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ENMAT101A Engineering Materials and Processes
Associate Degree of Applied Engineering
(Renewable Energy Technologies)
Lecture 6 – Mechanical Deformation of Metals
www.highered.tafensw.edu.au
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Mechanical Deformation of Metals
Reference Text
Section
Higgins RA & Bolton, 2010. Materials for Engineers and Technicians,
5th ed, Butterworth Heinemann
Ch 6
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
EMMAT101A Engineering Materials and Processes
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Elastic / Plastic Strain
ELASTIC strain: At first, crystals
within the metal are distorted
elastically and strain increases
proportionally with stress. If the
stress is removed during this stage,
the metal returns to its original
shape, which is elastic.
PLASTIC strain: As stress increases
beyond the yield point, the binding
forces between atoms are
overcome, and layers of atoms
begin to slide over each other. This
process of 'slip', is not reversible, so
if the stress is removed, deformation
remains. This is plastic.
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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Slip (plastic strain). Higgins 6.2
A demonstration of slip. Polish some pure copper, deform it, then look at
the surface under a microscope. A large number of parallel, hair-like lines
on the polished and etched surface show that layers of atoms within each
crystal slid over each other (Figure 6.2). These lines are called slip bands.
doitpoms.ac.uk
Higgins Fig 6.1
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A scanning electron micrograph of a
single crystal of cadmium deforming by
dislocation slip on 100 planes, forming
steps on the surface
http://www.doitpoms.ac.uk/tlplib/slip/intro.php
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Higgins Fig 6.2
The formation of slip bands:
(i) indicates the surface of the specimen before straining and
(ii) The surface after straining. The relative slipping along the crystal
planes produces ridges
(iii) Microscope view of shadows cast by the ridges on the surface.
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Slip Plane
Watch the bubble raft carefully:
Can you see a slip plane?
What angles does the slip plane
make to the axis of the applied
stress? (Note: The applied stress is
vertical)
This is not unusual. The same
happens with ductile failure (slip) in
metals.
(Bubble raft represents a plane of HCP.
i.e. honeycomb, not square grid).
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/
EMMAT101A Engineering Materials and Processes
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45o Slip planes
An examination of the fracture surface of a tensile test piece can
show whether the part was ductile or brittle.
Ductile specimen showing “cup-andcone” failure, where shearing occurs
at 45o to the applied force. SLIP !
Brittle specimen displays an almost
flat fracture surface, perpendicular to
the applied force.
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Dislocation
Higgins 6.2.1
Animation of slip by
dislocation glide.
Dislocation glide
allows plastic
deformation to occur
at a much lower
stress than would be
required to move a
whole plane of atoms
at once.
A perfect crystal (in
theory) would be
1000 times stronger.
You Tube
Offline (mp4)
http://www.msm.cam.ac.uk/doitpoms/tlplib/dislocations/
dislocation_glide.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/
EMMAT101A Engineering Materials and Processes
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Work Hardening (Higgins 6.2.3)
A particular slip plane will not stop unless it hits something - a fault or
obstacle within the crystal. That slip plane is now locked up.
As stress increases, another slip plane moves until it locks up too.
This process continues until all the available slip planes in the piece of
metal are used up. The metal is then said to be work-hardened.
It is no longer ductile, any further increase in stress will fracture it.
In this condition, the metal is hard and strong; but it has lost its ductility,
and, if further shaping is required, must be softened by annealing.
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Tensile test on 0.4% C steel in
as-drawn state (already has
some work hardening).
Work hardening occurs each
time the test piece is stretched
beyond the (new) yield point.
(Compare YS@1 to YS@3).
Image: Tim Lovett
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Annealing (Higgins 6.3)
A cold-worked metal becomes work
hardened and has internal stresses due to
elastic strains internally balanced within
the distorted crystal structure.
Annealing removes these stresses. The
heat makes the atoms more mobile, so
they move (migrate) into more comfortable
positions, improving ductility.
Annealing progresses in three stages:
1 The relief of stress
2 Recrystallisation
3 Grain growth
Wrinkling of steel pipe that was not
annealed
http://www.sciencedirect.com/science/article/pii/S
0924013605003225
Wikipedia: Annealing
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Annealing of gas cylinders to reduce cracking at welds:
http://metallurgyfordummies.com
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Recrystallisation
Recrystallisation temperatures of some metals.
These are well below the melting point.
Higgins Table 6.1
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Grain growth (Higgins 6.3.3)
Both the time and temperature of annealing must be controlled, in order to
limit grain growth.
Highly work-hardened material has more internal stress points. These form
many nuclei of the new grains, hence grains will be smaller.
Alloy elements can also help to initiate grain formation during
recrystallisation – hence finer grains.
Recrystallisation
Properties and Grain Structure BBC (1973)
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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 vs 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
(recrystallisation),
certain grains will
grow at the expense
of their neighbours.
This is BAD!
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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Cold-working processes (Higgins 6.4)
• STRONGER: To obtain the necessary combination of strength,
hardness and toughness for service. Mild steel and most non-ferrous
materials can be hardened only by cold-work.
