PART 1- INTRODUCTION

UNIVERSITY OF MAURITIUS FACULTY OF ENGINEERING MECHANICAL AND PRODUCTION ENGINEERING DEPARTMENT ( MECH 2001Y & MECH 2006Y ) LECTURER: Mr. Main Topics to be Covered Atomic Structure and Inter Atomic Bonding Crystalline Structure of Solids Imperfections in Solids Physical and Mechanical Properties of Metals Plastic Deformation of Crystalline Materials Failure Mechanism of Materials in Service PART 1- INTRODUCTION ENGINEERS MUST BE AWARE OF; Types of materials available Understand their general behaviour and capabilities Failure mechanism of the materials in service Metals and Alloys: e.g lead, steel, zinc, aluminium, stainless steel Semiconductors: e.g. Si, Ge, Gallium Arsenide Ceramics: e.g. brick, glass, refractories, abrasives Polymers: Produced by creating large molecular structures from organic molecules by polymerisation. Composites: Formed from 2 or more materials producing properties not found in any single material The Structure-Properties-Processing relationship Structure: Atomic Structure Crystal Structure Grain Structure 1

Properties: Mechanical properties e.g creep, fatigue, wear, strength, ductility, etc. Physical properties e.g electrical, magnetic, optical, thermal etc. Material processing: Casting Joining Machining Forming Powder metallurgy 2

The Dalton Atomic theory. PART 2- THE ATOMIC STRUCTURE Atom Molecule Element, Compound Mixture - Constituents cannot be physically separated -Composition may vary -Properties are the sum of properties of each component Symbol, formula Relative Atomic Mass Avogadro s Constant (6.023 * 1023 atoms). One mole of an element or compound contains 6.023 * 1023 particles. Isotopes Atomic Structure - Each atom consists of a very small nucleus composed of protons and neutrons, encircled by moving electrons Atomic number, Mass number Ionisation Energy Atomic Mass = # Protons + # Neutrons The atomic mass of carbon = 12 The atomic # of carbon = 6 = the # of protons # neutrons = Atomic Mass - # protons # neutrons =12-6 = 6 The energies of electrons are quantized that is the electrons are permitted to have only specific values of energy. An electron may change energy, and it must then make a quantum jump to either an allowed higher energy (with absorption of energy) or a lower energy (with emission of energy) The orbital is an electron probability density cloud surrounding the nucleus. Each orbital is a quantum state with a set of quantum numbers: Principal Quantum Number (n) Orbital Angular Momentum Quantum Number (l) Magnetic Quantum Number (m l ) Electron Spin Quantum Number (m s ) The smaller the principal quantum number,m, the lower the energy level Within each shell, the energy of the sub shell increases with the value of l 3

Energy Sublevels (Orbitals) Principal Quantum # Sublevel #, 1 0 s 2 0,1 s, p Sublevel Letter 3 0,1,2 s, p, d 4 0,1,2,3 s, p, d, f 5 0,1,2,3 s, p, d, f 6 0,1,2,3 s, p, d 7 0 s As more elements are identified, the sublevels of 5,6 and 7 will fill. E l e c t r o n E n e r g y L e v e l s Energy Level 2n 2 1 2(1 2 ) 2 2 2(2 2 ) 8 3 2(3 2 ) 18 4 2(4 2 ) 32 Possible # of electrons 5 2(5 2 ) 50 (theoretical, not filled) 6 2(6 2 ) 72 (theoretical, not filled) 7 2(7 2 ) 98 (theoretical, not filled) 4

Bohr Atomic Model Electron are assumed to revolve around the nucleus in discrete orbitals and the position of any particular electron is more or less well defined in terms of its orbitals The energies of the electrons are quantized. An electron may make a quantum jump with emission or absorption of energy Nuclear atom model in which the massive nucleus is surrounded by planetary electrons Revolving around it. The centripetal force required for rotation is provided by electrostatic attraction between the electrons and the nucleus. An electron can revolve only in a few widely separated permitted orbitals. While moving in The permitted orbits the electron does not radiate energy. The permissible orbits of an electron revolving round a positive nucleus are those whose angular momentum of the electron is an integral multiple of h/2π. The emission and absorption of energy takes place when an electron jumps from one permitted orbit to another. Wave Mechanical Atomic Model Electron considered to exhibit both particle-like and wavelike characteristics. The orbital is an electron probability density cloud surrounding the nucleus. Electrons no longer considered as particles in discrete orbitals but rather position is considered to be the probability of an electron s being at various locations around the nucleus. The Heisenberg Uncertainty principle states that the momentum and position cannot be specified precisely. 5

