2. INTRODUCTION TO ENGINEERING
MATERIALS
Common type of materials
Metals Ceramics Polymers
Composites
Structure
Properties
Processing
Performance
The Materials Tetrahedron
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3. METALLURGY
PHYSICAL MECHANICAL ELECTRO-
CHEMICAL
TECHNOLOGICAL
• Extractive
• Casting
• Metal Forming
• Welding
• Powder Metallurgy
• Machining
• Structure
• Physical
Properties
Science of Metallurgy
• Deformation
Behaviour
•Chemistry
• Corrosion
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5. Crystal Structure
• An ideal crystal is a periodic array of
structural units, such as atoms or molecules.
• It can be constructed by the infinite repetition
of these identical structural units in space.
• Structure can be described in terms of a
lattice, with a group of atoms attached to
each lattice point. The group of atoms is the
basis.
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6. • Unit cell: smallest repetitive volume which
contains the complete lattice pattern of a
crystal.
• 7 crystal systems of varying
symmetry are known,
These systems are built by
changing the lattice parameters:
a, b, and c are the edge lengths,
, , and are inter axial
angles
Fig. 3.4, Callister 7e.
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7. Basic types of crystal structures
SC BCC FCC
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8. • APF for a simple cubic structure = 0.52
APF =
a3
4
3
p (0.5a) 3
1
atoms
unit cell
atom
volume
unit cell
volume
Atomic Packing Factor (APF)
APF =
Volume of atoms in unit cell*
Volume of unit cell
*assume hard spheres
Adapted from Fig. 3.23,
Callister 7e.
close-packed directions
a
R=0.5a
contains (8 x 1/8) =
1 atom/unit cell Here: a = Rat*2
Where Rat is the ‘handbook’ atomic
radius
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9. • Coordination # = 8
Adapted from Fig. 3.2,
Callister 7e.
(Courtesy P.M. Anderson)
• Atoms touch each other along cube diagonals.
--Note: All atoms are identical; the center atom is shaded
differently only for ease of viewing.
Body Centered Cubic Structure (BCC)
ex: Cr, W, Fe (), Tantalum, Molybdenum
2 atoms/unit cell: (1 center) + (8 corners x 1/8)
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10. Atomic Packing Factor: BCC
a
APF =
4
3
p ( 3a/4)3
2
atoms
unit cell atom
volume
a3
unit cell
volume
length = 4R =
Close-packed directions:
3 a
• APF for a body-centered cubic structure = 0.68
a
R
Adapted from
Fig. 3.2(a), Callister 7e.
a
2
a
3
Prepared by Dr. Tanuja Vaidya
11. • Coordination # = 12
• ABAB... Stacking Sequence
• APF = 0.74
• 3D Projection • 2D Projection
Adapted from Fig. 3.3(a),
Callister 7e.
Hexagonal Close-Packed Structure (HCP)
6 atoms/unit cell
ex: Cd, Mg, Ti, Zn
• c/a = 1.633 (ideal)
c
a
A sites
B sites
A sites
Bottom layer
Middle layer
Top layer
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12. CRYSTALLOGRAPHIC DEFECT
Crystalline solids exhibit a periodic crystal structure. The positions of
atoms or molecules occur on repeating fixed distances, determined by the
unit cell parameters. However, the arrangement of atoms or molecules in
most crystalline materials is not perfect. The regular patterns are
interrupted by crystallographic defects
Electron microscopy of antisites (a, Mo
substitutes for S) and vacancies (b, missing
S atoms) in a monolayer of molybdenum
disulfide. Scale bar: 1 nm.[1]
Prepared by Dr. Tanuja Vaidya
13. Point defects
• Point defects are defects that occur only at or
around a single lattice point. They are not
extended in space in any dimension.
• However, these defects typically involve at most a
few extra or missing atoms.
• Larger defects in an ordered structure are usually
considered dislocation loops.
• These dislocations permit ionic transport through
crystals leading to electrochemical reactions.
point defect types in a monatomic solid
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14. Vacancy defects
•are lattice sites which would be occupied in a perfect crystal, but are vacant.
• If a neighboring atom moves to occupy the vacant site, the vacancy moves in the
opposite direction to the site which used to be occupied by the moving atom.
•The stability of the surrounding crystal structure guarantees that the neighboring
atoms will not simply collapse around the vacancy.
•In some materials, neighboring atoms actually move away from a vacancy,
because they experience attraction from atoms in the surroundings.
•Interstitial defects
•are atoms that occupy a site in the crystal structure at which there is usually not
an atom. They are generally high energy configurations.
•Small atoms in some crystals can occupy interstices without high energy, such
as hydrogen in palladium.
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15. Line Defects
•Dislocations are linear defects, around which (the atoms of the crystal lattice
are misaligned. There are two basic types of dislocations, the edge dislocation
and the screw dislocation. "Mixed" dislocations, combining aspects of both
types, are also common.
•Edge dislocations are caused by the termination of a plane of atoms in the
middle of a crystal. In such a case, the adjacent planes are not straight, but
instead bend around the edge of the terminating plane so that the crystal
structure is perfectly ordered on either side.
•The screw dislocation is more difficult to visualise, but basically comprises a
structure in which a helical path is traced around the linear defect (dislocation
line) by the atomic planes of atoms in the crystal lattice.
•The presence of dislocation results in lattice strain (distortion). For an edge
type, b is perpendicular to the dislocation line, whereas in the cases of the
screw type it is parallel. In metallic materials, b is aligned with close-packed
crystallographic directions and its magnitude is equivalent to one inter-atomic
spacing.
The dislocation line is presented in blue
Prepared by Dr. Tanuja Vaidya
16. PLANAR DEFECTS
•Grain boundaries occur where the crystallographic
direction of the lattice abruptly changes. This usually
occurs when two crystals begin growing separately
and then meet.
•Antiphase boundaries occur in ordered alloys: in this
case, the crystallographic direction remains the same,
but each side of the boundary has an opposite
phase:
Origin of stacking faults: Different stacking
sequences of close-packed crystals
Prepared by Dr. Tanuja Vaidya
