Transcript Chapter 3
Why do we care about crystal structures, directions, planes ?
Physical properties of materials depend on the geometry of crystals
ISSUES TO ADDRESS...
• How do atoms assemble into solid structures?
(for now, focus on metals) • How does the density of a material depend on its structure?
• When do material properties vary with the sample (i.e., part) orientation?
Chapter 3 1
Electronic Structure
Bohr atom Bohr-Sommerfeld Quantum numbers Aufbau principle Multielectron atoms Periodic table patterns Octet stability
The Big Picture
Bonding
Primary: •Ionic •Covalent •Metallic
State of aggregation
Gas Liquid
Solid Classification of Solids:
1. Bonding type 2. Atomic arrangement 2
Chapter 3-
Atomic Arrangement
SOLID
: Smth. which is dimensionally stable, i.e., has a volume of its own
classifications of solids by atomic arrangement atomic arrangement order name ordered
regular long-range crystalline “crystal”
disordered
random* short-range amorphous “glass” 3
Chapter 3-
Energy and Packing
• Non dense, random packing
Energy
typical neighbor bond length typical neighbor bond energy • Dense, ordered packing
Energy
typical neighbor bond length
r
typical neighbor bond energy Dense, ordered packed structures tend to have lower energies.
Chapter 3-
r
MATERIALS AND PACKING
Crystalline • typical of: materials...
• atoms pack in periodic, 3D arrays -metals -many ceramics -some polymers
LONG RANGE ORDER
crystalline SiO 2 Adapted from Fig. 3.18(a), Callister 6e.
Noncrystalline materials...
• atoms have no periodic packing • occurs for: -complex structures -rapid cooling " Amorphous " = Noncrystalline
SHORT RANGE ORDER
noncrystalline SiO 2 Adapted from Fig. 3.18(b), Callister 6e.
Chapter 3 3
Metallic Crystal Structures
• How can we stack metal atoms to minimize empty space?
2-dimensions
vs.
Now stack these 2-D layers to make 3-D structures 6
Chapter 3-
6
Robert Hooke – 1660 - Cannonballs
“Crystal must owe its regular shape to the packing of spherical particles”
7
Chapter 3-
Niels Steensen ~ 1670
observed that quartz crystals had the same angles between corresponding faces regardless of their size. 8
Chapter 3-
SIMPLE QUESTION:
If I see something has a macroscopic shape very regular and cubic, can I infer from that if I divide, divide, divide, divide, divide, if I get down to atomic dimensions, will there be some cubic repeat unit? 9
Chapter 3-
Christian Huygens - 1690
Studying calcite crystals made drawings of atomic packing and bulk shape. 10
Chapter 3-
BERYL Be 3 Al 2 (SiO 3 ) 6 11
Chapter 3-
Early Crystallography
René-Just Haüy
(1781): cleavage of calcite • Common shape to all shards: rhombohedral • How to model this mathematically?
• What is the maximum number of distinguishable shapes that will fill three space? • Mathematically proved that there are only 7 distinct space-filling volume elements 12
Chapter 3-
The Seven Crystal Systems
BASIC UNIT Specification of unit cell parameters 13
Chapter 3-
Does it work with Pentagon?
14
Chapter 3-
August Bravais
• How many different ways can I put atoms into these seven crystal systems, and get distinguishable point environments? When I start putting atoms in the cube, I have three distinguishable arrangements.
SC BCC FCC And, he proved mathematically that there are 14 distinct ways to arrange points in space.
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Chapter 3-
16
Chapter 3-
Last Day: Atomic Arrangement
SOLID
: Smth. which is dimensionally stable, i.e., has a volume of its own
classifications of solids by atomic arrangement atomic arrangement order name ordered
regular long-range crystalline “crystal”
disordered
random* short-range amorphous “glass” 17
Chapter 3-
MATERIALS AND PACKING
Crystalline • typical of: materials...
• atoms pack in periodic, 3D arrays -metals -many ceramics -some polymers
LONG RANGE ORDER
crystalline SiO 2 Adapted from Fig. 3.18(a), Callister 6e.
Noncrystalline materials...
• atoms have no periodic packing • occurs for: -complex structures -rapid cooling " Amorphous " = Noncrystalline
SHORT RANGE ORDER
noncrystalline SiO 2 Adapted from Fig. 3.18(b), Callister 6e.
Chapter 3 3
Energy and Packing
• Non dense, random packing
Energy
typical neighbor bond length typical neighbor bond energy • Dense, ordered packing
Energy
typical neighbor bond length
r
typical neighbor bond energy Dense, ordered packed structures tend to have lower energies.
