the stored internal strain energy is relieved by dislocation motion due to enhanced atomic diffusion at the elevated temperature. Even after recovery is complete, the grains are still in a relatively high strain energy state. 32 Recrystallization Recrystallization occurs when new strain-free, equiaxed grains (equal dimensions in all directions) form, showing low dislocation densities and characterizing the precold-worked condition. The new grains form as very small nuclei and grow until they completely consume the parent material 33 34 For pure metals, the recrystallization temperature is normally 0.4Tm, where Tm is the absolute melting temperature; for some commercial alloys it may run as high as 0.7Tm. 35 Grain growth After recrystallization is complete, the strain-free grains will continue to grow if the metal specimen is left at the elevated temperature; this phenomenon is called grain growth. Grain growth does not need to be preceded by recovery and recrystallization; it may occur in all polycrystalline materials, metals and ceramics alike. 36 Fracture Toughness The ability of a material containing a flaw to withstand an applied load. 37 Failure of material by crack initiation and crack propagation. Crack initiation Crack propagation Final Fracture 38 The ability of a material to resist the growth of a crack depends on: 1. Higher ductility 2. Low rate of application of load 3. Thin materials 4. Increasing the temperature 5. Small grain size And more 39 40 41 Materials Science for Engineering Technology 1 Course Content: Introduction Evolution of engineering materials, Classification of materials. Structures of Materials Atomic Structure, Bonding forces and energies, Primary interatomic bonds and secondary bonding. Crystalline and non-crystalline materials Phase diagram and microstructure Electrical and Optical Properties of Materials Conductors, semiconductors, and insulators. Mechanical Properties of Materials Tensile, compression, impact energy, fracture toughness. Mechanical behavior of Materials Stress-strain behavior, Elastic and plastic properties of materials, dislocations and strengthening mechanisms. Introduction to Failure Analysis and Prevention Fundamentals of Fracture Fractures Types (ductile, brittle, fatigue and creep), Corrosion, Nondestructive Testing and Techniques for failure analysis and Prevention. Selection of Engineering Materials 2 Characterization of Materials, Design and safety factors Fracture types 3 Fracture Toughness The ability of a material containing a flaw to withstand an applied load. 4 Failure of material by crack initiation and crack propagation. Crack initiation Crack propagation Final Fracture 5 The ability of a material to resist the growth of a crack depends on: 1. Higher ductility 2. Low rate of application of load 3. Thin materials 4. Increasing the temperature 5. Small grain size And more 6 7 Stress at a crack tip 𝜎!"#$!% 𝑎 ≅ 2𝜎 ⁄𝑟 Applied stress causes an elastic strain; when a crack propagates, this energy is released. The energy is used to create 2 new surfaces. 8 Critical stress required to propagate a crack (Griffith equation) 2𝐸𝛾 𝜎"&'#'"!% ≅ 𝜋𝑎 𝛾 is the surface energy per unit area. 9 Assume that an advanced ceramic (silicon aluminum oxynitride has a tensile strength of 60 MPa. Let us assume that this value is for a flaw-free ceramic. Before the part is tested, a thin crack 0.254 mm deep is observed. The propagation of the crack unexpectedly fails the part at a stress of 5 kPa. Estimate the radius of the crack tip. 10 Microstructural features of fracture in metallic materials: Ductile Fracture Ductile fracture normally occurs in a transgranular manner (through the grains) in metals that have good ductility and toughness. In many cases, a significant amount of deformation, including necking, is noticed in the failed component. 11  In a simple tensile test, ductile fracture begins with the nucleation, growth, and coalescence of microvoids near the center of the bar. Microvoids form when high stress causes separation of the metal at grain boundaries or interfaces between the metal and small impurity particles. As the local stress increases, the microvoids grow and coalesce into larger cavities. Eventually, the area of metal- to-metal contact becomes too small to support the load, and fracture occurs. 12 When a ductile material is pulled in a tensile test, necking begins and voids form. This process starts near the center of the bar and involves nucleation at grain boundaries or inclusions. As deformation continues, a 45° shear lip may form, resulting in a final cup and cone fracture. 13 14 Microstructural features of fracture in metallic materials: Brittle Fracture Brittle fracture occurs in high-strength metals and alloys or metals and alloys with poor ductility and toughness. Even metals that are normally ductile may fail in a brittle manner at low temperatures, at high strain rates (such as impact), or when flaws play an important role. Brittle fractures are frequently observed when impact, rather than overload, causes failure. 