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Engineering materials are normally split into
4 categories - metals, polymers, ceramics
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and composites.
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Understanding the different types of materials,
their properties and how to use them effectively
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is a crucial part of engineering.
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In this video we’ll explore metals, their
microstructure, and different techniques like
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alloying and heat treatment that can be used
to improve their properties.
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Around two thirds of the elements in the periodic
table are metals, although for engineering
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purposes we’re particularly interested in
just a handful of them.
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Iron is probably the most important of them
all, because it’s used to create steel,
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a high strength material with a wide range
of engineering applications.
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Aluminum is commonly used because its alloys
have high strength-to-weight ratios.
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It has a relatively low melting temperature,
which makes it easier to process and use for
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casting, and it’s relatively inexpensive.
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Like Aluminum, Titanium has excellent strength-to-weight
properties, although it is even stronger,
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making it a popular choice for aerospace applications.
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Its high melting point makes it suitable for
applications at high temperatures, but makes
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processing more difficult.
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It’s also much more expensive than Aluminum.
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Other important metals include Magnesium,
Copper, and Nickel.
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The key to using these metals effectively
lies in understanding how they’re structured
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at the atomic level.
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The atoms of a pure metal are packed together
closely, and are arranged in a very regular
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grid.
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Because of this regular structure, metal is
what we call a crystalline material, and the
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grid the atoms are arranged in is called the
crystal lattice.
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Not all materials have a regular structure
like this.
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In glass for example the atoms are arranged
randomly, so it’s an amorphous material,
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not a crystalline one.
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We can think of the crystal lattice as a repeating
number of identical units, that we call the
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unit cell.
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There are several different ways the atoms
of a metal can pack together, which means
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that there are several different types of
unit cell.
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At room temperature, copper atoms for example
pack together as shown here, where there is
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an atom at the corner of each unit cell and
one at the centre of each face.
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We can see this better if we shrink the size
of the atoms and display the bonds between
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them.
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This is called the face-centred cubic structure,
or FCC.
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But iron atoms prefer to pack together in
a structure where the atoms at the centre
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of each face are replaced by a single atom
in the middle of the unit cell.
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This is the body-centred cubic structure,
or BCC.
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And titanium atoms prefer to pack together
in what’s called the hexagonal close-packed
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structure.
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These are the three most common crystal structures
in metals.
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Both the FCC and the HCP structures have a
packing factor of 74%, meaning that the atoms
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occupy 74% of the total volume of the unit
cell.
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The BCC structure is slightly less closely
packed, with a packing factor of 68%.
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The close packing of the atoms is one of the
reasons metals have much higher densities
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than most other materials.
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In reality lattices aren't perfect like the
one shown here, but contain numerous defects,
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of which there are several different types.
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A vacancy defect occurs when an atom is missing
from the lattice.
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An interstitial defect occurs when an atom
squeezes into the gap between existing atoms
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in the lattice.
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This is a self-interstitial defect, since
the extra atom is of the same element as the
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lattice, but interstitial defects can also
be created by impurity atoms of a different
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element.
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And then we have substitutional defects, where
certain atoms in the lattice are completely
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replaced by impurity atoms.
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These are all point defects, because they
affect a single location within the lattice.
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Lattices also contain linear defects, called
dislocations, where a number of atoms are
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offset from their usual position in the lattice.
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The first type of dislocation is an edge dislocation,
where the lattice contains an extra half plane
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of atoms.
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Let’s shrink the atom size so that we can
show the atomic bonds.
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This is a stable configuration, but when a
stress is applied to the lattice, the atomic
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bonds break and re-form, allowing the extra
half plane of atoms to glide through the lattice.
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Another type of dislocation is the screw dislocation,
where an entire block of atoms is shifted
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out of alignment with the perfect lattice
structure.
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It gets its name because if you follow a path
of atoms around the dislocation, it will spiral
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down through the lattice like the thread of
a screw.
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Again when a shear stress is applied the atoms
rearrange into a new stable configuration.
