Hello and Welcome to the newest thing in materials
engineering to hit the web since a photon hit an atom! Hopefully I can shed a
bit more light on a few subjects that
I’ve written about over the past few years as a Materials Engineering Student
at Loughborough University. The Idea of this is to spread a bit of the
knowledge that I have gained over my time as a student and also to help others
in their work themselves.
Here is a few to get you started, I hope they are up to the standard
of what you come to expect from university students, many thanks for looking,
and I hope you enjoy the content!
The common DVD player works in much the same way as a
CD player. Just like its sister product, the information on a DVD is coded as a
series of pits on the disks surface. These pits are arranged in a spiral
structure in order to be read off by a laser within the DVD player. This laser
is the key reason why DVDs have superseded CD’s in the world of data storage.
DVD’s use a shorter wavelength for the laser that enables it to place the pits
in a much denser arrangement, therefore enabling the same area of disk to hold
much more data.
Within a DVD player, its main components consist of a
disk drive mechanism, which uses a spindle to hold the disk and a motor to spin
it, a printed circuit board, which contains all of the electrical components to
convert the data being read into a usable format, and an optical system
assembly, which is the laser that reads the information off the DVD. All of
these parts can be broken down into their component pieces, which will consist
of a range of metals and polymers that have been specifically chosen for their
use by their physical and sometimes electrical properties.
There are two types of plastics in production today,
the first of which are classified as thermoplastics, they fall under this
classification if they possess the properties of being remouldable under heat
and pressure. Thermoplastics, such as polyethylene and polypropylene are used
in production in low stress applications where they are not put under any load
as thermoplastics have a tendency to display creep when under extreme loads.(1)
The other category is Thermosetting plastics, these are polymers which form a
highly crosslinked molecule upon heating. The reaction is irreversible but the highly
crosslinked structure adds properties of heat and chemical resistance, and also
a high dimensional stability due to the chains being unable to slip therefore
reducing ductility.
In this experiment, 9 parts have been taken
from the internals of a DVD player to be analysed with respect to what material
they are made from and how they were processed during production. The metallic
parts of the DVD player are to be mounted in Bakelite on a metallographers
mounting press, whilst the plastic elements can be Identified using the process
of Simple Identification, whilst any hard to identify metals can be analysed
using the Transmission Electron Microscope. The TEM works on the same basic
principle as a light microscope but instead of light, it uses electrons. A TEM
uses the much lower wavelength of electrons to get a resolution nearly a
thousand times better than a conventional microscope. It uses electromagnetic
lenses to focus a beam of overhead electrons into a very thin beam.(3) This
beam then travels through the specimen you want to study and then any
unscattered electrons hit a fluorescent screen which shows the image as dark
and light density patches.Four metallic samples and five polymeric samples were
chosen to give a wide range of materials, giving a large spectrum of possible
results.
Experimental Procedure
The testing of the polymeric samples was
done using a process called Simple Identification. This involves a number of
simple and conclusive tests that, when referenced against a table of results, a
clear material can easily be seen.
The first test was a density test to
measure whether it floated or not. Care was taken to ensure that surface
tension was broken and that no air was trapped inside the specimen. The next
test for the polymers was a test called the Beilstein test. This involved
heating the end of a copper wire in a Bunsen flame until the flame turned
colourless, using this hot wire to burn through the polymer on test left an
amount of residue on the wire. When put back in the flame, the presence of
halogen within the polymer could be seen if the flame turned green/blue.
Testing after this was more of a visual test rather than chemical, firstly the
specimens were tested to see if they could be torn or cut easily, this gave
another property that could be cross referenced to eliminate some polymers. The
heating test was also used, this was useful to gauge whether or not the polymer
was a thermoplastic i.e. whether it melted or not, and also, using litmus
paper, the pH of the reaction in the escaping vapours could be measured. Lastly
the sample was left in the Bunsen flame to burn, this allowed us to see whether
the sample was self extinguishing, what colour the smoke that was given off
was, whether it gave off particles, and also what kind of smell the smoke had
as this is an easy way to detect some polymers, such as PE smelling like candle
wax.
An alternative and altogether more accurate
way of testing a polymer to determine its molecular make up is by means of
Infra Red Spectroscopy.The use of
infrared rays is based on the principle first given by the Beer-Lambert law.(2)
This states that the amount of rays absorbed is wholly dependant upon the
chemical composition of a given material. When the material is hit with the
rays, the outer electrons get excited. This only occurs at differing energy
levels. When testing polymers, infrared is used at a frequency of 0.8-25 µm,
which correlates with the frequency required to excite the outer electrons.
This leads to the infrared light getting absorbed by the outer electrons, which
creates a lower density of that frequency which can be detected and set on a
graph.(3) This
printout can then be compared to past printouts to determine by which peaks and
troughs are evident, which material it is. This process was needed for a sample
that was an elastomer band used for driving a pulley system within the DVD
drive.
In order for the metallic samples to be
properly analysed under the microscope to determine structure and composition,
they had to be mounted and polished. The samples were first cut into small
pieces, mounted in Bakelite using the metallographers mounting press, and
finely polished. The polishing process was time consuming but necessary,
starting off with coarse silicon carbide paper and working onto finer and finer
grades, the scratches were finally rubbed out of the surface of the metal using
the polishing wheels. Care was taken to ensure the correct polishing wheel was
used depending on whether or not the metal was ferrous or not. This was first tested
using a magnet. Using initial estimates as to what the metal was, the correct
etching liquid was used to finely etch the metal to bring out the internal
structure more clearly. The samples were then viewed and photographed under a
microscope to determine what the metals were by their structure.
