Download Materials Science

Types of Materials
• Metals:
– Strong, ductile
– high thermal & electrical conductivity
– opaque
• Polymers/plastics: Covalent bonding 
sharing of e’s
– Soft, ductile, low strength, low density
– thermal & electrical insulators
– Optically translucent or transparent.
• Ceramics: ionic bonding (refractory) –
compounds of metallic & non-metallic elements
(oxides, carbides, nitrides, sulfides)
– Brittle, glassy, elastic
– non-conducting (insulators)
Material Density
Material Stiffness
Material Resistance to Fracture
Material Electrical Conductivity
The Materials Selection Process
Composition
Mechanical
Electrical
Thermal
Optical
Etc.
Environment
Load
Applications
Functions
Properties
Structure
Shape
Materials
Processes
Structure, Processing, & Properties
• Properties depend on structure
ex: hardness vs structure of steel
(d)
Hardness (BHN)
600
500
400
(c)
(a)
(b)
4 mm
300
200
30 mm
30 mm
100
0.01 0.1
30 mm
1
10 100 1000
Cooling Rate (ºC/s)
• Processing can change structure
ex: structure vs cooling rate of steel
Steel with 0.4 wt% C
d) Martensite
c) Martensite
(tempered at 371 C)
b) Fine pearlite
a) Spheroidite
ELECTRICAL
• Electrical Resistivity of Copper:
6
(10-8 Ohm-m)
Resistivity, r
5
4
3
2
1
0
-200
-100
0
T (°C)
• Adding “impurity” atoms to Cu increases resistivity.
• Deforming Cu increases resistivity.
THERMAL
• Space Shuttle Tiles:
--Silica fiber insulation
offers low heat conduction.
• Thermal Conductivity
of Copper:
Thermal Conductivity
(W/m-K)
--It decreases when
you add zinc!
100 mm
400
300
200
100
0
0
10
20 30 40
Composition (wt% Zinc)
MAGNETIC
--Recording medium
is magnetized by
recording head.
• Magnetic Permeability
vs. Composition:
--Adding 3 atomic % Si
makes Fe a better
recording medium!
Magnetization
• Magnetic Storage:
Fe+3%Si
Fe
Magnetic Field
OPTICAL
• Transmittance:
--Aluminum oxide may be transparent, translucent, or
opaque depending on the material structure.
single crystal
polycrystal:
low porosity
polycrystal:
high porosity
DETERIORATIVE
• Stress & Saltwater...
• Heat treatment: slows
--causes cracks!
crack speed (m/s)
crack speed in salt water!
10-8
10-10
“as-is”
“held at
160ºC for 1 hr
before testing”
Alloy 7178 tested in
saturated aqueous NaCl
solution at 23ºC
increasing load
--material:
7150-T651 Al "alloy"
(Zn,Cu,Mg,Zr)
4 mm
Properties From Bonding: Tm
• Bond length, r
• Melting Temperature, Tm
Energy
r
• Bond energy, Eo
ro
Energy
r
smaller Tm
unstretched length
ro
r
Eo =
“bond energy”
larger Tm
Tm is larger if Eo is larger.
Properties From Bonding : a
• Coefficient of thermal expansion, a
length, L o
coeff. thermal expansion
unheated, T1
DL
= a (T2 -T1)
Lo
DL
heated, T 2
• a ~ symmetry at ro
Energy
unstretched length
ro
E
o
E
o
Larger a
Smaller a
r
a is larger if Eo is smaller.
Summary: Primary Bonds
Ceramics
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Polymers
(Covalent & Secondary):
Large bond energy
large Tm
large E
small a
Variable bond energy
moderate Tm
moderate E
moderate a
Secondary bonding dominates
small Tm
small E
large a
Brief of Metal
The Periodic Table
• Columns: Similar Valence Structure
H
He
Li Be
O
F Ne
Na Mg
S
Cl Ar
Se
Kr
K Ca Sc
Rb Sr
Cs Ba
Fr Ra
Y
Te
I
Xe
Po At Rn
Energy and Packing
• Non dense, random packing
Energy
typical neighbor
bond length
typical neighbor
bond energy
• Dense, ordered packing
r
Energy
typical neighbor
bond length
typical neighbor
bond energy
Dense, ordered packed structures tend to have
lower energies.
r
Materials and Packing
Crystalline materials...
• atoms pack in periodic, 3D arrays
• typical of: -metals
-many ceramics
-some polymers
Noncrystalline materials...
• atoms have no periodic packing
• occurs for: -complex structures
-rapid cooling
"Amorphous" = Noncrystalline
crystalline SiO2
Si
Oxygen
noncrystalline SiO2
Types of Imperfections
• Vacancy atoms
• Interstitial atoms
• Substitutional atoms
Point defects
• Dislocations
Line defects
• Grain Boundaries
Area defects
• Vacancies:
Point Defects
-vacant atomic sites in a structure.
Vacancy
distortion
of planes
• Self-Interstitials:
-"extra" atoms positioned between atomic sites.
selfinterstitial
distortion
of planes
Point Defects in Alloys
Two outcomes if impurity (B) added to host (A):
• Solid solution of B in A (i.e., random dist. of point defects)
OR
Substitutional solid soln.
(e.g., Cu in Ni)
Interstitial solid soln.
(e.g., C in Fe)
• Solid solution of B in A plus a new phase (usually for a larger
amount of B)
Second phase particle
--different composition
--often different structure.
Line Defects
Dislocations:
• are line defects,
• slip between crystal planes result when dislocations move,
• produce permanent (plastic) deformation.
Schematic of Zinc (HCP):
• before deformation
• after tensile elongation
slip steps
Imperfections in Solids
Edge Dislocation
Imperfections in Solids
Screw Dislocation
Screw Dislocation
b
Dislocation
line
Burgers vector b
(b)
(a)
Edge, Screw, and Mixed Dislocations
Mixed
Edge
Screw
Dislocations & Crystal Structures
• Structure: close-packed
planes & directions
are preferred.
view onto two
close-packed
planes.
close-packed plane (bottom)
close-packed directions
close-packed plane (top)
• Comparison among crystal structures:
HCP: few slip systems/directions;
FCC: many slip systems/directions;
BCC: the most slip systems/directions
• Specimens that
were tensile
tested.
Mg (HCP)
tensile direction
Al (FCC)
Planar Defects in Solids
• External Surfaces
The most obvious
• Grain Boundary
Different crystal
orientation between
grains
• twin boundary (plane)
– Essentially a reflection of
atom positions across the
twin plane.
• Stacking faults
– For FCC metals an error in
ABCABC packing
sequence
– Ex: ABCABABC
• Phase boundary
– In multiphase materials
Polycrystalline Materials
Grain Boundaries
• regions between crystals
• transition from lattice of
one region to that of the
other
• slightly disordered
• low density in grain
boundaries
– high mobility
– high diffusivity
– high chemical reactivity