Chapter 3-
r
Three Types of Solids according to atomic arrangement
20
Chapter 3-
Unit Cell Concept
• The unit cell is the smallest structural unit or building block that uniquely can describe the crystal structure. Repetition of the unit cell generates the entire crystal. By simple translation, it defines a lattice .
• Lattice: The periodic arrangement of atoms in a Xtal.
b a
Lattice Parameter : Repeat distance in the unit cell, one for in each dimension Chapter 3-
Crystal Systems
a • Units cells and lattices in 3-D: – When translated in each lattice parameter direction, MUST fill 3-D space such that no gaps, empty spaces left.
b c Lattice Parameter : Repeat distance in the unit cell, one for in each dimension Chapter 3-
The Importance of the Unit Cell
• One can analyze the Xtal as a whole by investigating a representative volume.
• Ex: from unit cell we can – Find the distances between nearest atoms for calculations of the forces holding the lattice together – Look at the fraction of the unit cell volume filled by atoms and relate the density of solid to the atomic arrangement – The properties of the periodic Xtal lattice determine the allowed energies of electrons that participate in the conduction process.
23
Chapter 3-
Metallic Crystal Structures
• How can we stack metal atoms to minimize empty space?
2-dimensions
vs.
Now stack these 2-D layers to make 3-D structures
Chapter 3-
Crystal Systems
Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal.
7 crystal systems 14 crystal lattices Fig. 3.4,
Callister 7e.
a, b,
and
c
are the lattice constants
Chapter 3-
26
Chapter 3-
SIMPLE CUBIC STRUCTURE (SC)
• Rare due to poor packing • Close-packed directions are cube edges.
Closed packed direction is where the atoms touch each other
• Coordination # = 6 (# nearest neighbors) (Courtesy P.M. Anderson) Chapter 3- 5
ATOMIC PACKING FACTOR
• APF for a simple cubic structure = 0.52
Adapted from Fig. 3.19, Callister 6e.
Chapter 3- 6
BODY CENTERED CUBIC STRUCTURE (BCC)
• Close packed directions are cube diagonals.
--Note: All atoms are identical; the center atom is shaded differently only for ease of viewing.
ex: Cr, W, Fe ( ), Tantalum, Molybdenum
• Coordination # = 8
2 atoms/unit cell: 1 center + 8 corners x 1/8
(Courtesy P.M. Anderson) Chapter 3 7
ATOMIC PACKING FACTOR: BCC
• APF for a body-centered cubic structure = 0.68
R a
3
a
2
a a
Chapter 3- 8
FACE CENTERED CUBIC STRUCTURE (FCC)
• Close packed directions are face diagonals.
--Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing.
ex: Al, Cu, Au, Pb, Ni, Pt, Ag • Coordination # = 12 Adapted from Fig. 3.1,
Callister 7e.
4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8
(Courtesy P.M. Anderson) Chapter 3- 9
ATOMIC PACKING FACTOR: FCC
?
HW Chapter 3-
1. Finish reading Chapter 3.
3. Fill in the blanks in Table below
HW
2.On a paper solve example problems:3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9
SC BCC FCC
Unit Cell Volume a 3 Lattice Points per cell 1 Nearest Neighbor Distance Number of Nearest Neighbors Atomic Packing Factor a 6 0.52
THEORETICAL DENSITY,
Example: Copper Data from Table inside front cover of Callister (see next slide): • crystal structure = FCC: 4 atoms/unit cell • atomic weight = 63.55 g/mol (1 amu = 1 g/mol) • atomic radius R = 0.128 nm (1 nm = 10 cm) Result: theoretical
Cu = 8.89 g/cm3 Compare to actual:
Cu = 8.94 g/cm3 Chapter 3- 14
Theoretical Density,
• Ex: Cr (BCC)
A
= 52.00 g/mol
R
= 0.125 nm
n
= 2
R a
atoms unit cell = volume unit cell 2 52.00
a
3 6.023 x 10 23
a
= 4
R
/ 3 = 0.2887 nm g mol theoretical actual atoms mol = 7.18 g/cm 3 = 7.19 g/cm 3 Chapter 3 -
Characteristics of Selected Elements at 20C Element Aluminum Argon Barium Beryllium Boron Bromine Cadmium Calcium Carbon Cesium Chlorine Chromium Cobalt Copper Flourine Gallium Germanium Gold Helium Hydrogen Symbol Al Ar Ba Be B Br Cd Ca C Cs Cl Cr Co Cu F Ga Ge Au He H At. Weight (amu) 26.98 39.95 137.33 9.012 10.81 79.90 112.41 40.08 12.011 132.91 35.45 52.00 58.93 63.55
19.00 69.72 72.59 196.97 4.003 1.008
Density (g/cm3) 2.71 ------ 3.5 1.85 2.34 ------ 8.65 1.55 2.25 1.87 ------ 7.19 8.9 8.94
------ 5.90 5.32 19.32 ------ ----- Atomic radius (nm) 0.143 ------ 0.217 0.114 ------ ------ 0.149 0.197 0.071 0.265 ------ 0.125 0.125 0.128
Adapted from Table, "Charac teristics of Selected Elements", inside front cover, Callister 6e.
