1. Engineering Materials and Processes Lecture 4 – Crystal Structure of Metals
Prescribed Text:
Ref 1: Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. ISBN:
Readings:
Callister: Callister, W. Jr. and Rethwisch, D., 2010, Materials Science and Engineering: An Introduction, 8th Edition, Wiley, New York. ISBN
Ashby 1: Ashby, M. & Jones, D., 2011, Engineering Materials 1: An Introduction to Properties, Applications and Design, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN:
Ashby 2: Ashby, M. & Jones, D., 2011, Engineering Materials 2: An Introduction to Microstructures and Processing, 4th edition, Butterworth-Heinemann, Oxford UK. IBSN:
Lecture (2 hrs):
Ref 1, Ch 1: Engineering materials;
Ref 1 Ch 2: Properties of materials.
Laboratory 1 (2 hrs): Hardness test
Callister: Ch 1, 2, 18-21
Ashby 1: Ch 1, 2
Ashby 2: Ch 1
Grains after acid etching: Wikipedia 1

2. Crystal Structure of Metals
Reference Text
Section
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann
Ch 4
Additional Readings
Section
Sheedy, P. A, 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
Engineering Materials and Processes

3. 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
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4. 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
At the other end of the scale, tungsten melts at 3410°C and boils at 5930°C.
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 1.2.1
Mercury vapour streetlights: theage.com.au
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5. 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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6. Melting / Boiling of Elements
A classifiaction by atomic structure of all the elements (different types of atoms)
Go to website for PT animation of temperature…
Engineering Materials and Processes

7. 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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8. 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.
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 1.2.1
Gas Animations: Tim Lovett 2012
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9. 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.
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
Lava Solidification:
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10. Shrinkage
Metal
Contraction (%)
Grey cast iron
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
Zinc
2.5
Manganese steel
2.6
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.
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
Based on;
Titanic iceberg (maybe): Wikipedia
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11. 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
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
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12. 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).
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
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13. 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 RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
Higgins: Fig 4.2
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14. Crystals
The unit structures of the 3 crystal structures are a bit misleading.
They are different SIZES!
The packing difference is only a few %.
A very 3 dimensional problem, so best explained in 3D video.
Lattice structures: Part 1 thechemprofessor
Local (mp4)
Crystals
Univ of Florida
Local
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
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15. 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.
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
BCC body-centred cubic (a-iron)
FCC above 910°C
(g-iron).
Iron
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16. 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.
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
Cracking of steel due to quenching
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17. 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).
BCC Unit: Higgins Fig 4.3
The branched crystal is called a 'dendrite‘ (Greek for tree).
Snowflake: Wikipedia
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
Higgins Fig 4.4
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18. Dendrite of Silver: Wikipedia
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19. VIDEO: Excerpt from BBC Properties and Grain Structure 1973
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
Higgins RA & Bolton, Materials for Engineers and Technicians, 5th edition, Butterworth Heinemann. Ch 4
Grains after acid etching: Wikipedia
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20. 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
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. 20

21. 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
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22. 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.
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23. 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, Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-Share Alike licence
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24. 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.
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25. 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, p Van Nostrand Reinhold (UK) Co. Ltd. ISBN:
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26. 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 Courtesy of DoITPoMS, The University of Cambridge. Released under Creative Commons Attribution-Non-Commercial-Share Alike licence
You Tube
Offline (mp4)
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27. 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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28. Make sense now? Engineering Materials and Processes
Grains in cast aluminium
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29. Forging
Forging improves steel – because it mixes up the bad grain structure of a casting.
Forging of Samurai sword.
You Tube
Offline (mp4)
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30. 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
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31. 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,
DoITPoMS, The University of Cambridge.
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32. VIDEO: Crystals and Grain Structure
Properties and Grain Structure BBC (1973)
What is a grain?
Recrystalisation
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33. 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
h ttp://
S how this website on screen. Will be using this later.
Lattice Structure BCC, FCC,HCP
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34. Body-centred cubic (BCC) Coordination number Crystalline
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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35. Define all the glossary terms.
QUESTIONS
Callister: Ch3 (Mostly about calculating atomic packing factors - too esoteric)
Moodle XML: Some questions in Classification and Steel
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.
Determine the coordination number (number of direct neighbours) for BCC, FCC and HCP lattice packing.
As pure iron cools, at 910oC it switches from ….. lattice to …..
Explain why grain size is important in engineering metals, and which is best.
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