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Most real dislocations will actually be a
combination of edge and screw dislocations.
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Because dislocations move through the lattice
by the breaking and re-forming of atomic bonds,
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the process is irreversible - a dislocation
doesn’t return to its original position
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when the applied shear stress is removed.
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This is the underlying mechanism behind plastic
deformation in metals - it’s essentially
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the motion of a large number of dislocations
at the atomic level.
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Elastic deformation is caused by the stretching
of atomic bonds.
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Unlike the motion of dislocations, this stretching
is completely reversed when the load is removed.
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This graph shows how a material’s yield
strength changes with dislocation density.
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Materials that contain a large number of dislocations
have improved strength, because dislocations
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can get tangled, preventing each other from
moving through the lattice.
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The motion of dislocations through the lattice
is also affected by how the atoms pack together.
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It's easiest for dislocations to move along
planes where the atoms are closest to each
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other, since it’s easier for those bonds
to break and re-form.
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This corresponds to different planes depending
on the structure of the unit cell.
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In reality even pure metals don’t maintain
a regular crystalline structure over long
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distances.
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Let’s zoom in to some molten metal and see
how it solidifies.
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As the metal cools down, atoms group together
and a lattice structure begins to form in
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several different locations at the same time.
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Each of these lattices has its own orientation,
and as the metal cools down the lattices continue
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to grow until it has completely solidified.
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We end up not with one continuous lattice,
but with multiple lattices that are oriented
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in different directions.
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This creates what we call grains within the
metal’s structure, and materials made up
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of a collection of these grains are said to
be polycrystalline.
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The grains are separated by grain boundaries.
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Since each grain has specific planes along
which it’s easier for slip to occur, the
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presence of grains impedes the motion of dislocations,
and so polycrystalline materials tend to be
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stronger than materials made up of a single
uniform crystal.
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The smaller the grain size, the stronger the
material will be.
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This is captured in the Hall-Petch equation.
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We can use this information to intentionally
strengthen metals, by controlling the size
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of the grains that form as the metal is cooled.
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Impurities called inoculants can intentionally
be added to the molten metal so that crystal
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nucleation occurs at more sites than it otherwise
would have, leading to smaller grain sizes.
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Another way we can do this is by controlling
how fast the metal is cooled.
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If a metal is cooled very rapidly, nucleation
occurs at more locations and the crystals
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don’t have much time to grow, so the metal
will end up with a finer grain structure,
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and will be stronger as a result.
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Controlling grain size to strengthen a metal
is called grain boundary strengthening.
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This is just one of many strengthening techniques.
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We can also strengthen a metal by plastically
deforming it, using techniques like cold rolling
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or forging.
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This increases the number of dislocations,
and so increases the strength of the material,
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at the cost of reducing its ductility.
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This is called work hardening.
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One very useful quality of metals is that
they can be mixed with small quantities of
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other metallic and non-metallic elements to
improve the properties of the base metal in
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some way.
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Metals that are created by combining different
elements in this way are what we call alloys.
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We typically split metals and their alloys
into ferrous and non-ferrous categories, depending
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on whether or not the base metal of the alloy
is iron.
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Brass for example is a non-ferrous alloy of
copper and zinc.
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It typically contains 65% copper and 35% zinc,
although other alloying elements are sometimes
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added.
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It is used for its attractive appearance and
the ease with which it can be machined.
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Aluminum alloys are important in engineering
and are often used for the good strength properties
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they provide at a light weight and reasonable
cost.
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Common alloying elements are Copper, Manganese,
Silicon, Zinc and Magnesium.
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Aluminum alloys are classified according to
whether they’re designed to be used for
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casting, or to be worked, and are designated
using specific numbering systems.
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But steel is probably the most important engineering
alloy of all.
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Pure iron is too soft for it to be used for
structural purposes, but it can be combined
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with small amounts of carbon and in some cases
other elements to produce steels that have
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incredibly useful properties.
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Steels are separated into a few different
categories, depending on the amount of carbon
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and other alloying elements.