One
metallic sample that appeared at first glance to be a steel of some kind did
not respond to the etching agent of 2% Nital, so the agent was changed for 10%
Nital. Again, the metal remained unchanged and the structure did not appear, at
this point the metal could’ve been a number of things including a type of
Stainless Steel or possibly a Chromium alloy, so to determine exactly what it
was it was sent to the Transmission Electron Microscope.
Results
On looking at the first of the
metallic samples, which was a screw threaded piece that rotated to move the
laser reader, it was noticed that where the sample had snapped off of the
original piece, the break revealed a structure not dissimilar to that of a ceramic
which led us to believe that the process by which it was made was that of a
powdered metal possibly by injection forming. It was not until the molecular
structure was revealed after the etching in Carapellas Reagent , which is made
up for ferric chloride, hydrochloric acid and alcohol, that this was found not
to be the case. The screw was found to be a Brass of composition 60% Copper,
40% Zinc.
(1)(2)
The second piece that was also suspected to
be brass was an element of the electrical side of the DVD player, during the
process of mounting it in Bakelite and polishing it, a coating was rubbed off
leaving a brass coloured metal, the coating can be seen on the side of figure
(2), but due to the extremely thin section of metal that was left on the side,
the composition of the metal coating could not be deduced. Looking at the
molecular structure when comparing it to the other brass section we can see
there is only one phase, leading us to the conclusion that it is a brass of
composition 70% Copper, 30% Zinc.
Brass in its standard form is mostly a
solid solution of zinc, dissolved into copper. It can be seen from the
microstructures of the second brass sample that around the 30% zinc mark, the
zinc atoms can fit in amongst the copper atoms without disrupting the
crystallographic arrangement of the pre-existing copper atoms. As the
percentage of zinc increases within the brass, the mass density, colour and
average molecular spacing gradually change, saying this, it is worth noting
that the shape of the actual structure and how the atoms are positioned stays
the same. Looking at the first brass sample, where the amount of copper gets to
around the 40% mark a new phase starts within the copper-zinc solid solution.
The first phase no longer increases their zinc content as the percentage of
zinc increases, it merely makes the molecules in the second phase larger and
more numerous
An interesting thing to notice on the 70:30
copper in figure (1) is the twinning that has occurred on some of the grain
boundaries. This is when a part of the atomic lattice is deformed so that it
forms a mirror image of the undeformed lattice next to it. This process leaves
small but well-defined regions of the crystal deformed. This does not distort
the properties of the metal by too large an amount but in the process of
forming twins within the structure it places some slip systems in a better
direction for shear stress to affect the metal, so can indirectly increase the
chances for additional slip within the structure. This can create a more
ductile metal which is not always wanted.(4)
The first of the steels is the outer
casing for the internal motor that was used to drive the wheel that moved the
laser along a screw thread. This was found to be magnetic and upon inspection
the results pointed to low carbon steel. Looking closer at the shape of the
grains, they are not elongated or equiaxed, which leads us to believe that the
steel has been annealed. Annealing has the effect of getting rid of any
internal stresses in a metal, creating a metal with better stress resistance
properties. You can see on the structure photograph the sample contains both
pearlitic and ferritic regions, although being a low carbon steel, there is a
lot more ferrite. The darker areas of the picture are the pearlite regions, and
the lighter areas, the ferrite.
The second steel that was tested had to be
sent to the TEM to get analysed due to the fact that none of the etching
techniques managed to bring out a usable structure. The results were as
follows:
Element
Wt %
At %
SiK
0.59
1.16
MoL
0.69
0.4
CrK
12.61
13.38
FeK
86.1
85.06
The results of the scan show the metal to
be a ferritic stainless steel. Stainless steel is defined to be an iron-carbon
alloy with at least 10.5% Chromium content, it becomes highly resistant to
corrosion after 12% due to a protective film of chromium oxide that forms on
the metal surface.
For the polymeric samples, most were tested
using the process of simple identification, but due to the nature of the
elastomer band, Infra red spectroscopy was used to gain a better understanding
of what the material was. Below are the results for the simple identification
tests that were carried out on the other polymer samples.
Sample 1 – Mounting base
Sample 3 – Front Display
Hard to cut, Brittle
Brittle
Bubbling flame.
Floats
Sinks
Black smoke
Flammable
Self extinguishing
Sample 2 - Fan Casing
Sample 4 - Motor housing
Cuts but brittle
Sinks
self extinguishing
Yellow flame upon burning
Green flame on Beilstein test
Cuts easily
Sinks
Turns litmus Red
For the first sample that was tested you
could tell from the marks made by the ejector sprues that the piece was
injection moulded, so immediately this suggests a thermoplastic. When flexed,
the material showed signs of whitening, again indicating a thermoplastic.
Another indication of this is that it softened when heated, indicating that is
was a semi crystalline thermoplastic. When put into water, the polymer showed
signs of having higher density than water so we can rule out it being a
polyolefin. From this information and the fact that it was brittle this sample
is simply Polystyrene.
The second sample is slightly anomalous,
the signs point to a polyolefin, but this clashes with the fact that it is
self-extinguishing. On closer inspection it was found to be a three part
lamination which incorporated a film of aluminium foil which would explain the
polymers ability to self extinguish.
