Transcript The Structure of Metals” Group # 3 2/06/2006 Keith Dager

“The Structure of Metals”
Group # 3
2/06/2006
Keith Dager
David Fuller
Grant Thomas
Austin Weddington
Jared Martinez
Introduction
 Many questions about metal can be answered
by knowing their ATOMIC STRUCTURE
(the arrangement of the atoms within the metals).
The Crystal Structure of Metals
 Metals and Crystals

What determines the strength of a
specific metal.
 Three basic atomic arrangements
1of 3 basic atomic arrangements
1. Body-centered cubic (bcc)
1. A portion of the structure of a body-centered cubic metal (b.c.c.)
2 of 3 basic atomic arrangements
2. Face-centered cubic (fcc) also known as Cubic close packing
3 of 3 basic atomic arrangements
3. Hexagonal close-packed (hcp)
Review of the three basic
Atomic Structures
B.C.C.
F.C.C
H.C.P
Basic terminology
 Hard-ball / Hard-sphere - the small spheres used to
display the unit cell and show the individual atomic
arrangement.
 Basal planes- This is the orientation/layout of the
atomic arrangement specifically in the h.c.p. layout
of the ABAB pattern.
 Alloying- this is formed by adding atoms of one
metal/metals to some other metal/metals.
 Allotropism/Polymorphism (meaning many
shapes)- the appearance of more than one type of
crystals.
 When a single crystal is subjected to an external force, it goes through
Elastic Deformation
 If the force on the crystal is increased sufficiently, the crystal goes
through Plastic/Permanent Deformation

The amount of stress required for a crystal to permanently deform is
the called the Critical Shear Stress
Cross-Section
F2
 Shear Stress is the ratio of the applied
shearing force to the cross-sectional
area being sheared
 When this occurs, one plane of atoms
slips across an adjacent plane of
atoms
F1
Tensile Force2
Tensile Force1
Video here
 b/a ratio – Proportional to the amount of shear stress needed to cause
slip in single crystals
Atoms
b is inversely proportional to
the atomic density in the atomic plane
a is the spacing
of atomic planes
 Anisotropic – The different properties of a single crystal
when tested in different directions

Examples : Plywood & Cloth
 Twinning – The crystal forms a mirror image of itself
across the plane of twinning
Tensile Force1
Tensile Force2

A slip system is the combination of a slip plane and its direction of
slip

Body-Centered Cubic structure

48 slip systems

Highly probable for any shear stress to act on one of these
systems, but because of a high b/a ratio, the shear stress
required must be high

Metals with these structures have good strength and moderate
ductility (flexibility)

Face-Centered Cubic structure




12 slip systems
Moderate probability for a shear stress to act on one
of these systems
Low b/a ratio, the shear stress required is low
Metals with these structures have moderate strength
and good ductility

Hexagonal Close-Packed structure




3 slip systems
Low probability for a shear stress to act on one of
these systems
More systems become active at elevated
temperatures
Metals with these structures are usually brittle at
room temperature


The actual strengths of metals are lower than theoretical
calculations because of defects and imperfections in
crystal structures
This includes:

Grain/Phase Boundaries (Next Section)

Volume/Bulk Imperfections



Voids, Cracks, Inclusions ( nonmetallic elements)
Point Defect
Dislocations



Vacancy - a missing atom
Interstitial Atom - an extra atom in the structure
Impurity – a foreign atom that has replaced an atom of
metal

Dislocations are defects in the orderly arrangement of the
atomic structure including:
Screw Dislocations
Edge Dislocations

Dislocations can lower the required shear stress to cause
slip
They can also interfere with each other and be impeded by
barriers ( grain boundaries, impurities, inclusions ) which
can cause the required shear stress for slip to go up.
This is referred to as Work/Strain Hardening




