CHAPTER 3: STRUCTURE OF METALS

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CHAPTER 3 STRUCTURE OF METALS

• What happens when many atoms come together to
  form a solid?
• Regular structures (crystalline) or ? Irregular
  structures (amorphous)

• Crystalline
• A solid characterized by periodic or repeating
  arrays over large atomic
• distances
• long-range order , repetitive 3D pattern
• Often opaque
• Most metals
• Portions of some polymers (semi-crystalline)
• Amorphous (Non-crystalline)
• A solid which may have short-range order, but no
  long-range order.
• without form
• Often transparent
• Ceramic glasses
• Amorphous metals
• Some polymers are completely amorphous

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What is the difference between quartz and glass?
Quartz crystalline SiO2
Glass amorphous SiO2
Long-range order
Only short-range order
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Crystal Structures

• Describes the way atoms (ions) arranged in 3D or
  lattice space in terms of unit cell geometry
  atom position within a unit cell.
• Allotropes gt1 crystal structure
• Unit Cell
• - basic building block (simplest structural
  unit) of crystal structure
• - indicates crystal structure geometry atom
  position

repeats
Many unit cells form a crystalline solid
repeats

• FCC, BCC, and HCP crystal structures

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Crystal Systems

• Scheme by which crystal structures are
  classified only by unit cell geometry (not atom
  position).
• 7 possible unit cell geometries
• - Ex cubic, hexagonal
• 7 possible crystal systems
• See Table 3.2 and Fig. 3.4
• Unit cell geometries determined by 6 lattice
  parameters
• 1. 3 edge lengths (a, b, c)
• 2. 3 interaxial angles (a, b, ?)

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FCC crystal structure BCC crystal structure
all angles
all edges
HCP crystal structure
Table 3.2 and Fig. 3.4
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Types of Models to Describe Crystal Structures

• Hard Sphere Model (a)
• - big atoms
• Atoms are like spheres
• Atoms touch neighbor
• Radius of hard sphere atomic radius
• Atomic radius distance between two nuclei
  of two touching atoms

d
r d/2

• Reduced Sphere Model (b)
• Center of atoms represented as small circles
• Aggregate of many atoms (c)

3 Predominate Crystal Structures for Metals FCC,
BCC, and HCP
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Face-centered cubic crystal structure (FCC)
1, 2, 3, 4 4 corner nearest atoms
1
2
Consider this atom
5, 6, 7, 8 4 face atoms in contact from behind
3
4
9, 10, 11, 12 Not shown
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f02_03_pg42
Body-centered cubic crystal structure (BCC)
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Hexagonal close-packed crystal structure (HCP)
G, H, E, F, J, A, B, C, D (only)
Traditional HCP crystal structure
Expanded HCP crystal structure
All atoms
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f13_03_pg62
BOTH FCC and HCP are CLOSE-PACKED (APF 0.74)
Both generated by stacking of close packed planes
on top of one another difference is the stacking
sequence
Triangular vertix up B
Portion of a close-packed plane of A atoms
Form 2 types of triangular vertices B and C
Triangular vertix down C
A plane of close-packed B atoms is added on
top of the close-packed plane of A atoms over
the B vertices
AB stacking sequence for close-packed atomic
planes
At this point (2 layers), AB or AC stacking
sequences are equivalent
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f14_03_pg62
HCP ABAB Stacking Sequence
Centers of 3rd layer of A atoms are positioned
directly above the A atoms of the 1st layer
3
2
(ACACAC would be equivalent)
1
2D Projection (looking down)
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f15_03_pg63
FCC ABCABC Stacking Sequence
2D Projection (looking down)
3
2
1
FCC unit cell
Centers of 3rd close-packed layer of C atoms
are positioned directly above the C vertices
of 1st layer
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Models

• FCC
• All atoms are identical corner atoms are shaded
• differently only for ease of viewing
• Atoms touch each other along face diagonal

• BCC
• All atoms are identical the center atom
• is shaded differently only for ease of viewing
• Atoms touch each other along cube (body) diagonal

HCP
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Metallic Crystal Structures
Tend to be densely packed
vs.
FCC and HCP close-packed
BCC not close-packed
- Minimize empty space
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Densities of Material Classes
In general
Graphite/
Metals/
Composites/
Ceramics/
Polymers
gt
gt
Alloys
fibers
Semicond
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Why?
B
ased on data in Table B1, Callister
2
0
GFRE, CFRE, AFRE are Glass,

Metals have... close-packing
(metallic bonding) often large atomic
masses
Carbon, Aramid Fiber-Reinforced

Epoxy composites (values based on

60 volume fraction of aligned fibers

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in an epoxy matrix).

Ceramics have... less dense packing
often lighter elements
5

3
4

(g/cm )
3

r
2
Polymers have... low packing density
(often amorphous) lighter elements
(C,H,O)
1
0.5

Composites have... intermediate values
0.4

0.3

Data from Table B1, Callister 7e.