The third sample, the fan casing was
similar to the first sample in the fact that it was injection moulded. This
therefore indicates a thermoplastic. Also, looking at the fact that the polymer
could be cut relatively easily points us to a polymer which is highly amorphous
in its composition. The red litmus paper is a sign that the fumes given off are
of an acidic nature, this combined with the green flame in the Beilstein test
leads us to the conclusion that it must be Polyvinyl Chloride.
The fourth sample was a surround for a
small electric motor, again it was injection moulded and showed signs of being
a semi crystalline thermoplastic. On the same premise as the first sample I
would be confident to say this sample was Polystyrene as well.
The fifth polymer to be tested was tested
using Infra Red Spectroscopy. It is a highly cross linked elastomer which
proved very difficult to determine what it was. Below is the graph and results
of the IR test.
Initially it had a definite resemblance to
raw butadiene, but due to the fact this material was not used in these kinds of
applications, that was ignored. Nitrile rubber was another possibility that
would be very suitable to the application, but there was no peak representing
that. There is a large peak at 1000 suggesting some kind of silicon rubber, but
this had to be discounted for being too far out of the plausibility range. The
only plausible material it could be is a highly crosslinked butadiene polymer.
Discussion
When designing a product to go into
production, many different things have to be looked at when trying to pick the
correct material for your application. What stresses are going to be applied to
it, how the component might fail, how it will be made to a high level of
quality but with the lowest level of cost, and many other factors. In this
experiment just 9 pieces were taken from the hundreds that were in the DVD
player to analyse.
Starting with the first piece, the 60/40
brass was most likely rolled on the production like due to the fact that when
you look at the grain structure inside, all the grains are in 1 direction. This
creates a strong and standardised structure within the screw that has high
tensile properties and as they will be put under constant torsional load as
they turn. This piece is a key part to the DVD player and so has to be designed
so it doesn’t corrode or break. Brass is a good choice as it is non corrosive
and yet still cheap to manufacture.
The second piece of Brass was part of an
electrical connection which had a metallic coating to it which was most likely
applied by the process of electroplating. Making the brass element the cathode
in the reaction allows it to be evenly coated with whichever metal the producer
deems necessary.
The steel casing for the motor is a very
simple press forming involving probably thousands of identical parts being
punched out of a large sheet of thin steel sheet at the same time. Low carbon
steel was used for this application due to its low cost and ductile structure
being easy to form into different shapes.
The stainless steel element was part of a
runner that the laser moved upon inside the DVD reader. This is a vital
component that needs to be smooth at all times, even if some moisture were to
get into the DVD player, if this part were to corrode, the player would stop
working. Stainless was used to stop any corrosion, and although it is slightly
more expensive, the cost is worth the peace of mind. The stainless steel would
be made through a process of drawing.
Both of the Polystyrene pieces will have
been made in the same way. Injection moulding, although the initial cost is
high for the mould tool, is a very quick and effective way of producing many
identical and intricate parts. Injection moulding works best with
thermoplastics of a lower molar mass than average, ensuring a steady flow is
maintained.
Conclusion
Upon breaking down a DVD player into its
smaller component parts you can tell by very simple tests what the material is
and why the manufacturer chose it. The process of Simple ID and IR can tell
most polymers apart, but not always conclusively, additives can not always be
recognised.
References
William D
Callister Jr – Materials
science and engineering, an introduction – ISBN 0471320137
Vernon John – Introduction
to engineering materials – ISBN 033394917x
Wiliam F Smith
– Foundations
of materials science and engineering –
ISBN 0070592920
Chapman and
Hall – Materials
Science – ISBN 0412341506
Dietrich Braun
– Simple
methods for identification of plastics –
When looking at Aluminium alloys, the process of
Precipitation Hardening is a relatively fast and cheap way of increasing the
strength and hardness. This comes hand in hand with reduced ductility and
toughness, and if aged for too long, the alloy can be subject to a process
called over-aging whereby a visible precipitation appears in the microstructure
and the hardness levels start to decrease.[1]
As the samples were left in the furnace for longer and
longer amounts of time, precipitates of Mg2si appear within the structure of
the Al. This precipitate is much harder and more brittle than the Aluminium
which explains the rise in hardness figures.
Introduction
Precipitation hardening is a widely used process to
increase the hardness in a usually non-ferrous metal that have high level of
malleability. It was first introduced in the early 20th century on a
material called Duralumin by a professor in America. In essence, the process
produces a uniform dispersion of a very fine coherent precipitate in a softer,
more ductile base matrix.
The
process has three stages, Solution treatment, Quenching, and Ageing. In the
Solution treatment stage, the alloy is heated and held above the solvus
temperature to create a solid solution. This stage dissolves the precipitate
and creates a homogeneous un-segregated solution that is the starting point of
the process.[2]
The
quenching stage is just cooling the alloy very rapidly. This forces the
particles in solution to solidify as they are as they don’t have time to
diffuse to potential nucleation sites. After the quench, the structure contains
Aluminium particles with Magnesium silicate in solid solution. The Ageing
process is where the supersaturated solid solution is heated below the solvus
temperature, this allows some magnesium silicate atoms to diffuse to nucleation
sites and grow.[2] The best properties can be obtained by stopping this process
before it gets to the stage of Over-ageing.
The
industrial process of making aluminium drinks cans utilises this process
extensively. To obtain the perfect can, the compromise between the amount of
hardness needed to keep the can stable and the amount of ductility needed in
order that the can form properly is vitally important.
