ENMAT101A Engineering Materials and Processes
Associate Degree of Applied Engineering
(Renewable Energy Technologies)
Lecture 4 – Crystal Structure of Metals
Grains after
acid etching:
Wikipedia
www.highered.tafensw.edu.au
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Crystal Structure of Metals
Reference Text
Section
Higgins RA & Bolton, 2010. Materials for Engineers and Technicians,
5th ed, Butterworth Heinemann
Ch 4
Additional Readings
Section
Sheedy, P. A, 1994. Materials : Their properties, testing and selection
Ch 1
Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and
Engineering: An Introduction, 8th Ed, Wiley, New York.
Ch 3
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Crystal Structure of Metals
Crystals are the lattice structure
that metal atoms form when they
become solid.
In engineering, crystals of metal are
called grains.
Grains in cast
aluminium
www.spaceflight.esa.int
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Solid, Liquid, Gas
Everything can exist as either solid, liquid or
gas. (state). This depends on temperature and
pressure.
Mercury freezes at -9°C and boils (to form a
gas or vapour) at 357°C at 1 atm.
Liquid Mercury: Wikipedia
Mercury vapour streetlights: theage.com.au
At the other end of the scale,
tungsten melts at 3410°C and
boils at 5930°C.
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Classification of Materials by State
• Solid: Rigid structure of atoms (or molecules). A regular pattern
of atoms is called a crystal (ceramics) or a grain (metals).
Random is called amorphous (some plastics). Low temperature
all materials freeze (become solid).
• Liquid: A liquid is a substance that flows (fluid) but does not
compress easily. Solids become liquid when heated to their
melting point (fusion)
• Gas: A gas is a compressible fluid, that expands to fill its
container. At high temperature all materials vapourise to a gas.
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Melting / Boiling of Elements
www.ptable.com
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Kinetic Theory of Gases
In any gas, the particles (whether atoms or
molecules) are in constant motion. As these
particles bounce off the walls it pushes them
– which makes pressure exerted by the gas.
As the temperature increases, the velocity
and number of impacts increases, so the
average pressure on the wall of the vessel
increases.
This is the kinetic theory of gases.
Gas Animation
Volume vs temperature
Tim Lovett 2012
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Gas to Liquid. (Metal Vapour Condensing)
The temperature of a metal vapour (gas) falls until it reaches the boiling
point where it starts to turn into liquid (condense).
In a liquid the atoms are randomly mixed together and are free to slide
around. The atoms are held together only by weak forces of attraction at
this stage, the liquid lacks cohesion and will flow.
Gas Animations: Tim Lovett 2012
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Liquid to Solid. (Solidification)
To turn the liquid metal into solid, each atom must lose more energy.
For a pure metal this occurs at a fixed temperature (melting point).
During solidification, the atoms arrange themselves according to some
regular pattern, or 'lattice structure' which is called a crystal – or in the case
of metals – a grain.
Each atom is now connected to
its neighbours like springs. This
spring stiffness causes the
Modulus of Elasticity E.
Lava Solidification:
http://www.geol.umd.edu
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Shrinkage
Grey cast iron
Contraction
(%)
0.7 to 1.05
White cast iron
2.1
Malleable iron
1.5
Steel
2.0
Brass
1.4
Alluminium
1.8
Aluminium alloys
1.3 to 1.6
Bronze
1.05 to 2.1
Magnesium
1.8
Zinc
2.5
Manganese steel
2.6
Metal
Atoms in a solid fit closer together than in
a liquid, so shrinkage occurs.
A few substances expand when they
solidify because they have a very loose
packing arrangement. E.g. Water (ice),
and Silicon.
Based on;
http://www.calculatoredge.com
Titanic iceberg (maybe): Wikipedia
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Latent Heat
A pure metal solidifies at a fixed
temperature (melting point).
The liquid resists cooling below the
melting point until the liquid has
solidified. This requires removal of
the Latent Heat. This energy is
called the latent heat of fusion
(solidification in this case).
Alloys (metal mixtures) can have a
range of melting temperatures.
Higgins: Fig 4.1
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Crystals
During solidification,
the atoms fit into a
regular pattern, or
'lattice structure'
called a crystal.
(or a metal grain).
In a 2D plane, the
tightest arrangement
is like a honeycomb.