15  In brittle fracture, no plastic deformation is required. Initiation of the crack normally occurs at microcracks, which cause a concentration of stress. The crack may propagate at a rate approaching the velocity of sound in the metal. Normally, the crack propagates most easily along specific crystallographic planes by cleavage. In some cases, however, the crack may take an intergranular path (along the grain boundaries), particularly when segregation (preferential separation of different elements) or inclusions weaken the grain boundaries. 16 Metal Fracture Surface Normally, the fracture surface is flat and perpendicular to the applied stress in a tensile test. If failure occurs by cleavage, each fractured grain is flat and differently oriented, giving a crystalline or “rock candy” appearance to the fracture surface 17 The Chevron pattern occurs when distinct cracks propagate at varying depths within the material. A radiating pattern of surface markings, or ridges, fans away from the origin of the crack. The Chevron pattern helps to identify the brittle nature of the failure process and its origin. 18 Microstructural features of fracture in ceramics, glasses, and composites. Most crystalline ceramics fail by cleavage along closely packed planes. Glasses display a conchoidal fracture surface, which includes a smooth mirror zone near the origin of the fracture and tear lines for the rest of the surface. 19 20 Microstructural features of fracture in metallic materials: Fatigue Fatigue is the lowering of strength or failure of a material due to repetitive stress which may be above or below the yield strength. It is a common phenomenon in load-bearing components such as turbine blades, crankshafts, and other machinery and consumer products such as shoes that are constantly subjected to repetitive stresses in the form of tension, compression, bending, vibration, thermal expansion and contraction, or other stresses. When the stress occurs a sufficient number of times, it causes failure by fatigue 21 Fatigue failures of metals typically occur in three stages. First, a tiny crack initiates. Normally, cracks are located at or near the surface, where the stress is at a maximum, and include surface defects. Next, the crack gradually propagates as the load continues to cycle. Finally, a sudden fracture of the material occurs when the remaining cross-section is too small to support the applied load. 22 Metal Fracture Surface The surface of a metal fracture, especially near the starting point, is usually smooth. As the initial crack grows, the surface becomes rougher and may appear fibrous during final crack propagation. Microscopic and macroscopic examinations reveal a fracture surface including a beach mark pattern and striations (Figure 7-16). Beach or clamshell marks (Figure 7-17) are normally formed when the 23 load is changed during service or when the loading is intermittent, Metal Fracture Surface The surface of a metal fracture, especially near the starting point, is usually smooth. As the initial crack grows, the surface becomes rougher and may appear fibrous during final crack propagation. 24  Beach marks always imply fatigue failure, but their absence does not rule it out. 25 Microstructural features of fracture in metallic materials: Creep and Stress Rupture. The time-dependent permanent deformation under constant load or stress at high temperatures is known as creep. When a material is exposed to stress at a high temperature, it may stretch and fail, even if the applied stress is lower than the yield strength at that temperature. Creep in metallic materials can be caused by diffusion, dislocation glide or climb, and grain boundary sliding. 26  A material is considered failed by creep even if it has not actually fractured. When a material does creep and then ultimately breaks, the fracture is defined as stress rupture. Normally, ductile stress-rupture fractures include necking. Furthermore, grains near the fracture surface tend to be elongated. Ductile stress-rupture failures generally occur at high creep rates and relatively low exposure temperatures and have short rupture times. Brittle stress-rupture failures usually show little necking and occur more often at smaller creep rates and high temperatures. Equiaxed grains are observed near the fracture surface. Brittle failure typically occurs through the formation of voids at the intersection of three-grain boundaries and the precipitation of additional voids along grain boundaries. 27 Materials Science for Engineering Technology 1 Corrosion 2 Deteriorative mechanisms are different for each material type. Metals, Actual material lost by dissolution (corrosion) or by formation of non metallic film (oxidation), Ceramics are generally resistant to deterioration, which usually occurs at elevated temperatures or in rather extreme environments; this process is frequently also called corrosion. Polymers, the term degradation is most frequently used. Polymers may dissolve when exposed to a liquid solvent, or they may absorb the solvent and swell; also, electromagnetic radiation (primarily ultraviolet) and heat may cause alterations in their molecular structures. 