Cans
are produced by a method called Impact-Extrusion, whereby a punch is driven
into a circular slug of Aluminium creating the base and sides all in one piece.[1]
If the Aluminium alloy used is too hard, there will be cracks appearing in the
furthest corners of the base, but if the aluminium is too soft, a phenomenon
called ‘earing’ occurs, where metal is forces out the top and over the edge,
forming a lip that needs to be trimmed off before the can can be used.
Experimental
Procedure
Eight identical pieces of
Aluminium alloy HS30, with composition within the limits of 0.4-1.5% Magnesium,
0.6-1.3% Silicon, 0.6% maximum Iron, 0.4-1.0% Manganese (the balance being
Aluminium) were placed in a holding furnace at 525°C.
These were solution treated in the furnace for 15 minutes,
whereupon seven of the samples were removed from the furnace and quenched in
cold water. The furnace was then switched off and the eighth specimen allowed
to cool inside the furnace very slowly. Care was taken to ensure that the
temperature did not rise above the maximum solution temperature for the alloys
as it is very close to the solidus of the Aluminium alloy.
One specimen was tested as a freshly quenched sample, the
others were put in another furnace at 200°C and individually taken out to be
tested at intervals of 2.5, 5, 10, 20, 40, and 80 minutes. These samples were
cooled in water before each test.
The samples were tested using a Vickers Hardness machine
with a 5kg load. Three hardness impressions were made on the outside of each
sample, taking the average as our usable result.
The ductility of each sample was obtained using an Erichson
cupping tester. Each sample was tested by forcing a ball bearing to deform the
surface, stopping when the first cracks appeared. The height of the deformation
was measured, thus giving a ductility result.
Results
1
2
3
Avg
Ductility (mm)
0 (Water Quenched)
49.70
48.00
52.40
50.03
9.61
2.5
53.90
55.30
56.80
55.33
9.59
5
63.80
65.80
64.10
64.57
9.47
10
84.30
82.40
82.10
82.93
9.08
20
91.30
92.50
90.90
91.57
8.92
40
91.40
94.20
95.80
93.80
8.85
80
89.90
91.30
88.70
89.97
9.50
160 (Furnace Cooled
37.60
37.20
36.90
37.23
10.57
Discussion
The process of precipitation hardening is a solution
treatment process. In this process, the alloys is first heated above the solvus
temperature and held until a homogeneous solid solution is produced. This step
reduces the segregation within the original alloy.
When
the aluminium alloy is heated to 525°C in this process, the Magnesium and
silicon molecules react and combine to form Magnesium silicate or Mg2Si. At
this point, the Mg2Si is in solution with the Aluminium, much like salt in
water. When the first sample is quenched in water, the magnesium silicate
particles do not have time to diffuse to potential nucleation sites so are
trapped as a solid solution. This mean the Aluminium is at its purest form in
the freshly quenched sample as the impurities are trapped outside the aluminium
grains. This sample is different to the others as putting the others in an oven
at 200°C artificially age-hardens them.
As
can be seen in the graph, at the 2.5 minute stage, the hardness increases, this
is due to very fine particles of Mg2Si coming out of solution and appearing
within the Aluminium grains. These extremely fine grains are much harder and
more brittle than Aluminium, and as these are now within the grains of
Aluminium, the properties of the alloy start to change.
As
the time continues, more and more Mg2Si appears creating a harder and harder alloy,
with ever decreasing ductility. This continues until a point on both of the
graphs at around the 50 minute mark where the hardness starts to decrease and
ductility starts to increase. This is the point where the alloy moves into the
stage of Over-ageing. This is where the alloy has been in the furnace for too
long, and the particles of Mg2Si come together and form course particles.[4]
The particles are larger than they were in previous stages, but the process of
clumping together means that there is less surface area exposed. This in turn
means that there is more Aluminium particles exposed, so the alloy starts to
return to the original properties of Aluminium, hence the higher ductility and
lower hardness.
Conclusion
As the process continues, the alloy
releases Mg2Si
Mg2Si is harder and more brittle, so
as more is produced, hardness goes up, ductility goes down.
This happens until over-Ageing occurs
at roughly the 50 minute stage.
After this, more Al is exposed aiding
ductility but hampering hardness.
References
The
Metallurgy of Aluminium and Aluminium Alloys -Robert John
Anderson,
The
Science and Engineering of Materials -Donald Askeland
An
Introduction to MaterialsEngineering and Science for
Chemical and Materials
Engineers - Brian S. Mitchell
Foundations
of Materials Science and Engineering
- William Fortune Smith
Glass
is used abundantly in everyday life all through the world. It is a versatile
and useful material which can be formed into different shapes and sizes very
easily. Glass in its purest form is an exceptionally strong material,
unfortunately, pure, unflawed glass is extremely hard to produce and is not
available on a large scale. What gives glass the brittle properties we know it
to have are the surface flaws. These small cracks are caused by chemical or
mechanical damage to the surface. These cracks are weak spots and can propagate
causing the glass to fail.
These
cracks have a very small tip radius and create high stress concentrations at
the tip end. This is calculated by the formula:
1 + 2 (√l/R)
Wherel = depth of the crack
R
= radius.
Experiment Procedure
Involving acid
in an experimental procedure meant that safety rules had to be adhered to at
all times. These involved the used of rubber protective gloves at all time as
Hydrofluoric acid is a highly dangerous and corrosive substance which can
injure easily. Spillages need to be made visible by covering the work surfaces
in tissue. Any contact with the acid must be washed thoroughly and be treated
with magnesium paste and glycerol.