• FINISH: To produce a smooth, clean surface finish in the final
operation. Hot-working generally leaves an oxidised or scaly surface,
which necessitates 'pickling' the product in an acid solution.
• ACCURATE: To attain greater dimensional accuracy than is possible
in hotworking processes.
• MACHINABLE: To improve the machinability of the material by
making the surface harder and more brittle.
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Cold-working uses (Higgins 6.4.1)
• The cold-rolling of metal plate, sheet and strip.
• Spinning and flow-turning, as in the manufacture of aluminium
kitchenware.
• Stretch forming, particularly in the aircraft industry.
• Cold-heading, as in the production of nails and bolts.
• Coining and embossing.
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Hot-working processes (Higgins 6.5)
• DEFORMATION: Large deformations possible. E.g. Rolling of billets
into I beam. Breaking down initial slabs, billets and plate.
• NO WORK HARDENING: Since processing above recrystallisation
temperature.
• LOWER FORCES: Large parts might be limited to hot rolling only
since machinery could not attain adequate force for cold forming.
• HARD MATERIALS: Some materials will not cold form, or will work
harden excessively – e.g. forming of large springs.
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Hot-working uses (Higgins 6.5.1)
• Hot-rolling, for the manufacture of plate, sheet, strip and shaped
sections such as rolled-steel joists.
• Forging and drop-forging, for the production of relatively simple
shapes, but with mechanical properties superior to those of castings.
• Extrusion, for the production of many solid and hollow sections
(tubes) in both ferrous and non-ferrous materials.
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Grain flow and fibre (Higgins 6.6)
As metal is shaped, grains get distorted. Impurities that
accumulated at grain boundaries now form 'flow lines' or 'fibres' in
the direction of deformation.
When new crystals grow (independently of these fibres), they do not
weakens the structure as much as the original inter-crystalline
impurity films.
This makes the
material stronger and
tougher, particularly
along the direction of
the fibres.
At right angles to the
fibres, the material is
still weaker.
Higgins
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Grain flow and fibre examples (Higgins 6.6)
See discussion of forged bolt design vs machined bolt:
Higgins 6.6
Failure of replacement gears:
Discussion:
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Diffusion
The two main diffusion methods
are:
Vacancy Diffusion: A new atom
works it’s way into the metallic
lattice by taking vacant positions.
Interstitial Diffusion: A new
(small) atom migrates between
atoms.
Temperature increases the rate
of diffusion.
Stress encourages diffusion by
opening up gaps.
Diffusion of new type of atom into a metallic lattice.
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Diffusion: Carburising
Mild steel cannot be hardened unless
there is carbon in the lattice.
Adding carbon to steel is called
carburising.
There are several ways to do this, but
the oldest and simplest is to heat the
mild steel in the presence of carbon
(charcoal) – for a long time at high
temperature.
This allows carbon to diffuse into the
surface for a mm or so.
Pack Carburising. A few minutes excerpt
from BBC Video Heat Treatment:
Heat treatment [videorecording] / producer Brian Davies.
[B.B.C.], 1981.
Video: Discusses the use of heat which changes the
properties of metals. Outlines different techiques including
hardening, tempering, annealing, normalising as well as a
non-heat process, coldworking.
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Diffusion to
Dislocations
Dislocation slip can
be hindered:
Here, an interstitial
atom migrates into
the stress zone,
hindering
dislocations.
Otherwise the slip
continues to the
grain boundary,
which distorts the
grain, hence plastic
deformation.
You Tube
Dislocation and the effect of
migration of interstitial atoms
Offline (mp4)
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Metallurgical Furnaces (Higgins 6.7)
See text. Higgins 6.7
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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
Failure of replacement gears
Video: Heat Treatment: BBC 1981
Pack Carburising only – or salt and plasma it time allows
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GLOSSARY
Binders
Hot forming
Cold forming
Recrystallisation
Annealing
Work Hardening
Stress Relieving
Grain Growth
Slip
Dislocation
Elastic vs Plastic Strain
Interstitial atom
Substitutional atom
Diffusion
Carburising
EMMAT101A Engineering Materials and Processes
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QUESTIONS
Callister: Ch3
Moodle XML: 10107 Processing and 10105 Steel
1. Define all the glossary terms.
2. Define the glossary terms
3. Explain the difference between elastic and plastic deformation in terms of
movements of atoms.
4. Why are smaller grains stronger and tougher?
5. Describe 2 ways to reduce slip in a crystal.
6. What happens if annealing is done at excess temperature and/or for too long?
7. What can be done to keep grains small when heat treating?
8. Why does forging make a stronger part than casting?
9. What is the composition, in atom percent, of an alloy that contains 98 g tin and
65 g of lead?
10. What is diffusion? Describe two types of diffusion.
11. List several factors that encourage diffusion.
12. Describe what is Carburising does to mild steel and how it can be achieved.
EMMAT101A Engineering Materials and Processes
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