lecture 1: review and crystal structure
TRANSCRIPT
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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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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