Increases strength of metal
Increases hardness of metal
Grains and Grain Boundaries
and their effects on a metal
Grain size has a
significant effect on the
strength of metals.
Grain boundaries have a
major influence on metal
behavior.
What are grains?
 Grains- individual, randomly oriented
crystals within a metal
Grain Structure of metal alloys
How are grains formed?
 Molten metal begins to solidify
 Crystals begin to form independently of
each other
 Each crystal has random unrelated
orientation to the other
 Each of these crystals grows into a
crystalline structure (a grain)
What difference does Grain Size make?
 Significantly influences mechanical properties of
the metal
 Large grain size is generally associated with
- Low strength
- Low hardness
- Low ductility (extent of deformation before fracture)
 Small grain size is generally the opposite
What determines Grain size?
Three factors that influence the median size of
developed grains
• Rate of nucleation (initial rate of formation
of the individual crystals)
• Number of sites where crystals begin to
form
• Rate crystals grow
Calcite crystal (~50 µm) grown at T = 800
°C and p = 300 MPa within 10 hours
What determines Grain
number?
 High rate of nucleation (comparatively)
- Number of grains per unit volume of metal will be high
- Grain size will be small
 High growth rate (comparatively)
- Fewer grains per unit volume of metal
- Grain size will be large
Wear-resistant
nanometals
could make
sporting goods
more durable.
The grain size of a
nanocrystalline
metal, right, is
about 1,000 times
smaller than
conventional metal,
above. When grain
size is cut in half,
the company says,
hardness
quadruples.
Controlling grain size
In general:
Rapid cooling produces smaller grains.
Slow cooling produces larger grains.
The smaller the grain size, the
stronger the metal
How is grain size measured?
1. Counting the grains in a given area
2. Counting number of grains that intersect a
length of a line (microscopic)
3. Determined by comparing to a standard
chart (ASTM Chart)
ASTM Chart
 American Society for
Testing and Materials
 Grain number is
determined by
formula:
N = 2^(n-1)
N- Number of grains
n- grain size number
Per square inch at 100x
magnification
TABLE 1.1
ASTM No.
–3
–2
–1
0
1
2
3
4
5
6
7
8
9
10
11
12
Grains/mm2
1
2
4
8
16
32
64
128
256
512
1,024
2,048
4,096
8,200
16,400
32,800
Grains/mm3
0.7
2
5.6
16
45
128
360
1,020
2,900
8,200
23,000
65,000
185,000
520,000
1,500,000
4,200,000
Examples of grain size
 Grain sizes between 5 and 8 are generally
considered fine grains
 Grain size of 7 is acceptable for sheet metal
of car bodies, kitchen utensils, and
appliances
 Grain size can also be so large as to be seen with
the naked eye as in zinc on the surface of
galvanized sheet steels
What are grain boundaries?
 Surfaces that separate individual grains
1. Influence strain hardening
(boundaries interfere with
the movement of
dislocations)
2. More reactive than the
grains themselves
(atoms along grain
boundaries are more
disordered and packed less
efficiently)
3. Great influence on the
strength and ductility of the
metal
Cast Iron grain boundaries
What can happen at grain
boundaries?
1. Grain boundary sliding
2. Grain boundary
embrittlement
3. Hot shortness
Grain boundary sliding
 Process by which grains will begin to slide
along one another at the boundary
Possible Effects on Metal:
1.
2.
Plastic deformation
Creep mechanism (elongation under stress over period of
time)
Note: These types of deformation
are usually accompanied with high
temperatures as well
Illustration of Creep Mechanism
Grain Boundary embrittlement
 Generally: weakening of the grain
boundaries by embrittling elements
1.
2.
3.
Liquid-metal embrittlement (elements are in liquid state)
Solid-Metal embrittlement (elements are in the solid state)
Temper embrittlement (in alloy steels -caused by segregation
(movement) of impurities to grain boundaries)
Hot Shortness
 Generally: softening and/ or melting of metal along grain
boundaries
 Cause: local melting of impurity in the boundary at a
temperature below the melting point of the metal itself
 Effect: when subjected to plastic deformation at elevated
temperatures, (hot working) the piece of metal crumbles and
disintegrates along the metal boundary
 Prevention: metal is worked at a lower temperature
Plastic Deformation of
Polycrystalline Metals
 Polycrystalline Metals
 Equiaxed grains
 Plastic deformation
 Strain (deformation)
 Anisotropy
 Preferred Orientation
 Mechanical Fibering
Polycrystalline metals
What is Plastic Deformation of Polycrystalline Metals and all that goes with it?
polycrystalline materials: metals, alloys, intermetallic compounds,
ceramic materials, compound materials, polymers, semiconductors,
nanocrystals, supraconductors, rocks;
•Metals commonly used in manufacturing various products
Consist of many individual, randomly oriented crystals (grains);
Thus, metal structures typically are not single crystals but
Polycrystals. (“many crystals”)
•The most natural and artificial solids (rocks, ceramics, metal
alloys or polymers) are polycrystalline. They contain many
crystallites of different size, shape and different orientations.
Equiaxed grains
•Equiaxed grains
•Having equal dimensions in all
Directions, as shown in the in
Fig.1.12a
Reduction operation resulting in directionality or anisotropy
Grain Sizes
Equiaxed grains
(having equal
dimensions in all
Directions, as shown
in the in Fig.1.12a)
TABLE 1.1
ASTM No.
–3
–2
–1
0
1
2
3
4
5
6
7
8
9
10
11
12
Grains/mm2
1
2
4
8
16
32
64
128
256
512
1,024
2,048
4,096
8,200
16,400
32,800
Grains/mm3
0.7
2
5.6
16
45
128
360
1,020
2,900
8,200
23,000
65,000
185,000
520,000
1,500,000
4,200,000
Cold Working
 If a polycrystalline metal with uniform
equiaxed grains is subjected to plastic
deformation at room temperature(cold
working), the grains become deformed and
elongated.
Homologous Temperature Ranges for Various Processes
TABLE 1.2
Process
Cold working
Warm working
Hot working
T/Tm
< 0.3
0.3 to 0.5
> 0.6
Deformation Process
 Compressing the metal:

Compression stresses develop within a material when forces
compress or crush the material. A column that supports an overhead
beam is in compression, and the internal stresses that develop within
the column are compression.

As is done in forging to make a turbine disk

"Stretching"

Stretching is a process where sheet metal is clamped around its
edges and stretched over a die or form block. This process is mainly
used for the manufacture of aircraft wings, automotive door and
window panels.

As is done in stretching sheet metal to make a car body.
Compressing and stretching
(Turbine Disk and Car Body)

Types of compressions and
strains

ANISOTROPY (Texture)
As result of Plastic deformation, the grains have elongated in
one direction and contracted in the other. Consequently, this
piece of metal has become anisotropic.
Anisotropic surface will change in
appearance as it is rotated about its
geometric normal, as is the case with
velvet.
anisotropic properties: plasticity, elasticity, hardness, strength,
cleavability, thermal expansion and conductivity, electric conductivity,
magnetization, corrosion resistance, optical properties.
ANISOTROPY
 Texture and Anisotropy of Crystalline Materials
 A crystal is characterized by the periodic arrangement of its elements
(atoms, ions) in space. This always generates a dependence of the crystal
properties on the chosen direction, which is called anisotropy. Thus, the
modulus of elasticity can vary by the factor 22 in a graphite crystal
depending on the direction.

Anisotrop
y
(b)
Figure 1.13 (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging
(caused by, for example, pushing a steel ball against the sheet). Note the orientation of the crack
with respect to the rolling direction of the sheet; this sheet is anisotropic. (b) Aluminum sheet with a
crack (vertical dark line at the center) developed in a bulge test; the rolling direction of the sheet was
vertical. Source: J.S. Kallend, Illinois Institute of Technology.
Anisotropy
The most important parameter describing the anisotropy of polycrystalline
materials is their texture. Via the anisotropy of physical properties due to the
lattice structure, a regular texture in which the crystallites of one phase have
only a few preferred orientations produces anisotropy of the polycrystalline
material as well.
.
Anisotropy
 In a single crystal, the physical and mechanical
properties often differ with orientation. It can be
seen from looking at our models of crystalline
structure that atoms should be able to slip over one
another or distort in relation to one another easier in
some directions than others. When the properties of
a material vary with different crystallographic
orientations, the material is said to be anisotropic.
Preferred Orientation
 Also called crystallographic
anisotropy can best be described by
referring to fig. 1.6a.When a metal
crystal is subjected to tension, the
sliding blocks rotate the direction of
the pulling force direction.
 Note:for polycrystalline with grains
in random orientation, all slip
directions tend to align themselves
with the direction of the pulling
force while slip planes under
compression tend to align
themselves in a direction
perpendicular to the direction of the
compressing force.
Preferred Orientation
1.12
Equiaxed grains (having equal
dimensions in all
Directions, as shown in the in
Fig.1.12a)
(equiaxed) grains in a
specimen subjected to
compression (such as occurs
Figure 1.12 Plastic
deformation of idealized in
the rolling or forging of
metals): (a) before
deformation; and (b) after
deformation. Note hte
alignment of grain
boundaries along a
horizontal direction; this
effect is known as preferred
orientation.
Mechanical Fibering
 This type of anisotropy results from the
alignment of inclusions (stringers),
impurities and voids in the metal during
•Inclusions
(Stringers)- Usually non-metallic particles
deformations.
contained in metal. In steel they may consist of simple or
complex oxides, sulphides, silicates and sometimes
nitrides of iron, maganese, silicon, aluminium and other
elements. In general they are detrimental to mechanical
properties buy much depends on the number, their size,
shape and distribution.
Mechanical Fibering
impurity effects on metals (graph
explanation)
 The first model proposed that elements that draw
charge from the neighbouring metal–metal bonds weaken them and
embrittle the materials1,2.
 The second model states that the tendency to
embrittlement is determined by whether the impurity is more likely
to segregate to a grain boundary or a surface3,4.The model
suggests that
this parameter predicts the relative likelihood of forming a sharp
crack
(brittle fracture) over a blunt crack (as in a tough material).Using this
criterion, the segregation data of Miolinari et al.5 would predict that Bi
would not embrittle Cu.
 The third model states that embrittlement
occurs when impurities segregate to the grain boundaries and make
Effects of impurities (graphs)
1
3
2
Perfect analogy:
Mechanical fibering
 Major consequence: If the spherical grains in fig 1.12a were
coated with impurities, these impurities would align
themselves generally in a horizontal direction after
deformation. Because impurities weaken the grain
boundaries. This piece of metal will be weak and less ductile
when tested in the vertical direction. Such as a plywood.
Recovery, Recrystallization,
Grain Growth
Recovery:
 The removal of residual stresses by localized
plastic flow as the result of low-temperature
annealing operations.
Recrystallization:
 The change from one crystal structure to
another, as occurs on heating or cooling through
a critical temperature.
Grain Growth:
 The enlarging or coarsening of the
individual grains within the metal or
alloy during heating at a
temperature above the
recrystalization temperature.
Cold, Hot and Warm Working
Cold Work
 Altering the shape
or size of a metal
by plastic
deformation. it is
carried out below
the
recrystallisation
point usually at
room temperature
Methods of cold working
 Processes include
rolling, drawing,
pressing,
spinning, and
peening
Effects of cold work
 Increased Strength
 Increased Hardness
 Decreased
Malleability
 Decreased Ductility
Hot Working
 The rolling,
forging or
extruding of a
metal at a
temperature
above its
recrystallisation
point.
Method of Hot work
 Rolling, Pressing,
Forging
 Quenching
Effects of Hot Work
 Undesirable surface
finish
 Less force required
than cold work
 Decreased yield
strength
Warm Working
 warm working: is carried
out at intermediate
temperatures. It is a
compromise between cold
and hot working.
Effects of Warm Working
The effects of this type of working
depend on how close is the
warm process to be a cold or hot
process.
The type of process chosen depends
on the physical and mechanical
properties needed for the
product, meaning the product
itself and its uses.
Annealing
Annealing consists of:
(1) recovery (stressrelief ),
(2) recrystallization,
(3) grain growth
Annealing reduces
the hardness, yield
strength and tensile
strength of the steel.
The benefits of
annealing are:
Improved ductility
Removal of residual
stresses that result from
cold-working or
machining
Improved machinability
Grain refinement
A Real World Application
Conclusions
1. The terms cold, warm and hot working are relative. The difference between them
is the temperature at which the process is carried out.
2. There is not a “best” process. It all depends on what will be the use of the product,
or what is the product going to be.
Definitions