4 Corrosion of Metals: Electrochemical consideration The corrosion of metals is primarily an electrochemical process, involving a chemical reaction where electrons are transferred from one chemical species to another. During the process, metal atoms typically lose or give up electrons in what is known as an oxidation reaction. 5 Oxidation (Anode reaction) The site where oxidation takes place is called anode. !" # M → M + ne $" # Fe → Fe + 2e %" # Al → Al + 3e 6 Reduction (Cathode reaction) Electrons generated from each metal must be transferred to and become a part of another chemical species termed reduction. 7 In an acidic solution " # 2H + 2e → H$ In an acidic solution having dissolved oxygen O! + 4H " + 4e# → 2H! O In a neutral or basic solution having dissolved oxygen O! + 2H! O + 4e# → 4(OH)# 8 Any metal ions present in the solution may also be reduced M !" + e# → M (!#')" M !" + ne# → M 9 The overall electrochemical reaction must include at least one oxidation and one reduction reaction, which are typically called half-reactions. It's important to note that there can't be a net electrical charge accumulation from the electrons and ions. This means that all electrons generated through oxidation must be consumed by reduction. 10 Zinc metal in an acidic solution Zn → Zn!" + 2e# 2H " + 2e# → H! ______________________ " !" Zn + 2H → Zn + H! 11 Electrode potentials Not all metals oxidize to form ions with the same degree of ease. $" # Fe → Fe + 2e $" # Cu + 2e → Cu $" $" Fe + Cu → Cu + Fe 12 Standard emf (electromotive force) series 13 14 Environmental Effect on Corrosion The environment in which corrosion occurs, including factors like fluid velocity, temperature, and composition, can significantly impact the corrosion properties of the materials in contact with it. Higher fluid velocity increases the rate of corrosion. Additionally, the rates of most chemical reactions increase with higher temperaturese, and increasing the concentration of corrosive species (e.g., H+ ions in acids) often leads to a more rapid rate of corrosion. Cold-worked metal is more susceptible to corrosion. 15 Forms of Corrosion: Galvanic Corrosion Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The more reactive metal in the particular environment experiences corrosion, and the more inert metal, the cathode, is protected from corrosion. For example, copper and steel tubing are joined in a domestic water heater; thee steel corrodes in the vicinity of the junction. 16 17 Prevention of Galvanic Corrosion 1. If coupling of dissimilar metals is necessary, choose two that are close together in the galvanic series. 2. Avoid an unfavorable anode-to-cathode surface area ratio; use an anode area as large as possible. 3. Electrically insulate dissimilar metals from each other. 4. Electrically connect a third, anodic metal to the other two; this is a form of cathodic protection. 18 Forms of Corrosion: Crevice Corrosion Electrochemical corrosion may occur due to differences in ion or dissolved gas concentrations within the electrolyte solution and between two areas of the same metal piece. This leads to corrosion in the zone with the lower concentration. 19 20 Crevice corrosion may be prevented by, Using welded instead of riveted or bolted joints, Using nonabsorbing gaskets when possible, Removing accumulated deposits frequently, 21 Forms of Corrosion: Pitting Pitting is another form of very localized corrosion attack in which small pits or holes form. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction. The mechanism for pitting is the same as for crevice corrosion, in that oxidation occurs within the pit itself, with complementary reduction at the surface. It is supposed that gravity causes the pits to grow downward, the solution at the pit tip becoming more concentrated and dense as pit growth progresses. 22 23 Prevention of Pitting It has been observed that specimens with polished surfaces exhibit greater resistance to pitting corrosion. Stainless steels are somewhat susceptible to this form of corrosion; however, alloying with about 2% molybdenum significantly enhances their resistance. 24 Forms of Corrosion: Intergranular Corrosion Intergranular corrosion occurs preferentially along grain boundaries for certain alloys and in specific environments, causing a macroscopic specimen to disintegrate along its grain boundaries. 25 Intergranular corrosion is an especially severe problem in the welding of stainless steel when it is often termed weld decay. Stainless steels may be protected from intergranular corrosion by the following measures: 1. Subjecting the sensitized material to a high-temperature heat treatment in which all the chromium carbide particles are redissolved, 2. Lowering the carbon content below 0.03 wt% C so that carbide formation is minimal. 3. Alloying the stainless steel with another metal such as niobium or titanium, which has a greater tendency to form carbides than does chromiu, so that the Cr remains in a solid solution. 