Lecture Notes Of Engineering Materials

Prepared by: Dr.D.K.Hurreeram August 04 B. Engg. Mechanical Engineering Level 2 MECH 2104 Material Science Lecture Notes 1999/2000 Hurreeram D K 1.0 Atomic Structure and Inter Atomic Bonding 1.1 Course Perspective 1.2 Atomic and Crystalline Structure of Metals 1.3 Atomic Structure 1.4 Electrons in an atom 1.5 Atomic Bonding in Solids 2.0 Crystalline Structure of Solids 2.1 Introduction 2.2 Crystal Structure 2.3 Imperfection in Solids 3.0 Physical and Mechanical Properties of Metals 3.1 Testing Techniques DT and NDT (Tension, Compression, Torsion, Surface) 3.2 Physical Properties 3.3 Mechanical Properties 3.4 Other Properties 4.0 Deformation of crystalline Materials 4.1 Dislocations and Plastic Deformation 4.2 Slip Systems 5.0 Strengthening Mechanism in Metals 5.1 Equilibrium Phase Diagrams 5.2 Iron-Carbon System 6.0 Thermal Processing of Metals and Alloys 6.1 Cold and Hot Working 7.0 Failure Mechanism of Materials in Service 7.1 Brittle and Ductile Failure 7.2 Creep and Fatigue 7.3 Corrosion Prepared by: Dr.D.K.Hurreeram August 04 1.0 Course Perspective Materials involved in all aspects of human life • Transportation, Housing, Clothing, Communication, Food Production, Recreation • Civilization designated by level of materials development • Stone, wood, clay, skins, pottery, metals • New developments in materials production and use • ability to alter properties according to requirement • selection of materials out of specifications • aim to provide comfort to man • Material Science - investigate relationship between structures and properties of materials • Structure relate to arrangement of its internal components • Microscopic (using microscope) and macroscopic (viewed with naked eye) structures • Property relate to the response of materials exposed to external stimuli • Force, heat, environment • Basically six properties • mechanical, thermal, electrical, magnetic, optical, deteriorative • Why study material science? • design problems, material selection problems, economic or cost problems • Classification of materials • Metals, Ceramics, Polymers, Composites, Semiconductors • Modern Materials needs

• Energy consumption • Nuclear Power Usage • Environment quality (pollution control) • Renewable resources 1.1 Atomic and Crystalline Structure of Metals 1.2 Atomic Structure Properties of solid materials - Function of geometrical atomic arrangement and interaction between constituent atoms or molecules • The Atom - Nucleus (Neutrons, Protons), Electrons, • Electron, Proton are electrically charged ( magnitude of 1.6E-19 C) • Neutron - electrically neutral • Neutron and Protons have approximately the same mass (1.67E-27 kg) Cloud of electrons Neutrons and protons (in nucleus) Prepared by: Dr.D.K.Hurreeram August 04 • Electron Mass (9.11E-31 kg, negligible compared to the above) • Atomic Number (Z) - Number of Protons in the nucleus (same for all atoms of a given element), 1 for Hydrogen to 94 for Plutonium (Periodic Table) • Atomic Mass (A)- Sum of Masses of Protons and Neutrons within nucleus. The number of neutrons(N) may be variable (isotopes) • Atomic Weight - weighted average of the atomic masses of the atom’s naturally occurring isotopes • Atomic Mass Unit - used for computations of atomic weight. 1 AMU = 1/12 of the atomic mass of the most common isotope of Carbon, Carbon 12, 12C (A=12) A=Z+N • Atomic Weight of an element or molecular weight of a compound can be specified on the basis of AMU’s per atom or mass per mole of material. In one mole of a substance there are 6.023E23 atoms or molecules (Avogadro’s Number) 1 AMU/atom (or molecule) = 1 g/mol 1.3 Electrons in an atom • Quantum Mechanics (Bohr atomic model - Fig 1 above) • electrons assumed to revolve around nucleus in discrete orbital (position well defined in terms of its orbital) • energy of electrons are quantised (change of energy levels or states) • Calculations concerning Bohr’s model • momentum equation • energy emission or absorption • force of attraction between electrons and nucleus • radii of orbits • velocity of electrons • orbital frequency • electron energy • energy levels • Wave mechanical Model (position defined by the probability of electrons being at various locations around the nucleus - electron cloud instead of orbit, Fig 1b above) • Quantum Numbers - each electron characterised by four parameters; the size, shape and spacial orientation of an electron’s probability density are specified