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Low-carbon or “mild” steel contains up
to 0.25% carbon.
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It doesn't have particularly high strength,
but is ductile and relatively low-cost.
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Medium-carbon steel contains between 0.25
and 0.6% carbon, and high-carbon steel contains
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between 0.6% and 2% carbon.
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Since these steels contain a larger amount
of carbon, they are stronger and can be more
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easily strengthened using different heat treatment
methods like quenching and tempering.
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Between 2% and 4% carbon we obtain cast iron.
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It has good fluidity and the additional carbon
lowers the melting point of the alloy, making
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it good for casting, although it tends to
be brittle.
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We can add additional elements to the iron-carbon
mix to obtain specific properties.
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Stainless steel for example incorporates chromium
to provide resistance to corrosion, the most
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common being type 304 stainless steel, that
contains 18% Chromium and 8% Nickel.
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Alloys are created by melting the base metal
and various alloying elements together.
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They can either be substitutional or interstitial,
depending on the relative size of the atoms.
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Steel is an interstitial alloy, because the
atomic radius of carbon is much smaller than
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the atomic radius of iron.
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The presence of alloying elements distorts
the crystal lattice, which tends to impede
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the motion of dislocations, and so has a strengthening
effect.
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This is called solid solution strengthening.
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But the alloying elements aren’t always
able to fully dissolve into the base metal's
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lattice.
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If an alloying element is added beyond a certain
saturation point, it can separate out and
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produce a distinct homogeneous phase within
the metal’s microstructure that has a different
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composition.
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There are several different ways the particles
making up the second phase can be incorporated
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into the material and, unsurprisingly, they
can significantly affect the properties of
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the material.
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Like grain boundaries, the boundaries between
phases impede the motion of dislocations,
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and increase a material’s strength.
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Using heat treatment to intentionally produce
a phase of uniformly dispersed particles with
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the goal of strengthening a material is called
precipitation hardening.
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Pure iron goes through several phase transformations
with changes in temperature.
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Below 912 degrees celsius it’s in BCC form,
which is called ferrite.
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Above 912 degrees it changes from BCC to FCC,
which is called austenite.
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It then changes back to BCC at 1394 degrees,
and the melting point is at 1538 degrees,
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so above that it’s a liquid.
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The different solid phases are called allotropes
of iron, and for convenience a Greek letter
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is assigned to each one.
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We can extend this diagram to show how the
phases within the material change with the
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presence of different amounts of carbon.
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This is what is called the phase diagram for
the iron-carbon alloy.
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Because of the nature of the BCC structure,
ferrite can only hold a very small amount
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of interstitial carbon.
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When the solubility of ferrite is exceeded,
the extra carbon atoms have to go somewhere,
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and so a new phase called cementite forms
alongside the ferrite.
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Cementite is a hard, brittle compound made
up of one carbon atom for every three iron
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atoms, which corresponds to 6.7% carbon by
weight.
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A two-phase ferrite-cementite material looks
something like this.
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The exact way in which the two phases combine
together within the material will depend on
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the amount of carbon and other factors like
how fast the material has been cooled.
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Because of its FCC structure, austenite can
hold a much larger amount of interstitial
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carbon than the BCC structure of ferrite.
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But in the same way, if more carbon is added
we obtain a two-phase material with austenite
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and cementite phases.
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There are several other possible phase combinations
depending on the temperature and the amount
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of carbon present.
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The presence of a cementite phase can have
a significant strengthening effect, which
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is part of the reason steel is much stronger
than pure iron.
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If you’d like to learn more, you can check
out the extended version of this video over
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on Nebula, where I've covered phase diagrams
in a bit more detail, including how two techniques,
00:16:09
the tie-line method and the lever rule, can
be used to figure out the composition and
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proportion of each of the different phases,
and how it’s possible to obtain phases like
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martensite that don’t appear on the phase
diagram.
00:16:21
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that Nebula is a streaming platform I’ve
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And that’s it for this look at metals and
alloying.
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Thanks for watching!