6

PART 3- BONDING SOLID - Atoms vibrating about their mean positions on fixed atomic sites. LIQUID - Atoms have translational freedom and can slide past each other. The bonds are continuously broken and remade. GAS - The bonds are completely broken. Primary Bonds IONIC Bonds COVALENT Bonds METALLIC Bonds Secondary Bonds. Dipole interactions between; - Induced dipoles - Induced dipoles and polar molecules - Polar molecules HYDROGEN Bonds Van Der Waals Forces Directional and Non-Directional bonds MIXED BONDING: In Practice most materials consist of a mixture of bonding e.g. Metallic-covalent or ionic- covalent. Variation in bonding characteristics and properties Bonding and Properties of TRANSITION METALS 7

PART 4- THE CRYSTAL STRUCTURE A CRYSTALLINE MATERIAL IS ONE IN WHICH THE ATOMS ARE SITUATED IN A REPEATING OR PERIODIC ARRAY OVER LARGE ATOMIC DISTANCES. THE UNIT CELL IS THE BASIC STRUCTURAL UNIT OR BUILDING BLOCK OF THE CRYSTAL STRUCTURE AND DEFINES THE CRYSTAL STRUCTURE BY VIRTUE OF ITS GEOMETRY AND T HE ATOM POSITIONS WITHIN. ATOMIC HARD SPHERE MODEL : SEVEN CRYSTAL STRUCTURES 1. CUBIC (SIMPLE, FACE CENTRED, BODY CENTRED) 2. HEXAGONAL 3. MONOCLINIC ( SIMPLE, END-CENTRED) 4. TRICLINIC 5. RHOMBOHEDRAL 6. ORTHORHOMBIC (SIMPLE, BODY-CENTRED,END-CENTRED, FACE- CENTRED) 7. TETRAGONAL ( SIMPLE, BODY-CENTRED) NUMBER OF ATOMS ASSOCIATED WITH EACH UNIT CELL COORDINATION NUMBER ATOMIC PACKING FACTOR APF = VOLUME OF ATOMS IN A UNIT CELL TOTAL VOLUME OF UNIT CELL CRYSTALLOGRAPHIC DIRECTIONS: CRYSTALLOGRAPHIC PLANES: MILLER S INDICES & MILLER-BRAVAIS INDICES LINEAR DENSITY AND LINEAR PACKING FACTOR PLANAR DENSITY AND PLANAR PACKING FACTOR INTERPLANAR SPACING BRAGG S LAW 8

X-RAY DIFFRACTION CRYSTALS WITH IONIC, COVALENT AND MIXED BONDING. The NaCl, CsCl, Zn S, Diamond Cubic structures NUMBER OF ATOMS ASSOCIATED WITH EACH UNIT CELL COORDINATION NUMBER ATOMIC PACKING FACTOR 9

PART 5 CRYSTAL IMPERFECTIONS CRYSTAL IMPERFECTIONS: Classified on basis of their geometry- - Point imperfections -Line imperfections (Dislocations) - Surface imperfections - Volume imperfections (pores, inclusions..) POINT IMPERFECTIONS (i) Vacancy (ii) Interstitial defects & Self interstitial (iii)substitutional defects (iv)frenkel Defect (v)schottky defect DISLOCATIONS: (I) Edge dislocations (ii) Screw dislocation The Burger s vector THE SLIP PROCESS: The process by which a dislocation moves and causes a material to deform is called the slip process. IMPORTANT FACTORS: The relation between stress required to cause the dislocation to move and Burger s vector and interplanar spacing Movement of dislocations in materials such as silicon or polymers which have covalent bonds. Movement of dislocations in materials with an ionic bond SIGNIFICANCE OF DISLOCATIONS: Slip explains why the strength of metals is much lower than the value predicted from the metallic bond. Slip provides ductility in the metal. Can control the mechanical properties by interfering with the movement of dislocations. CONTROL OF SLIP PROCESS (I)Strain hardening (II) Solid solution hardening (iii) Grain size strengthening 10