The main aim of
the experiment is to see the effect hydrofluoric acid had on the surface flaws
on glass. The Glass samples were sectioned into groups of three that would be
subjected to the acid for differing intervals. One group was left un-etched,
the other groups, etched for five, ten, and fifteen minutes respectively in
hydrofluoric acid.
To measure the
load the glass can take before breaking, the Hounsfield Tensometer with 1000N
and three point loading attachment was set up.
Using a
micrometer, the diameters of all the specimens were measured, creating an
average for each piece. This was needed in order to work out the tensile stress
for each sample.
The experimental
procedure was as follows;
Place group 1 samples in a beaker of 6%
Hydrofluoric acid solution for five minutes.
Remove samples from solution and wash thoroughly in
alcohol.
Dry samples with hot air dryer.
Repeat process for both the ten minute group, and
fifteen minute group.
Position samples in Hounsfield tensometer three
point loading attachment.
Cover with a sheet of polythene and test the sample
to breaking point.
To work out the tensile stress, the
following equation is used;
F =
(8WL)/( πD^3)
W =
Applied maximum load
L =
Distance between pair of supports = 30
D =
Diameter of sample
Results
Sample
Dia (mm)
Dia (mm)
Av Dia (mm)
Max Load (N)
Tensile Strength
(Nmm^-2)
Unetched
1
4.96
4.95
4.96
299.0
187.192
2
4.95
4.96
4.96
307.0
192.201
3
4.96
4.96
4.96
285.0
178.427
5 min etch
1
4.96
4.96
4.96
460.0
287.988
2
4.95
4.95
4.95
175.0
110.226
3
4.93
4.93
4.93
401.5
255.980
10 min etch
1
4.96
4.96
4.96
318.5
199.400
2
4.95
4.96
4.96
382.0
239.155
3
4.96
4.96
4.96
779.5
488.014
15 min etch
1
4.93
4.93
4.93
551.5
351.614
2
4.95
4.96
4.96
398.0
249.172
3
4.96
4.96
4.96
671.5
420.400
Conclusion
The
main conclusion that was found through doing the experiment was that the longer
the glass was exposed to the acid, the higher the tensile stress became, This
is due to the acid eroding away the tip of the crack to create a larger radius
at the end. This process of widening the radius of the tip lowers the stress
concentration at the point where the crack would normally propagate, so it
takes more to make the crack grow.
When
compared, glass and metals have roughly the same tensile properties, but due to
the fact that glass has a very small amount of plastic deformation, when a force
is applied, it acts brittle, whereas in metals, the plastic property allows the
cracks to propagate much slower and this will only happen when the elastic
limit is reached.
From
the graph we can see that there are a lot of scattered results. This is due to
the glass samples having many different crack arrangements within them. To have
a fair test the amount of samples would have to be vastly increased and then
averaged to create a better understanding of how much the Hydrofluoric acid
affects the tensile Properties of Glass.
References
Brian S Mitchell : An Introduction
to MaterialsEngineering and Science for Chemical
and Materials Engineers
The Microstructure of Cast Iron and
Copper-Silver Eutectic Alloys
Abstract
The microstructures of five samples of different forms
of cast iron were studied and drawn using an optical microscope. The
differences in microstructure were accounted for by the different heat
treatment processes, and additives present and the process by which the steel
was cooled.
Four samples of copper silver eutectic alloys of
differing compositions were also studied using an optical microscope and their
structures drawn.
Introduction
Cast irons are a class of ferrous alloys with a carbon
content of between 2.0 – 4.5% [1]. They are the most economical of
cast materials with respect to foundry costs which makes them desirable for
many engineering applications even though they tend to be quite brittle. There
is a high demand for cast irons due to their versatility. They vary in strength
but have good resistance to wear, abrasion and corrosion and are easily
machined. They have a high damping capacity and are also easily melted and
cast, which means they take good casting impressions[2].
The carbon within cast irons exists in two forms,
either in the combined form as cementite which is an unstable iron carbide, or
in the free form as graphite. Cementite is extremely hard and brittle which
makes the iron hard and difficult to machine. Graphite however is soft making
the iron softer and easier to machine. Graphite normally occurs in flakes which
weaken the metal by breaking up its continuity. The flakes also have sharp
edges which act as stress raisers in the metal causing localised concentrations
of stress. Due to the properties of these two forms of carbon, the shape,
distribution and relative amounts in the cast iron produce a variety of
different cast irons with differing properties [2].
There are a number of factors which affect the
structure of the cast iron. The rate of solidification determines the type of
iron to be formed, for example slow cooling will produce grey iron whereas more
rapid cooling will result in white iron structures. The carbon content of the
melt and the presence of other elements have a large effect on whether graphite
or cementite is formed and in what amounts. For example nickel and silicon
promote the formation of graphite in the iron structure. The type of heat
treatment will also affect the structure, as if cementite is heated and
decomposes to ferrite and graphite it produces a completely different structure
[3].
Copper and silver form a binary alloy which shows
three single-phase regions on the phase diagram.These are , and liquid. The
phase is a
solid solution rich in copper which has silver as the solute and an FCC
structure and is considered to include pure copper. The phase solid
solution has copper as the solute, has an FCC structure and is considered to
contain pure silver.
As silver is added to copper the temperature at which
the alloys become totally liquid decreases. This can also be said for the
addition of copper to silver. The composition where these two temperatures meet
is known as the eutectic. In the case of copper silver this is at 72% Ag 28% Cu
[1].