------ 0.122 0.122 0.144 ------ ----- Chapter 3- 15
DENSITIES OF MATERIAL CLASSES
metals •
ceramics •
polymers Why?
Metals have...
• close-packing (metallic bonding) • large atomic mass Ceramics • less dense packing (covalent bonding) • often lighter elements Polymers have...
have...
• poor packing (often amorphous) • lighter elements (C,H,O) Composites have...
• intermediate values Data from Table B1, Callister 6e.
Chapter 3 16
POLYMORPHISM & ALLOTROPY
• Some materials may exist in more than one crystal structure, this is called
polymorphism
. • If the material is an elemental solid, it is called
allotropy
.
An example of allotropy is carbon, which can exist as diamond, graphite, and amorphous carbon.
Chapter 3-
Polymorphism
• Two or more distinct crystal structures for the same material (allotropy/polymorphism) iron system titanium , -Ti liquid carbon BCC 1538ºC
-
Fe diamond, graphite FCC 1394ºC
-
Fe BCC 912ºC
-
Fe 39
Chapter 3-
Crystallographic Points, Directions, and Planes
• It is necessary to specify a particular point/location/atom/direction/plane in a unit cell • We need some labeling convention. Simplest way is to use a 3-D system, where every location can be expressed using three numbers or
indices
.
–
a , b , c
and α, β, γ z β γ α y x
Chapter 3-
Crystallographic Points, Directions, and Planes
• Crystallographic direction is a vector [uvw] – Always passes thru origin 000 – Measured in terms of unit cell dimensions
a, b, and c
– Smallest integer values • Planes with Miller Indices (hkl) – If plane passes thru origin, translate – Length of each planar intercept in terms of the lattice parameters
a, b, and c
.
– Reciprocals are taken – If needed multiply by a common factor for integer representation
Chapter 3-
x a
Section 3.8 Point Coordinates
z c
111 Point coordinates for unit cell center are
a
/2,
b
/2,
c
/2 ½ ½ ½
y
000
z b
2c
Point coordinates for unit cell corner are 111
b b
y
Translation: integer multiple of lattice constants identical position in another unit cell
Chapter 3-
z
Crystallographic Directions
y
Algorithm 1. Vector repositioned (if necessary) to pass through origin.
2. Read off projections in terms of unit cell dimensions
a
,
b
, and
c
3. Adjust to smallest integer values 4. Enclose in square brackets, no commas [
uvw
]
x
ex: 1, 0, ½ => 2, 0, 1 => [ 201 ] -1, 1, 1 => [ 111 ] where overbar represents a negative index families of directions <
uvw
>
Chapter 3-
Linear Density
• Linear Density of Atoms LD = Number of atoms Unit length of direction vector [110] ex: linear density of Al in [110] direction
a
= 0.405 nm
a
# atoms LD = length 2 2
a
= 3.5 nm 1
Chapter 3-
Crystallographic Planes
Adapted from Fig. 3.9,
Callister 7e.
Chapter 3-
Crystallographic Planes
• Miller Indices: Reciprocals of the (three) axial intercepts for a plane, cleared of fractions & common multiples. All parallel planes have same Miller indices.
• Algorithm 1. If plane passes thru origin, translate 2. Read off intercepts of plane with axes in terms of
a
,
b
,
c
3. Take reciprocals of intercepts 4. Reduce to smallest integer values 5. Enclose in parentheses, no commas i.e., (
hkl
)
Chapter 3-
Crystallographic Planes
z
example 1. Intercepts 2. Reciprocals 3. Reduction
a b c
1 1 1/1 1/1 1/ 1 1 0 1 1 0
a c
4. Miller Indices (110)
x z
example 1. Intercepts 2. Reciprocals 3. Reduction
a b
1/2 1/½ 1/
c
1/ 2 0 0 2 0 0
c
4. Miller Indices (100)
a x b b
Chapter 3-
y y
Crystallographic Planes
z
example 1. Intercepts 2. Reciprocals 3. Reduction
a b c
1/2 1 3/4 1/½ 1/1 1/¾ 2 1 4/3 6 3 4
a
4. Miller Indices (634)
x c
b y
Family of Planes {
hkl
} Ex: {100} = (100), (010), (001), (100), (010), (001)
Chapter 3-
Crystallographic Planes
• • We want to examine the atomic packing of crystallographic planes Iron foil can be used as a catalyst. The atomic packing of the exposed planes is important. a) Draw (100) and (111) crystallographic planes for Fe.
b) Calculate the planar density for each of these planes.