TWINNING: PART 6 PHASES AND PHASE CHANGES Phases. Grain Structure & Grain Size Equilibrium cooling Phase equilibrium diagrams Non equilibrium cooling Time Temperature Transformation Curves & Continuous Cooling Transformation Curves Recrystallization Heat treatable alloys Non heat treatable alloys Level Rule Heat Treatment processes Mechanical Working of Metals Fusion Welding 11

PART 6 - MECHANICAL PROPERTIES Properties: Strength, Hardness, Ductility, Hardenability, Formability, Impact resistance, Fatigue properties, Weldability, Appearance, Availability, Machinability, Cost Materials are selected by matching its properties to the service conditions required. Important Selection Factors Select a Material System Metals Plastics Ceramics Other Select a Specific Material Standard tests available: Destructive testing & Non destructive testing. The Tensile Test The Hardness Test : Rockwell, Brinell, Vickers, Knoop, Shore Scleroscope The Bend Test The Creep Test The Fatigue Test The Impact Test- Charpy s and Izod s DATA SOURCES: Data on the properties on materials is available from a range of sources such as: Specifications issued by bodies responsible for standards e.g. BS, ASM, ASTM, MS Data books e.g. ASM metal reference book (American Society for Metals, 1983), Metals Reference Book (by R. J. Smithells, Butterworth, 1987), Metals Databook (by C. Robb, The Institute of Metals, 1987), Handbook of Plastics and Elastomers (edited by C. A. Harper, Mc Graw Hill, 1975), Newnes Engineering Materials Pocket Book (by W. Bolton, Heinemann- Newnes, 1989) Computerised databases which give materials and their properties e.g Cambridge Materials Selector Trade Associations e.g. the Copper Development Association, Zinc Development Association, Aluminium Federation Data sheets supplied by the suppliers 12

PART 7 DEFORMATION OF METALS Stress-Strain relationship. Elasticity, Limit of proportionality, Limit of elasticity, Upper yield point & Lower yield point, Yield Strength, Ultimate tensile stress (Tensile strength), Young s modulus of elasticity, Proof stress, Ductility, % elongation, % reduction in area, Secant modulus, Modulus of resilience, Poisson s ratio Hooke s Law Ductile fracture & Brittle fracture The ductile to brittle transition : Work Hardening: 13

PART 8- CORROSION What is corrosion? Corrosion is the deterioration of a substance (most commonly a metal) or its properties when the substance/metal reacts with the surrounding environment. It is a process whereby a substance/metal forms a compound that is more stable, (for metals, this means returning to its natural ore form.) What causes corrosion? Corrosion is frequently caused by an electro-chemical reaction that is similar to the reaction that makes a battery work (the flow of electrons from the anode (positive end) to the cathode (negative end)). The surrounding environment most often provides the necessary ingredients (heat, moisture, acids, chemical contaminants, for example salt, nitrogen and sulfur oxides, etc.), that creates the electro-chemical reaction on the surface of substance/metal. Galvanic Cell may also occur when two pieces of the same metal are placed in 2 different electrolytes or the same electrolyte of different concentration and separated by a porous membrane/substance. ytwo different phases in the same metal may form a galvanic couple ygalvanic cell may be set up due to differences in concentration of the metal ion in the electrolyte ya galvanic cell may also arise due to differences in the oxygen concentration. THE RATE OF CORROSION Metal dependent factors Environment dependent factors. THE STANDARD ELECTRODE POTENTIAL THE STANDARD EMF SERIES AND THE GALVANIC SERIES FORMS OF CORROSION: Uniform attack Galvanic corrosion Crevice corrosion Pit corrosion Intergranular corrosion Selective leaching Erosion corrosion Stress corrosion 14

CONTROL OF CORROSION: Corrosion can be controlled by providing some means that would circumvent the electro-chemical reaction from occurring (stop corrosion) or drastically reduce the rate of the reaction (rate of corrosion). Selection of materials Modification to the design Modification to the environment Application of barrier coats Cathodic protection Anodic protection Passivation: POLARISATION 15