Experimental
Five prepared micro specimens were provided and the
microstructure of each studied and drawn using a microscope. A magnification of
400 x was used for each specimen to give the best representation of the
microstructure.
The specimens provided were:-
·Blackheart malleable cast
iron
·Ferritic spheroidal
graphite iron
·Pearlitic spheroidal
graphite iron
·White cast iron
·Phosphoric grey cast iron
Four polished and etched micro sections of copper silver alloys were
provided. These were inspected under a microscope at 400x magnification and
drawn.
The samples provided had the following compositions:-
·90%Ag 10% Cu
·72%Ag 28% Cu
·50% Ag 50% Cu
·30% Ag 70% Cu
The phase diagram for the copper-silver alloy system was constructed
from information provided.
Results
Copper Silver Phase diagram
The first set of drawings show the microstructures of the different cast
irons at 400 x magnification. The second set of drawings show the
microstructures of the copper silver alloys of differing compositions also at
400 x magnification.
Discussion
Different structures of cast iron can be produced by the way the metal
is cooled. Two of these structures are white and grey cast iron. On cooling the
liquid austenite begins to form and as the metal is cooled further the amount
of austenite in the matrix increases. At the eutectic temperature of 1130°C the
remaining liquid solidifies producing austenite in a eutectic matrix. At 723°C
this structure begins to decompose to form a structure consisting of cementite,
Fe3C and eutectoid iron. The eutectoid is a mixture of cementite and
ferrite, which contains areas of pearlite and cementite. The pearlite is formed
as a result of the decomposition of the austenite on cooling. The structure
described above is that of white cast iron which is formed when the rate of
cooling is rapid [3].
Grey cast iron is produced when the cementite in the structure
dissociates to form ferrite and precipitates of graphite. This leaves no pure
cementite in the structure and reduces the amount of pearlite, as the cementite
lamellae within the pearlite will also dissociate forming ferrite and graphite.
Incomplete dissociation means that there may still be traces of pearlite within
the grey iron structure. The cooling rate for this process is much slower than
that used for the white cast iron, as the cementite needs time to dissociate [2].
In order to produce grey cast iron instead of white cast iron a number
of variables must be controlled. The rate of cooling must be controlled and
made as slow as possible to form grey cast iron [3]. A high silicon
content will also promote the formation of grey cast iron as silicon has strong
graphitising tendencies [2]. Phosphoric grey iron is stronger, has a
lower melting point and better fluidity than normal grey cast iron [4].
The Blackheart malleablising process involves packing
white iron castings into pots with a neutral packing, such as sand or crushed
slag, and heating them to 900°C for three days. They are then cooled very
slowly, at about 3°C per hour. This results in the cementite in the white iron
being decomposed into ferrite and graphite being precipitated in a finely
dispersed form[1]. The structure is composed entirely of ferrite and
graphite. The graphite present in the structure is shown as ‘rosettes’ of
carbon in the ferrite. There is usually a small amount of pearlite left after
the process which does not affect the properties of the casting [4].
The steel produced in this process has good wear resistance and strength and
reasonable toughness [2].
To produce spheroidal graphite iron rather than flake
graphite iron a number of additives are needed. Magnesium amounting to 1-2% of
the weight of the iron is added in the form of a nickel magnesium alloy of
10-20% magnesium. The alloy is used to prevent an extremely violent reaction
from occurring. Magnesium can be replaced by Cerium, which has a similar
effect. The presence of silicon also assists the formation of the nodules
therefore ferro-silicon is added [2].
In order to produce a ferritic spheroidal graphite
iron from a pearlitic spheroidal graphite iron the steel must be heated to just
below the lower critical temperature. This has the effect of making the
cementite lamellae present in pearlite or free, to coalesce and become
spheroidal. This is possible due to the surface tension, because at this
temperature there is sufficient mobility to enable the particles to contract
their surfaces and become globular. The spheroids embedded in the ferrite
matrix reduce the hardness of the steel considerably [2].
The cooling of copper silver alloys of different compositions:-
·90%Ag 10%Cu
The liquid cools until it reaches approximately 870°C where the first
solidstarts to form.
The crystals
continue to grow until the melt reaches the eutectic temperature of 780°C. At
this temperature the liquid solidifies forming a eutectic intergrowth of and. The composition of andstays the same
as the solid cools to ambient temperature.
·72%Ag 28%Cu
The liquid cools until it reaches the eutectic temperature of 780°C at
which point all of the liquid solidifies forming a pure eutectic phase.
·50%Ag 50%Cu
The liquid cools until it reaches approximately 870°C where the first
solid forms. The crystals grow
until the eutectic temperature is reached when the remaining liquid solidifies.
This forms a eutectic intergrowth with the presence of crystals.
·30%Ag 70%Cu
The liquid cools until it reaches approximately 980°C where the first
solid forms. The
crystals continue to grow as in the 50-50% alloy, but due to the larger
temperature difference between the formation of the first crystals and
the eutectic, the amount of formed is
greater. At the eutectic temperature the remaining liquid solidifies to form
the eutectic intergrowth with large amounts of present.
Non equilibrium cooling produces the effect of coring. In a cored
crystal the composition is not the same at all points. The crystal lattice will
be continuous but there will be a gradual change in composition across each
crystal [3]. The dendrites within the structure represent the first
solid formed which will be rich in copper as it has the higher melting point.
