Lecture 1: Review and Crystal Structure

8/13/2019 Lecture 1: Review and Crystal Structure

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Physical Metallurgy

Crystal StructurePhysical and Mechanical Properties of Materials

Phase TransformationsFailure in Metals

Strengthening Mechanisms


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Crystal StructureDifferent Structure Levels

Review of Atomic Structure

Review of Types of BondingCrystalline and Amorphous SolidsCrystallography

Basic Types of Unit CellsUnit Cell Parameters


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PhysicalMetallurgy

branch of metallurgy

concerned with the physicaland mechanical propertiesof metals as affected by

composition, processing andenvironmental conditions


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Structurethe arrangement or

order of the internalcomponents of materials


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subatomicatomic

microscopic macroscopic


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AtomComposed of a nucleus and

electrons

Valence electrons determines theproperties of the elements.

Z, atomic numberA, atomic mass numberN, number of neutrons


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ElectronegativeElectropositiveGives up electrons,becomes positive.

Accepts electrons,becomes negative.


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Attractive forcestype of bonding

Repulsive forcesoverlap of orbitals


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Equilibrium Potential Well


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Depth of Potential Well, EoBonding Energy

Energy required to separate the two

atoms to an infinite distance

Affects

Melting temperatureMechanical stiffness (Modulus of elasticity) Coefficient of thermal expansion


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Primary Interatomic BondsInvolves the valence electrons

GoalAssume stable electron structure


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7

Ranges from 0.7to 4.0,

Smaller electronegativity Larger electronegativity

Large values: tendency to acquire electrons.

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of theChemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell

University.

ELECTRONEGATIVITY


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Predominant bonding in Ceramics

Give up electrons Acquire electrons

He-

Ne-

Ar

-

Kr-

Xe-

Rn-

F4.0

Cl

3.0

Br2.8

I2.5

At2.2

Li1.0

Na

0.9

K0.8

Rb0.8

Cs0.7

Fr0.7

H2.1

Be1.5

Mg

1.2

Ca1.0

Sr1.0

Ba0.9

Ra0.9

Ti1.5

Cr1.6

Fe1.8

Ni1.8

Zn1.8

As2.0

CsCl

MgO

CaF2

NaCl

O3.5

Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of theChemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell

University.

EXAMPLES: IONIC BONDING


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Ionic Bonding


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Covalent Bonding


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Valence electrons free to driftthrough the entire metal

sea of electrons orelectron cloud

Ion coresnucleus + non-valence

electrons

Good electrical and thermalconductivity

Metallic Bonding


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Secondary Bonding Forces

van der Waals bondsArise from atomic or molecular dipoles

Fluctuating induced dipole

Polar molecule-induced dipoleHydrogen bonds


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Crystal Structures


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CrystallographyBranch of science concerned with the relationships

of atomic arrangement to the behavior andproperties of metals


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Crystalline MaterialA material in which the atoms are situated in arepeating or periodic array over large atomic

distances.

A solid that contains a regular or repeating atomicor molecular arrangement.

Metals, most ceramics


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Amorphous MaterialsSolids with no long-range ordering of atoms or

molecules.

Non-crystalline solid. Non-dense, random packing.


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Crystalline Amorphouslong-range order short-range order


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Lattice

Three-dimensionalarray of points

coinciding with atompositions


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Unit Cell

Smallest unit thatcompletely describesthe crystal pattern


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Unit Cell

ParametersInteraxial angles angles between axes

Lattice constants edge lengths along

major axes


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Crystal Systems Axial Relationships Interaxial AnglesTriclinic a"b"c """90

Monoclinic a"b"c ==90"

Orthorhombic a"

b"

c ===90

Rhombohedral a=b=c =="90

Tetragonal a=b"c ===90

Hexagonal a=b"c ==90,=120Cubic a=b=c ===90

7 Crystal Systems


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triclinic monoclinic orthorhombic rhombohedral

tetragonal hexagonal cubic


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METALLIC CRYSTALS tend to be densely packed. have several reasons for dense packing:

-Typically, only one element is present, so all

atomic

radii are the same.-Metallic bonding is not directional.

-Nearest neighbor distances tend to be small in

order to lower bond energy.

have the simplest crystal structures.


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Basic Types of Unit Cells

Simple Cubic Body-centered Cubic Face-centered Cubic Hexagonal Close-packed


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Simple Cubic

Atoms are situated at thecorner of each unit cell

The corner atoms touch eachother along the unit celledge


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Body-Centered Cubic (BCC)atoms are situated at the

corners, as well as at the (body)

center of the cube The body atom touches each of

the eight corner atoms


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Face-Centered Cubic (FCC)

atoms are situated at thecorners of the unit cell, aswell as at the centers of eachface

each face atom touches itsnearest corner atoms


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Hexagonal Closed-Packed (HCP)

has two basal planes in the form ofregular hexagons with an atom ateach corner of the hexagon and one

atom at the center

e.g. Cd, Co, Ti, Zn

Principal Metallic Structures


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Principal Metallic StructuresBody-Centered CubicFe, Cr, Mo, V, W Face Centered CubicAl, Au, Pb, Ni, Ag, Cu, Pt Hexagonal Close-PackedCd, Co, Ti, Zn, Mg


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Unit Cell Parameters Atomic Radii hard-sphere model Coordination Number - Number of nearest neighbor atoms

Number of atoms/unit cell Ratio of Lattice Constant to atomic radius Atomic Packing Factor

the fraction of space filled by spherical atoms ratio of the volume occupied by atoms to the total availablevolume

Lecture 1: Review and Crystal Structure

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