26 Forms of Corrosion: Selective Leaching Selective leaching occurs in solid solution alloys when one element is preferentially removed due to corrosion processes. A common example is dezincification of brass, where zinc is selectively leached from a copper–zinc brass alloy. This process significantly impairs the mechanical properties of the alloy, leaving behind only a porous mass of copper in the dezincified region. 27 Forms of Corrosion: Erosion-Corrosion Erosion–corrosion arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. It is especially harmful to alloys that passivate by forming a protective surface film; the abrasive action may erode away the film, leaving exposed a bare metal surface. 28 Forms of Corrosion: Stress Corrosion Stress corrosion, sometimes referred to as stress corrosion cracking, happens when both an applied tensile stress and a corrosive environment act together. Both factors are required for this type of corrosion to occur. Some materials that are usually resistant to corrosion in a specific environment become vulnerable to stress corrosion when under stress. Cracks start to form and spread in a direction perpendicular to the applied stress, potentially leading to failure. These cracks can appear at stress levels much lower than the tensile strength of the material. The behavior of the material during failure resembles that of a brittle material, even if the metal alloy itself is ductile. 29 30 Forms of Corrosion: Hydrogen Embrittlement Certain metal alloys, particularly some types of steel, can undergo a significant decrease in ductility and tensile strength when atomic hydrogen (H) seeps into the material. This phenomenon is known as hydrogen embrittlement (hydrogen- induced cracking, hydrogen stress cracking). Atomic hydrogen (H) diffuses interstitially through the crystal lattice, and even low concentrations of several parts per million can cause cracking. Several mechanisms have been suggested to explain hydrogen embrittlement, most of which are based on the interference of dislocation motion by the dissolved hydrogen. 31  Hydrogen embrittlement is similar to stress corrosion, as both cause a normally ductile metal to undergo brittle fracture when exposed to both tensile stress and a corrosive atmosphere. However, these two phenomena can be distinguished because while cathodic protection reduces or ceases stress corrosion, it can lead to the initiation or worsening of hydrogen embrittlement. 32 Protection Against Electrochemical Corrosion Design Prevent the formation of galvanic cell | Larger anode than cathode | Design to close the fluid system | Avoid cavities between joined materials Coatings Isolate the anode and cathode and prevent diffusion of oxygen and water Inhibitors In the presence of chromate salt, produce a protective film on the anode or cathode Passivation or Anodic Protection If iron is dipped in very concentrated acid, the iron rapidly and uniformly corrodes to form a thin, protective iron hydroxide coating. The coating protects the iron from subsequent corrosion in nitric acid. 33  Cathodic Protection A sacrificial anode is attached to the material to be protected Materials selection and treatment 34 35 Materials Science for Engineering Technology 1 Course Content: Introduction Evolution of engineering materials, Classification of materials. Structures of Materials Atomic Structure, Bonding forces and energies, Primary interatomic bonds and secondary bonding. Crystalline and non-crystalline materials Phase diagram and microstructure Electrical and Optical Properties of Materials Conductors, semiconductors, and insulators. Mechanical Properties of Materials Tensile, compression, impact energy, fracture toughness. Mechanical behavior of Materials Stress-strain behavior, Elastic and plastic properties of materials, dislocations and strengthening mechanisms. Introduction to Failure Analysis and Prevention Fundamentals of Fracture Fractures Types (ductile, brittle, fatigue and creep), Corrosion, Nondestructive Testing and Techniques for failure analysis and Prevention. Selection of Engineering Materials 2 Characterization of Materials, Design and safety factors Failure analysis 3 Problem-solving model 4 The major steps in the model define the problem-solving process: 1. Identify: Describe the current situation. Define the deficiency in terms of the indicators. Determine the impact of the deficiency. Collect data to provide a measurement of the deficiency. 2. Determine root cause: Analyze the problem to identify the cause(s). 3. Develop corrective actions: List possible solutions to mitigate and prevent the recurrence of the problem. Develop an implementation plan. 4. Validate and verify corrective actions: Test the corrective actions in a pilot study and verify that the problem has been corrected. 5. Standardize: Incorporate the corrective action into the standards documentation system of the company, organization, or industry to prevent recurrence in similar products or systems. Monitor changes to ensure effectiveness.  