The regions between the dendrites are rich in silver as it has the lower
melting point and is the last to form a solid. The composition and properties
of the +phase differs
from one region to the next, which results in the casting having poorer
properties. Also if heated, melting of the regions between the dendrites may
occur at a temperature below the equilibrium solidus of the alloy.
Conclusion
·Cast irons have many
different structures each one caused by a different cooling rate, additives and
different heat treatments.
·Slow cooling rates and
high silicon content promotes the formation of grey cast iron over white cast
iron
·Magnesium and silicon help
to produce spheroidal graphite iron rather than flake graphite iron
·To produce a ferritic
spheroidal graphite iron from a pearlitic spheroidal graphite iron the steel
must be heated to just below the lower critical temperature.
·The composition of the
copper silver eutectic alloys has a very large effect on the microstructure of
the alloy, with different amounts of the phases being produced on cooling.
References
[1] William D. Callister
Jr. ‘Fundamentals of Materials Science and Engineering an integrated approach’
– Second edition, 2005, John Wiley and Sons Inc. p372-373, 539-540
[2] W. Dennis ‘Metallurgy
of the Ferrous Metals’ , 1963, Sir Isaac Pitman and Sons Ltd, Woking and London
p107-109, 234,309
[3] V. John ‘Introduction
to Engineering Materials’ - 4th Edition, 2003,Palgrave Macmillan, Gosport,
p165, 293,294
[4] Richard Henry Greaves
and Harold Wrighton ‘Practical Microscopical Metallography’ 1967, Chapman and
Hall, Oxford,
p143,146-147
[5] D.R. Askeland ‘The
Science and Engineering of Materials’ - 5th Edition, 2006, Chapman
and Hall, Oxford,
p371
For
determining crystal structures and for the general identification of materials,
X-ray diffraction is a very useful tool. Using the X-ray equipment involves
some very high intensity X-rays and very accurate recording equipment. At the
end of the experiment the computer produces a diffraction pattern, along with
values for the intensity, d spacing and 2θ. Using this information we can
identify the material by comparing to past data on many thousands of materials.
For powdered samples, there are many diffracted rays so the diffraction
patterns can be easily read, and using Bragg’s law, the crystal structure can
be identified. With solid samples, there are fewer rays that can be diffracted
so the diffraction pattern is harder to read.
Introduction
X-rays are
fundamentally part of the electromagnetic spectrum and can be generated one of
two ways. The first and more dangerous way is by being emitted by radioactive
materials. This is a danger to the users as it cannot be turned off. The second
and much safer way is by being produced by the use of electricity.
Electrical
X-ray production occurs by focusing a beam of high velocity electrons with a
series of slits onto the surface of a copper target within an X-ray tube.
Within an atom there are three different electron shells, k, l, and m. These
shells have specific energy levels and are stable when they have the correct
amount of energy and electrons in them.[1] To create an X-ray, an
electron is fired at the atom and knocks an electron straight out of the ‘k’
shell, taking the fired electron with it. This creates an excited atom that can
only stay in this state for a very short amount of time. The solution to this
excited state is to move an electron from the ‘l’ shell down into the ‘k’ shell
to lose some energy. In moving down to the ‘k’ shell the atom gives off energy
in the form of an X-ray photon. This radiation given off in the form of an
electron moving from the ‘l’ shell to the ‘k’ shell is known as Kα radiation,
which is excellent for taking readings as it has a short wavelength and lower
energy. There is another type of radiation that can be created by the movement
of electrons through the shells, and that is Kβ radiation. This is when the
electron moves from the ‘m’ shell to the ‘k’ shell. This has a bigger energy
gap and a larger wavelength with higher energy, and only occurs roughly once in
every seven X-ray exposures. To combat this, a nickel filter was used. This is
to not only to combat the Kβ rays to produce a monochromatic graph, but also
the white noise. When the X-ray slows down it emits what is referred to as
white noise radiation, along with other surrounding factors such as background
radiation, this creates a lot of unwanted graphical data. X-ray diffraction is a very inefficient
process. Only 1% of the power it takes for the whole process goes towards
creating the X-rays, the remaining percentage gets used for other processes
such as heat production and various other applications.
Diffraction itself is a phenomenon
which occurs when a wave hits a series of barriers with a regular spacing that
can interfere with the waves’ wavelength. This interference can either be
constructive or destructive depending on how the waves line up when they are
reflected out of the material. In order for the interferences to become
constructive, the waves have to enter a material, and exit it in phase. This
creates a larger amplitude wave as the two waves’ amplitudes are added together
thus creating a wave that is easily detected. Destructive interference occurs
when the wavelength of the X-ray wave is larger than that of the inter-planar
spacing within the material. This creates a wave that is not in phase and in
essence, cancels itself out. This is very hard to detect, and is usually
ignored.This is especially important
when talking about X-rays, as the wavelength for Kα rays are very small, and
not coincidentally, they are roughly the same size as the atomic spacing within
materials. The rules of whether the interference is going to be constructive or
destructive all comes down to whether the conditions satisfy the Bragg
equation: nλ = 2d sinθ, where n is the order of diffraction, d is the
interplanar spacing, θ is the Bragg angle and λ is the wavelength of the X-ray.
When all the variables are put into this equation and it is now satisfied the
interference becomes constructive. If the Bragg equation is not satisfied,
there will be destructive interference and the outcome will be a low intensity
diffracted beam. [2]
Experimental
Using an
X-ray Diffractometer set up at 40mA and 40kV, and with the use of a Nickel
filter to knock out some of the white noise and the Kβ radiation, three samples
were exposed to X-rays for roughly half an hour. Out of the three samples involved,
two were in a powder form and another was in a solid metallic sheet. It was
know that one of the powdered samples is the same material as the solid sample,
but it is not known which one this is. Once the process was completed, the
computer displayed the details of the intensity, d spacing and 2θ for each of
the samples.