by three of these quantum numbers. • Bohr’s energy levels separate into electron shells designated by Principal quantum number (n=1,2,3,4,5…or K,L,M,N,O….) • Second quantum number signifies subshells (l=s,p,d,f) • Third quantum number define energy states in each subshells • Fourth quantum number define associated spin moment of electron +1/2 or -1/2 orientation (up or down) Prepared by: Dr.D.K.Hurreeram August 04 Table 1 No. of Available Electron States in some of the Electron shells and subshells Principal Quantum No. Shell Designation Sub-shells Number of States Number of Electrons Per Subshell Per Shell 1 K s 1 2 2 2 L s 1 2 8 p 3 6 3 M s 1 2 18 p 3 6 d 5 10 4 N s 1 2 32 p 3 6 d 5 10 f 7 14 • Electron Configuration • Ground State and Exited state (Fig 2) Fig 2 Relative Energies of electrons for the various shells and sub-shells Table 2 Electron Configuration of some elements Element Symbol Atomic Number Electron Configuration Hydrogen H 1 1s1 Helium He 2 1s2 Carbon C 6 1s2 2s2 2p2 Fluorine F 9 1s2 2s2 2p5 Neon Ne 10 1s2 2s2 2p6 Aluminium Al 13 1s2 2s2 2p6 3s2 3p1 Iron Fe 26 1s2 2s22p6 3s2 3p6 3d6 4s2 Copper Cu 29 1s2 2s22p6 3s2 3p6 3d10 4s1 1 2 3 4 5 6 7 s s p s p d f f d Energy Prepared by: Dr.D.K.Hurreeram August 04 • Valence Electrons - electrons occupying outermost shells; most important as they participate in bonding between atoms and molecules. Many

physical and chemical properties are based on the valence electrons • Stable electron configuration (rare gases, Ne, Ar, Kr, He) • The periodic table 1.4 Atomic Bonding in Solids • knowledge of physical properties of materials • Interaction between two atoms; the ideal case (forces and potential energy models Fig 3) Figure 3 Repulsive, attractive and net forces (potential energy) as a function of interatomic spacing for two isolated atoms • Bonding energy (minimum potential energy) • In reality, three types of primary bonding + Secondary bonding • Ionic, Covalent and Metallic bonding, Van der Waals, Hydrogen bonding • Ionic bonding - transfer of valence electrons (formation of ions) • Covalent bonding - sharing of electrons • Metallic bonding - ion ores and electron cloud formation • Van der Waals bonding - from dipoles • Hydrogen bonding - special case of Van der Waals bonding Net force FA FR ro Force of attraction Repulsive force Interatomic separation Prepared by: Dr.D.K.Hurreeram August 04 Table 3 Bonding energies and Melting temperatures for various substances Bonding Type Substance Bonding Energy Melting Temperature kJ/mole eV/atom, ion, molecule oC Ionic NaCl 640 3.3 801 MgO 1000 5.2 2800 Covalent Si 450 4.7 1410 C (diamond) 513 7.4 >3550 Metallic Al 324 3.4 660 Fe 406 4.2 1538 W 849 8.8 3410 Van der Waals Ar 7.7 .08 -189 Cl 31 .32 -101 Hydrogen NH3 35 .36 -78 H2O 51 .52 0 2.0 CRYSTAL STRUCTURE OF MATERIALS 2.1 Introduction Why metals have different properties? Soft, hard, brittle, ductile Atomic Structure - electron structure of individual atoms; bonding mechanisms Other factors that predict properties: • crystal structure (solid state) • composition of metal • impurities and vacancies • grain size, boundaries, environment, surface condition • method of manufacture