Quenching: Rapid cooling from a high
temperature by immersion in a liquid bath
of oil or water. Used to harden metal.
Hardness: is the characteristic of a solid material

permanently without breaking or rupturing

Ductility: is the physical property of being capable
of sustaining large plastic deformation without
fracture


Peening: is the mechanical working of metals by
means of hammer blows or by blasting with shot
Malleability: is the
property that enables a material to deform by co
mpressive forces without developing defects
Tensile strength: measures the force required to
pull something such as rope, wire, or a structural beam
to the point where it breaks

Brittleness: is the opposite of the property of
plastic- ity. A brittle metal is one that breaks or shatters
before it deforms Brittleness is the opposite of the
property of plastic- ity. A brittle metal is one that
breaks or shatters before it deforms
expressing its resistance to permanent deformation

Plasticity: is the ability of a material to deform

Annealing: is the process by which the distorted
cold worked lattice structure is changed back to
one which is strain free through the application of
heat.
Glossary
 Ductility
 the property of a metal which allows it to be permanently
deformed, in tension, before final rupture.
•Elasticity
•The property which enables a material to return to its original shape and dime
•Fatigue Strength
•the unit stress that ruptures a bar after an enormous (around 40 million) num
•repetitions of a load covering a range of values.
•Rolling
•The process of shaping metals by passing it between rolls revolving at
the same peripheral speed and in opposite directions.
References







http://www.nd.edu/~manufact/figures.html
http://en.wikipedia.org
http://info.lu.farmingdale.edu/depts/met/met20
5/annealingstages.html
http://www.principalmetals.com/glossary/rdoc.
htm
info.lu.farmingdale.edu/.../
met205/coldwork.html
http://tech.clayton.edu/eddins/hotworki.htm
http://www.seas.upenn.edu/~chem101/ssc
hem/metallicsolids.html
Work cited
•http://www.ndted.org/EducationResources/CommunityCollege/Materials/Structure/anisotropy.
htm
 http://www.tpub.com/steelworker1/2.htm
Questions