Results
A theorem
from a previous student stated that if the sin2θ value for the set
of results was divided by the first value of sin2θ, it would yield a
number that is in correlation with the list of h,k,l planes and their relevant
number patterns. This sequence was always there, but sometimes it was necessary
to multiply by a factor to find this illusive pattern. This, when referenced
against the Miller indices h, k, and l, and their relevant set of numbers ascertaining
which structure they are can determine the crystal structure of the material.
Sample A – White Powder
2θ
d
spacing
Counts
sin2
θ
sin2
θ /0.10869
S
Value
38.500
2.33641
1853
0.10869
1.00000
3
44.748
2.02364
820
0.14489
1.33305
4
65.144
1.43082
587
0.28983
2.66657
8
78.302
1.22005
420
0.39862
3.66749
11
82.500
1.16828
131
0.43470
3.99944
12
99.148
1.01190
76
0.57949
5.33158
16
112.100
0.92860
156
0.68811
6.33094
19
116.650
0.90513
128
0.72426
6.66353
20
137.550
0.82635
148
0.86890
7.99457
24
Sample D – Black Powder
2θ
d
spacing
Counts
sin2
θ
sin2
θ /0.11897
S
Value
40.353
2.23331
2887
0.11897
1.00000
2
58.350
1.58017
362
0.23763
1.99739
4
71.249
1.29121
617
0.35589
2.99142
6
87.050
1.11854
167
0.47426
3.98638
8
100.700
1.00045
233
0.59283
4.98302
10
115.004
0.91332
58
0.71134
5.97915
12
131.271
0.84561
288
0.82980
6.97486
14
Sample F – Solid Metallic Sample
2θ
d
spacing
Counts
sin2
θ
sin2
θ /0.28987
S
Value
65.149
1.43074
2599
0.28987
1.00000
8
78.250
1.22073
526
0.39817
1.37361
11
137.550
0.82635
132
0.86893
2.99767
24
Discussion
The
samples tested all have a set of Miller Indices, which when referenced against
a table of Miller indices that represent the different structures, we can find
the specific materials’ crystal structure. Upon construction of this list, it
was found that for the integers 7, 15, 23 and 28 there were no Miller indices
to be found. This is due to the face that there are no reflections at these
integers. Taking into account these numbers allows you to think about how the
other structures such as simple cubic and face centred cubic’s structure is set
out when corresponding to the Miller indices. A simple cubic structure will
have all apart from the 4 impossible reflections, whereas a BCC structure will
not reflect where h+k+l is odd, and BCC lattices must have h,k and l all odd or
all even.
Looking at
the first sample, the white powder, it was necessary to multiply the findings
for the proposed Miller indices by three in order to create the S Number which
lined up in conjunction with the actual Miller indices. The pattern emerged
that the S Number we found worked perfectly with the Face Centred Cubic
structure.
The second
sample we tested, the black powder, gave us a set of S numbers that when
doubled, linked up directly with the set of numbers from the Miller indices for
a Body Centred Cubic structure. This, coupled with the fact that the numbers
were never odd, confirmed our thoughts. It is not necessary to look at the
structure of the solid sample as it is known to be the same material as one of
the powdered samples. One can easily find the crystal structures of the
powdered materials through the use of the Miller Indices, but to identify the
material itself the JCPDS Powder Diffraction file must be used to reference the
various statistics that were recorded against the examples in the book. The
statistics used to find the correct material were d spacings, relative
intensities and 2θ. Using this file and referencing the samples that were
tested it was found that the first sample, the white powder, was Aluminium, and
the second, darker powder, was Tungsten.
Looking at the solid metallic sample,
it is clear that we cannot use the file for our identification, with only three
reflections to compare in the file there are hundreds upon hundreds of
different materials that have those same three reflections. A point to notice
when looking at the number of reflections the materials have is that the
different cubic structures have different amount of slip systems, and therefore
will have differing amounts of slip planes for the X-rays to be reflected from.
The FCC structure has 12 slip planes for the X-rays to reflect off of, the FCC
structure has only 8, which is why it has fewer reflection results. To identify
the solid sample it is necessary to then look at the values for intensity, and
comparing them to the other powdered samples to see which one was closest. Upon
seeing the comparison the solid sheet correlated strongest to Aluminium.
X-ray
diffraction works so well with powders due to the fact that in a powder there
are thousands of particles which are all mixed up together, and therefore have
slip planes that are facing every angle. This means that there is a much higher
probability that the X-ray is going to reflect off of them as there is more
chance that they are facing the direction of the approaching X-ray.
In the solid metallic sample, you can see from
the graph that the third peak is really the only one that can be identified. This
is because the 1st and 2nd peaks are slip planeswithin the Aluminium that during the forming
and rolling process, have been deformed and elongated so that they are
ultimately no longer along the same plane as the third peak. The third peak we
are seeing in the graph as the larger black peak is the 2,2,0 plane that lines
up with the X-ray beam and allows the X-ray to be diffracted and reflected
back.
Conclusion
§Powdered
samples give much better diffraction patterns due to the random orientation of
the planes in the particles allowing a larger portion of the material to give
off a diffraction pattern.
§X-ray
diffraction is a conclusive way of identifying materials effectively