2.2 Crystal Structure • Solid materials classified according to regularity with which atoms or ions are arranged with respect to one another - over large atomic distances • atoms bonded to nearest neighboring atom • all metals, many ceramic materials, certain polymers form crystalline structures on solidification • Unit cell (small repeating entities) - basic structural unit or building block of the crystal structure and defines the crystal structure by virtue of its geometry and the atom positions within. • Solidification process • Amorphous material (non-crystalline) - atomic structure resembles that of a liquid (supercooled liquid); rapid cooling e.g. SiO2 , inorganic gases, polymers • Atomic hard sphere model ; Lattice Structure, Space lattice • Lattice parameters of a unit cell (Fig 2.1) Z Prepared by: Dr.D.K.Hurreeram August 04 Fig 2.1 Form of the unit cell is a function of the interfacial angles and dimensions Seven different crystal systems available cubic, monoclinic, triclinic, tetragonal, orthorhombic, rhombohedral, hexagonal, Examples of materials Table 3.1 14 possible types of space lattices in these seven systems of crystals (Bravais Lattice structure) • Metallic crystal structures • most common in metals BCC, FCC, HCP (simplest atomic structures using hard sphere model): Fig 3.2 • Co-ordination Number • defined as number of nearest atoms which are directly surrounding a given atom • Simple Cubic Structure = 6, BCC = 8, FCC = 12, HCP = 12 • Atomic Radius defined as half the distance between nearest neighbours in a crystal of a pure element (assume spherical model and calculate radius for each structure) • Number of atoms per unit cell (calculate) • Atomic Packing Factor • defined as ratio of volume of atoms per unit cell to total volume occupied by the unit cell (calculate) Type of cubic cell Atomic Radius Number of atoms per unit cell Atomic packing factor Coordination number Simple cubic a/2 1 0.53 6 Body centred cubic 3a/4 2 .68 8 Face Centred Cubic a/2 2 4 .74 12 • Worked examples • Crystal Directions, Planes and Miller Indices a

b c X Y ß Z Prepared by: Dr.D.K.Hurreeram August 04 3.0 Imperfection in Solids • Ideal crystal does not exit; all contain large no. of defects or imperfections based on theoretical and experimental results (e.g. Modulus of Elasticity) • Properties of material affected (tensile strength, deformation mechanism etc..) • Imperfections classified according to geometry of the defect • Point defect • Vacancy, self interstitial • Number of vacancies Nv=N exp (-QvkT) Qv= Activation Energy, k=Boltzman Constant (example Nv for Cu at 1000 o C. Qv= .9 eV/atom, atomic weight= 63.5 g/mol, density =8./4 g/cm3. ) • Impurities in solid solutions (alloys) • substitutional and interstitial solid solutions • Cu/Ni alloy, Fe alloys with Carbon. • Line defect (Dislocations-linear defects) • edge dislocations • screw dislocations • mixed dislocations • Surface defect (Interfacial defects) • External surfaces (liquids) • Grains and Grain boundaries • Twin boundaries • Bulk or Volume defects • Pores, cracks, foreign inclusions, other phases Y X Substitutional impurity atom Self interstitial atom Vacancy Interstitial impurity atom Edge dislocation Grain boundary Prepared by: Dr.D.K.Hurreeram August 04 3.1 Deformation of crystal structures • Change in dimension or forms of matter under the action of applied forces (permanent or temporary) • Types of metal deformation • Elastic deformation • Elastometric deformation (some polymers, rubber, elastomers) • Plastic deformation • Slip mechanism 4.0 Strengthening Mechanism in Metals • Plastic deformation - ability of dislocations to move • Restricting or hindering dislocation motion renders metal harder and stronger • Strengthening by grain size reduction

• Solid solution hardening • Strain hardening • Recovery, Recrystallisation and Grain Growth 5.0 Physical and Mechanical Properties of Metals • Physical properties • Types of forces (Tensile, compressive, shear, torsion) • Concepts of Stress and Strain • Relationship between applied load and deformation • Tensile, Compressive, Shear stress and strain • Tension, compressive, shear and torsional tests • Stress/Strain behaviour • Mechanical properties (Modulus of Elasticity, Hooke’s Law, Modulus of Rigidity or Shear Modulus, Limit of Proportionality, Elastic and Plastic deformation, Tensile Strength, Proof Stress, Permanent set, Necking, Ductility, Brittleness, Toughness, Poisson Ratio, Yielding and Yield Strength, Resilience, Stiffness, True Stress and True Strain Extension or Strain Force or Stress Elastic deformation Plastic deformation Elastometric deformation Prepared by: Dr.D.K.Hurreeram August 04 6.0 Hardness • Rockwell Hardness Tests • Brinell Hardness Tests • Knoop and Vickers Micro Hardness Tests • Impact Tests • Charpy and Izod Tests • Hardness and Tensile Strength • Safety Factor • Fatigue • Creep • Failure Modes