1

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

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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)

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Tension Testing Two distinct stages of deformation Elastic Deformation (Reversible Change in

Volume) Plastic Deformation (Irreversible Constant

Volume)

Elastic Deformation Hooke’s Law = E = Stress = Strain E = Young’s Modulus / Modulus of Elasticity

Plastic Deformation

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Plastic Deformation (Non-Linear)

Yield Stress = y

Off-Set Yield = 0.2%

Ultimate Tensile Strength = uts

Fracture Stress = f (f < uts)

Ductility

Work Hardening / Strain Hardening

Figure 3.1

Figure 3.2

Figure 3.3-4

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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.

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Slip Deformation - continued

Slip occurs when the resolved component of

Shear Stress R = P/A cos cos exceeds the critical value

Critical Resolved Shear StressR)crit

Dislocation Edges (Rcrit < 1/5 Theoretical) Dislocation Lines & Frank-Reed Source

Figure 3.6

Figure 3.7-8

Figure 3.9

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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).

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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

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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

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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.

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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.

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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

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Creep at High Temperature (Stress Rupture)

Creep - Progress deformation at constant stress

Negligible below 40% absolute melting point

Andrade’s Empirical Formula = 0(1 + t 1/3)e kt

= Strain 0 = Initial Elastic Strain and k Material Constants t = Time

Figure 3.13

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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

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Metallurgical Variables

Microstructural Conditions

Effects of Heat Treatment

Effects of Processing Variables

Effects of Service Conditions

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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

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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

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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.

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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

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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.

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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).

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Analysis of Phase Diagrams

Thermal Arrest (Freezing/Melting Point) Lever Rule Solid Solution Alloy Eutectic Notation = primarily A, small amount of dissolved

B = primarily B, small amount of dissolved

A

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Eutectics

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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

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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

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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

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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

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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

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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.

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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

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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

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Crystalline State and Crystallography

Unit Cell Lattice with atoms at each corner (6

parameters) Parallelepiped (, 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

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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 3.37

Figure 3.36

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Miller Indices

System of notation used for denoting planes and directions in crystalline structures (hkl).

Note: All integers, without common factors.

Figure 3.38

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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

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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 = is X-Ray Wavelength and is Reflection

Number

Figure 3.39

Figure 3.40

Figure 3.41