Early life failures: Failures during this phase are usually caused by manufacturing defects, insufficient initial quality, or improper handling and installation. Intrinsic failure period: the product or system operates with a stable and predictable failure rate. Failures can occur due to environmental stresses, or random component failures. Wear-out failure period: Components may degrade over time due to wear and deterioration beyond their design limits.  Steps in performing failure analysis Fatigue Failure Analysis of a Centrifugal Pump Shaft Mohd Nasir Tamin and Mohammad Arif Hamzah http://dx.doi.org/10.5772/intechopen.70672 7 1. Description of the failure situation It was reported that the shaft of a centrifugal pump used to pump the blending of hydrocarbons to deliver the final oil product in a refinery has failed during operation. The failure resulted in a fire of the pump and the piping works of the refinery within the unit, with an estimated total loss of USD 48,000. During the last 12 h before the final fracture of the shaft, a total of 12 start-and-stop operations of the centrifugal pump have been scheduled. The centrifugal pump was installed and commissioned some 30 years ago. There have been three major repairs of the pump involving leaking of the seal. A mechanical seal was installed on the threaded portion of the shaft with a preload corresponding to 25– 30% of the material yield strength. However, no reported abnormality on the shaft was recorded. A typical operation cycle of the centrifugal pump consists of a start-up and running of the pump at a nominal rotor speed of 2975 rpm for 14 h, with a 2-h complete shut- down interval. The pump operates between 5 and 7 days a week throughout the year. 8 2. Visual inspection 9 The drive end of the shaft is connected to the shaft of the motor using the coupler. The distance between the bearing supports is 982 mm. The middle section of the stepped shaft with the largest diameter of 65 mm carries the impeller that is positioned in place with a key. The key way has the dimensions of 9 mm radius, length width of 60 x18 mm2, and the depth of 9 mm. Both ends of this section are threaded with M65 x 1.5 threads to receive the mechanical seals (only the critical threaded portion, located on the right side of the section, is drawn). A greater portion of the surface was flattened and smeared off due possibly to the repeated grinding against the fracture surface of the mating part while the motor runs after the complete fracture. Thus, details of the fracture feature could not be extracted easily from the fractograph. However, traces of beach marks indicating fatigue failure are obvious. It is worth noting that the fracture plane is oriented almost perpendicular to the longitudinal plane of the shaft. Such orientation of the fatigue fracture plane is indicative of the Mode-I (opening) crack propagation under the induced flexural fatigue loading. 10 3. Mechanical design analysis Mechanical design analysis is performed to examine the adequacy of the design against yield and fatigue failure of the shaft material. The analyses consist of the stress calculations, particularly at the observed fractured section of the rotor shaft. The critical stress states are then compared to the respective strengths of the shaft material to establish the possible causes of failure. In this respect, the stress levels in the rotor shaft arising from three different load cases are considered, as follows: 1. Stresses during the pumping operation at the rated load. 2. Stresses during the transient start-and-stop operation: 3. Additional stresses due to preloading of the lock nut for the mechanical seal: 11 4. Chemical design analysis The chemical design analysis is performed to establish the conformance of the failed shaft material to the manufacturer’s materials specification. 12 5. Metallographic examination 13 6. Determine properties The required set of mechanical properties of the Cr-Mo steel for use in the stress analysis is obtained from published literature. The properties are based on data for AISI 4140, oil- quenched and tempered at 650 C to 285HB. The tensile strength (SU) and yield strength (SY) of the material is 758 and 655 MPa, respectively. The cyclic yield strength S0Y is estimated at 458 MPa. The endurance limit (S0e) is reported to be 420 MPa at 107 cycles. Since the reported fatigue limit is often established using smooth specimens, it should be corrected to account for the surface condition at the fracture location and the large diameter of the rotor shaft relative to the fatigue test specimens. The surface of the fractured threaded region was machine-finished, and thus the fatigue limit-modifying factor, ka = 0.72 (refer to Figure A1a of the Appendix). Consider the size effect based on the root diameter of the shaft with M65 1.5 threads, the corresponding fatigue limit-modifying factor, kb = 0.795, as determined from Figure A1b of the Appendix.