(based on 120o).
http://www.chem.ufl.edu/~itl/2045/lectures/lec_h.html
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Crystals
In 3D, the common
crystal packing
arrangements are;
BCC, FCC and
HCP.
FCC and HCP pack
the closest, while
BCC packs slightly
less dense (by a
few %).
Higgins: Fig 4.2
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Crystals
The unit structures of the 3
crystal structures are a bit
misleading.
Lattice structures: Part 1
thechemprofessor
Local (mp4)
They are different SIZES!
The packing difference is
only a few %.
Crystals
A very 3 dimensional
problem, so best explained
in 3D video.
Univ of Florida
Local
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Allotropy of Iron
Iron is polymorphic it forms more than
one crystalline form
(allotropes of Iron).
The two main crystal
structures are;
BCC (a-iron)
FCC (g-iron) above
910°C.
Iron
BCC body-centred cubic
(a-iron)
FCC above 910°C
(g-iron).
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Iron BCC vs FCC
Iron
Since FCC is tighter than BCC, there is a sudden volume change during
quenching. This can cause internal stresses, distortion or even cracking of
the component.
But, it is also the
reason steel can
be heat treated
at all… making it
by far the most
important metal
in engineering.
Need to quench
with care.
Cracking of steel due to quenching
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Dendritic solidification (Higgins 4.3.1)
As the molten pure metal cools below its freezing point,
crystallisation will begin.
It starts out with a single unit – (e.g. BCC for Tungsten).
New atoms will join the 'seed crystal' and grow onto the
structure much like a snowflake (except the metal is
forming in liquid, not a cloud of droplets).
The branched
crystal is called a
'dendrite‘ (Greek
for tree).
BCC Unit: Higgins Fig 4.3
Snowflake: Wikipedia
Higgins Fig 4.4
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Dendrite of Silver: Wikipedia
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Dendrites to Grains
Each dendrite forms independently, so
the outer arms of neighbouring
dendrites make contact with each other
at irregular angles and this leads to the
irregular overall shape of crystals.
VIDEO: Excerpt from BBC Properties and
Grain Structure 1973
Grains form
random
orientations
Wikipedia
Grains after
acid etching:
Wikipedia
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Impurities
Above:
Dendritic formation of
grains.
Below:
Etched metal showing
dendritic structure with
porosity where molten
metal was not
available to fill the
voids.
openlearn.open.ac.uk
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The jumbled, chaotic area between grains. The
irregular nature of the grain boundary is one
source of creep in metals but it is a barrier to
dislocation mobility. www.spaceflight.esa.int
Grains
A section through a cast aluminium
ingot, polished and etched in acid.
The crystals are all the same (pure
aluminium), but each crystal lattice is
randomly tilted and so reflects the light
differently. This is why they look bright
or dark.
Note a metal “grain” is not like the
“grain” in wood. It means crystal.
Grains in cast
aluminium
www.spaceflight.esa.int
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Grain Boundaries
In pure metals, dentrites
disappear once solidification is
complete.
If impurities were in the melt, they
may get left out of the growing
grain and end up at the grain
boundaries.
This is why a small amount of
impurity can destroy the
mechanical properties – causing
brittleness and cracks along the
crystal boundaries.
The jumbled, chaotic area between grains. The
irregular nature of the grain boundary is one
source of creep in metals but it is a barrier to
dislocation mobility. www.spaceflight.esa.int
EMMAT101A Engineering Materials and Processes
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Grain Boundaries
Bubble raft demonstrating grains
in a metal under stress.
Each bubble represents an atom.
Under low stress, atoms stretch
elastically.
With higher deformation plastic
deformation occurs. The bubble
raft rearranges by dislocation
motion.
You Tube
Offline (mp4)
From TLP: Introduction to dislocations,
http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/dislocation
_motion.php Courtesy of DoITPoMS, The University of
Cambridge. Released under Creative Commons Attribution-NonCommercial-Share Alike licence
http://creativecommons.org/licenses/by-nc-sa/2.0/uk/
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Porosity
Porosity occurs because the casting
shrinks on solidification.
High pressure can fix this, but it makes
the mould very expensive.
Another way is to use a reservoir of
molten metal that feeds more liquid as it
solidifies.
