Material Science Structures and Properties of Metallic Materials Ceramics Polymers Composites Encompasses - Electronic, Magnetic, Optical, Mechanical, and Chemical Properties FE/EIT Exam - Two Major Areas - Fundamentals of 1. Strength, Deformation, Plasticity of Crystalline Solids 2. Phase Equilibrium in Metallic Systems

Mechanical Properties of Metals and Alloys Experimental Techniques - Response to Applied Stress Capacity to withstand static load (Tension / Compression) Resistance to permanent deformation (Hardness) Toughness under shock loading (Impact) Useful life under cyclic loading (Fatigue) Elevated temperature behavior (Creep and Stress Rupture)

Tension Testing s = Stress e = Strain Two distinct stages of deformation Elastic Deformation (Reversible Change in Volume) Plastic Deformation (Irreversible Constant Volume) Elastic Deformation Hooke’s Law s = Ee s = Stress e = Strain E = Young’s Modulus / Modulus of Elasticity Plastic Deformation

Plastic Deformation (Non-Linear) Yield Stress = sy Off-Set Yield = s0.2% Ultimate Tensile Strength = suts Fracture Stress = sf (sf < suts) Ductility Work Hardening / Strain Hardening Figure 3.1 Figure 3.2 Figure 3.3-4

Nature of Plastic Flow For Crystalline Material (including metals and alloys) Plastic deformation involves sliding of atomic planes called slip deformation, analogous to shear. Slip System - Combination of a close-packed plane and a close-packed direction. Slip occurs along planes and are restricted in crystallographic directions that are the most densely packed. The greater the planes and directions, the easier it is to produce plastic slip without brittle fracture.

Slip Deformation - continued Slip occurs when the resolved component of Shear Stress tR = P/A cosf cosl exceeds the critical value Critical Resolved Shear Stress (tR)crit Dislocation Edges (tRcrit < 1/5 Theoretical) Dislocation Lines & Frank-Reed Source Figure 3.6 Figure 3.7-8 Figure 3.9

Compressive Strength Compressive Stress similar to Tensile Stress (except no necking in pure compression) quite useful for materials which are brittle in tension, but have significant compressive load bearing capabilities (concrete, cast iron, etc).

Hardness Test Determines resistance to penetration of a stylus. Useful for qualitative estimate of service wear, strength, and toughness. Brinell, Rockwell, Vickers, MicroHardness Table 3.1

Fatigue Test Cyclic Load - Fatigue Life Number of Cycles (N) to Failure with Cyclic Stress Amplitude (S) Steel - Critical Value of Stress = Scrit Endurance Limit Aluminum - No Endurance Limit Figure 3.10

Fatigue Testing - continued Fatigue fractures are progressive. Fatigue Strength Maximum Cyclic Stress Amplitude for a specified number of cycles until failure. Fatigue is a surface active failure. Surface defect (notch) can initiate crack. Rough surface reduces fatigue strength by 25%. Cold rolling/shot peening increases by 25%. Corrosive Fatigue important cause of service failure.

Fatigue Testing - continued Fatigue Life / Fatigue Strength improved by Highly Polished Surface Surface Hardening Carburizing, Nitriding, etc. Surface Compression Stresses Shot Peening, Cold Rolling, etc.

Toughness and Impact Testing Impact Value Simple evaluation of the notch toughness. Toughness A measure of energy absorption before failure. Charpy and Izod Machines Swinging pendulum loading with notched-bar samples. Figure 3.11

Creep at High Temperature (Stress Rupture) Creep - Progress deformation at constant stress Negligible below 40% absolute melting point Andrade’s Empirical Formula e = e0(1 + bt 1/3)e kt e = Strain e0 = Initial Elastic Strain b and k Material Constants t = Time Figure 3.13

Stress Rupture Test Stress Rupture Test similar to creep test but carried out to failure Design Data Reports include Elongation, Applied Load, Time to Failure, and Temperature Grain Boundary Sliding Failure mode for polycrystalline metals Creep rate lower for large-grain material Note: Oxides influence creep and stress rupture

Metallurgical Variables Microstructural Conditions Effects of Heat Treatment Effects of Processing Variables Effects of Service Conditions

Microstructural Conditions Grain Size Effect - Ordinary temperature - fine grain, more strength High temperature - larger grain, greater strength Single Phase vs Multiphase Alloys Second phases many add profound differences Porosity & Inclusions - Poor mechanical properties Directionality - Rolling direction vs transverse direction affect mechanical properties, introduce anisotropy

Effects of Heat Treatment Annealing - Softening, ductile behavior Quenching of Steel - Martensite formation, strong but brittle Tempering of Martensite - Hardness decreases, toughness increases Strength is sacrifice to avoid brittle failure Age Hardening - Fine scale precipitation, increased strength Case Hardening - Hard case, soft core by carburizing and nitriding Increased strength, better wear-resistance

Effects of Processing Variables Welding - Heat-affected zone, larger grain size, poorer mechanical properties. Local chemical changes, including loss of carbon in steel, quenching cracking due to rapid quenching. Flame Cutting - Drastic changes of microstructure near the flame-cut surface, affects properties. Machining and Grinding - Cold working results in stain hardening, may produce surface cracks.