Chapter 3-
Planar Density of (100) Iron
Solution: At T < 912 C iron has the BCC structure.
2D repeat unit (100)
a
= 4 3 3
R
Adapted from Fig. 3.2(c),
Callister 7e.
atoms 2D repeat unit 1 Planar Density = area 2D repeat unit
a
2 = Radius of iron 1 4 3 3
R
2
R
= 12.1
atoms nm 2 = 0.1241 nm = 1.2 x 10 19 atoms m 2
Chapter 3-
Planar Density of (111) Iron
?
HW
Chapter 3-
Single Crystals and Polycrystalline
• In a single crystal
Materials
atoms is PERFECT This extends throughout the entirety of the specimen without interruption. • Polycrystalline material , on the other hand, is comprised of many small crystals or grain boundaries .
grains
.
These regions are called The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet.
Chapter 3-
Example of Polycrystalline Growth
Chapter 3-
CRYSTALS AS BUILDING BLOCKS
• Some engineering applications require single crystals: --diamond single crystals for abrasives (Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.) --turbine blades Fig. 8.30(c), Callister 6e.
(Fig. 8.30(c) courtesy of Pratt and Whitney).
• Crystal properties reveal features of atomic structure.
--Ex: Certain crystal planes in quartz fracture more easily than others.
(Courtesy P.M. Anderson) Chapter 3- 17
• Most
POLYCRYSTALS
engineering materials are polycrystals.
Adapted from Fig. K, color inset pages of Callister 6e.
(Fig. K is courtesy of Paul E. Danielson, Teledyne Wah Chang Albany) 1 mm • Nb-Hf-W plate with an electron beam weld.
• Each "grain" is a single crystal.
• If crystals are randomly oriented, overall component properties are not directional.
• Crystal sizes typ. range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers).
Chapter 3- 18
SINGLE VS POLYCRYSTALS
• Single Crystals -Properties vary with direction: anisotropic .
-Example: the modulus of elasticity (E) in BCC iron: Data from Table 3.3, Callister 6e.
(Source of data is R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 3rd ed., John Wiley and Sons, 1989.) • Polycrystals -Properties may/may not vary with direction.
-If grains are randomly oriented: isotropic .
(E poly iron = 210 GPa) -If grains are textured , anisotropic.
200
m
m Adapted from Fig. 4.12(b), Callister 6e.
(Fig. 4.12(b) is courtesy of L.C. Smith and C. Brady, the National Bureau of Standards, Washington, DC [now the National Institute of Standards and Technology, Gaithersburg, MD].) Chapter 3- 19
Anisotropy and Texture
• Different directions in a crystal have a different APF .
• For example, the deformation amount depends on the direction in which a stress is applied, other properties are thermal conductivity, optical properties, magnetic properties, hardness, etc.
• In some polycrystalline materials, grain orientations are random, hence bulk material properties are isotropic, i.e. equivalent in each direction • Some polycrystalline materials have grains with preferred orientations (texture) , so properties are dominated by those relevant to the texture orientation and the material exhibits anisotropic properties.
Chapter 3-
X-RAYS TO CONFIRM CRYSTAL STRUCTURE • Incoming X-rays diffract from crystal planes.
Adapted from Fig. 3.2W, Callister 6e.
• Measurement of: Critical angles,
q
c , for X-rays provide atomic spacing, d.
Chapter 3- 20
SUMMARY (I)
• Atoms may assemble into crystalline amorphous structures.
or • We can predict the density of a material, provided we know the atomic weight , atomic radius , and BCC, HCP).
crystal geometry (e.g., FCC, • Material properties generally vary with single crystal orientation (i.e., they are anisotropic ), but properties are generally non-directional (i.e., they are isotropic ) in polycrystals with randomly oriented grains.
Chapter 3 23
• • • • • • • • • • • • • • • • • • •
Summary (II)
Allotropy Amorphous Anisotropy Atomic packing factor (APF) Body-centered cubic (BCC) Coordination number Crystal structure Crystalline Face-centered cubic (FCC) Grain Grain boundary Hexagonal close-packed (HCP) Isotropic Lattice parameter Non-crystalline Polycrystalline Polymorphism Single crystal Unit cell
Chapter 3-