In this low pressure casting, aluminium alloy was
poured down the runner into the mould. The
hollow in the top of the runner caused by liquid
flowing from the runner into the mould as the
casting solidified. As well as the hollow at the top,
you can see some holes in the runner and one
hole within the casting itself. The runners and
risers will later be cut off and discarded.
http://openlearn.open.ac.uk
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Cooling rates and
grain size
Slow cooling = more time to form
= larger grains.
Rapid cooling = fine grains.
For the same metal grain, finer
grains are stronger and tougher.
Grains are typically 0.1 to 100
microns.
Note: This is NOT referring to
quenching of Carbon steel.
Quenching produces a different type
of grain - Martensite.
Grain size vsd yield strength. Low C steel.
W.O. Alexander, G.J. Davies, K.A. Reynolds and E.J.
Bradbury: Essential metallurgy for engineers, p63-71. 1985.
Van Nostrand Reinhold (UK) Co. Ltd. ISBN: 0-442-30624-5
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Grain Growth
If solid metal is
above a certain
temperature
(recrystalisation),
certain grains will
grow at the
expense of their
neighbours.
Growth of a grain structure
You Tube
Offline (mp4)
http://www.doitpoms.ac.uk/tlplib/grainGr
owth/2dcomputersimulation.php
Courtesy of DoITPoMS, The University
of Cambridge. Released under Creative
Commons Attribution-Non-CommercialShare Alike licence
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Cooling of a large casting
When a large ingot solidifies, the outer
skin cools rapidly (small grains).
Cooling then travels inwards, creating
inward grain growth. (Columnar)
The left over molten metal at the centre,
cools slowly (large grains). Cooling in all
directions, so equi-axed.
So the grain structure makes castings weak.
Zones of different crystal forms in
an ingot. Higgins Figure 4.12
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Make sense now?
Grains in cast
aluminium
www.spaceflight.esa.int
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Forging
Forging improves
steel – because it
mixes up the bad
grain structure of
a casting.
Forging of Samurai sword.
You Tube
http://www.roninkatana.com
Offline (mp4)
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Rapid solidification (Higgins 4.5.1)
By cooling fast enough, tiny
crystals are formed – or
even none at all. (glassy
metal)
Any impurities dispersed in
the melt will remain so in the
amorphous solid.
The metal is more uniform
in composition, stronger and
tougher, but still malleable
and ductile.
But cooling so fast (a million
oC per sec) means only very
thin bits have been made.
Amorphous (Glassy) Metals
http://metallurgyfordummies.com/amorphous-metal/
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Glassy Metal
'Melt Spinning' or 'Splat
quenching' to produce
metallic glass ribbon.
Molten tin alloy is fed
through several nozzles
onto a rotating metal
drum, resulting in
extremely high cooling
rates (approx. 1 million
Kelvin per second).
High speed photography
at 4000 fps.
You Tube
Offline
From TLP: Casting, http://www.doitpoms.ac.uk/tlplibdev2008/casting/casting_other.php
DoITPoMS, The University of Cambridge.
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VIDEO: Crystals and Grain Structure
Properties and Grain Structure BBC (1973)
What is a grain?
Recrystalisation
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Online Properties Resources.
Graphical comparison of materials properties.
DoITPoMS: Dissemination of IT for the Promotion of Materials Science
Wikipedia: Materials properties
Metal Grains and processing
Lattice Structure BCC, FCC,HCP
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GLOSSARY
Allotropy
Amorphous
Anisotropy
Body-centred cubic (BCC)
Coordination number
Crystalline
Face-centred cubic (FCC)
Grain
Grain boundary
Hexagonal close-packed (HCP)
Isotropic
Lattice
Noncrystalline
Polycrystalline
Polymorphism
Single crystal
Unit cell
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QUESTIONS
Callister: Ch3 (Mostly about calculating atomic packing factors - too esoteric)
Moodle XML: Some questions in 10102 Classification and 10105 Steel
1.
2.
3.
4.
5.
Define all the glossary terms.
What is the difference between atomic structure and crystal structure?
What is a dendrite?
Explain why ice floats.
As a pure metal constantly cools, what happens to the temperature at the
melting point? Explain.
6. Determine the coordination number (number of direct neighbours) for BCC, FCC
and HCP lattice packing.
7. As pure iron cools, at 910oC it switches from ….. lattice to …..
8. Explain why grain size is important in engineering metals, and which is best.
EMMAT101A Engineering Materials and Processes
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