Effects of Service Conditions Extreme Low Temperature Ductile-brittle transition occurs in steel. Extreme High Temperature Causes corrosion and surface oxidation Surface cracks may form Results in corrosion fatigue, creep, and rupture Impact Loading Notch sensitivity, surface scratches, corrosion pits can initiate brittle failure Corrosive Environment - Stress corrosion, pitting corrosion, corrosion fatigue

Equilibrium Phase Diagrams Alloy composition expressed as weight (wt.%) or atomic (at.%) percentage. Determining equilibrium phase diagrams - X-Ray Diffraction, Optical Microscopy, Calorimetric Analyses, and Thermal Analyses. Phase - Bounded volume of material of uniform chemical composition, with fixed crystalline structure, and thermo-plastic properties at a given temperature.

Equilibrium Equilibrium between Phases Gibb’s Phase Rule P + F = C + 2 P = number of phases, C = number of elements F = degrees of freedom, 2 = external variables (temperature and pressure).

Analysis of Phase Diagrams Thermal Arrest (Freezing/Melting Point) Lever Rule Solid Solution Alloy Eutectic Notation a = primarily A, small amount of dissolved B b = primarily B, small amount of dissolved A

Eutectics

Atomic Bonding and Solids Three Forms of Matter Gaseous, Liquid, Solid Solid - Amorphous, Crystalline, Mixture Amorphous Molecules randomly without any periodicity Crystalline Molecules organized in distinct three dimensional patterns (motif = unit cell) Atomic Bonding Ionic, Covalent, Metallic

Electronic Structure of Atoms Quantized = Orbiting (Shell) Electron Energy Levels Quantum Numbers (Three Indicators) Quantum Number n = Energy Level # of electrons per shell = 2n2 Sub-Levels l = 0, 1, … , n-1 l = 0, 1, 2, 3 = s, p, d, f for n=1, l =0 and shell = 1(s) for n=2, l =0,1 and shell = 2(s) and 2(p) Magnetic Quantum Number m = -l to +l (0) Spin Quantum Number s = + 1/2 or -1/2

Pauli’s Exclusion Principle Each quantum state can accommodate 2 electrons of opposite spin (- 1/2 & + 1/2 {up & down}) No more than 2 electrons per state Applies to states, not energy levels Valence Electrons = Outermost s & p states

Ionic Bonding Electropositive and Electronegative Elements Example: Due to “exchanged” electrons Sodium (Na+) and Chlorine (Cl-) Opposite charges attract Electron clouds repel Potential energy minimum at balance distance Potential Well = Preferred Site Figure 3.26 Figure 3.27

Covalent Bonding Homopolar (Covalent) Bonding = Electron Sharing Bonding Pairs = Number of Shared Electrons = 8 - N ( N=Valence) Carbon (Atomic Number 6) Electron Configuration 1(s)22(s)22(p)2 Valence Electrons = 2 (from 2s) + 2 (from 2p) = 4 Bonding Pairs = 8 - 4 = 4

Metallic Bonding Metallic Elements (Valence = 1 or 2) Valence Electrons “free” to migrate and are not “localized” to individual atoms in as in the case of ionic or covalent bonding. The “sea” of migrating electrons and the attraction between positively charged atoms producing three-dimensional periodic lattices.

Electrical Properties Ionic and Covalent Bonding Localized Electrons = Insulators Conductivity increases with temperature Metallic Bonding Free Migrating Electrons Collide with Oscillating Lattices Higher Mean Free Path = Higher Conductivity Conductivity decreases with temperature

Energy Bands Pauli’s Exclusion Principle (2 per state) Energy bands have quasi-continuous levels Fill from lowest to highest energy levels Additional energy (thermal or electric field) Kinetic energy increases Electrons move up an energy level but only at the highest level Conduction Band - Valence Band - Energy Gap Semiconductors Figure 3.28 Energy Gap

Crystalline State and Crystallography Unit Cell Lattice with atoms at each corner (6 parameters) Parallelepiped (a, b, g, a, b, c) Seven distinct shapes Bravais Lattice Fourteen constructions are possible where each atoms has an identical surrounding. Figure 3.30 Table 3.2 Figure 3.33

Body-Centered Cubic Lattice Body-Centered Cubic Lattice BCC (9) Face-Centered Unit Cell FCC (12) Closed Packed Plane Hexagonal Closed Pack Lattice HCP (13) Figure 3.34 Figure 3.35 Figure 3.36 Figure 3.37

Miller Indices System of notation used for denoting planes and directions in crystalline structures (hkl). Note: All integers, without common factors. Figure 3.38

Primitive Cells Only Corner Atoms Cubic Lattice, Hexagonal Lattice BCC, FCC, HCP are not primitive cells. Number of Atoms per Cell Simple Cubic (1/8 * 8) = 1 per cell FCC (1/8 * 8 + 1/2 * 6) = 4 per cell BCC (1/* * * + 1) = 2 per cell

Interplaner Spacing Interplaner Distance (dhkl) Perpendicular distance between equivalent planes Measured in Angstrom Units A = 10-8 cm Atomic Packing Factor = Volume of Atoms Volume of Space FCC APF = 0.74V BCC APF = 0.68 X-Ray Crystallography Bragg’s Law 2dhkl = sin q = h g g is X-Ray Wavelength and h is Reflection Number Figure 3.39 Figure 3.40 Figure 3.41