Thanks to CuriosityStream for sponsoring this
video. Engineering materials are normally split into
4 categories - metals, polymers, ceramics and composites. Understanding the different types of materials,
their properties and how to use them effectively is a crucial part of engineering. In this video we’ll explore metals, their
microstructure, and different techniques like alloying and heat treatment that can be used
to improve their properties. Around two thirds of the elements in the periodic
table are metals, although for engineering purposes we’re particularly interested in
just a handful of them. Iron is probably the most important of them
all, because it’s used to create steel, a high strength material with a wide range
of engineering applications. Aluminum is commonly used because its alloys
have high strength-to-weight ratios. It has a relatively low melting temperature,
which makes it easier to process and use for casting, and it’s relatively inexpensive. Like Aluminum, Titanium has excellent strength-to-weight
properties, although it is even stronger, making it a popular choice for aerospace applications. Its high melting point makes it suitable for
applications at high temperatures, but makes processing more difficult. It’s also much more expensive than Aluminum. Other important metals include Magnesium,
Copper, and Nickel. The key to using these metals effectively
lies in understanding how they’re structured at the atomic level. The atoms of a pure metal are packed together
closely, and are arranged in a very regular grid. Because of this regular structure, metal is
what we call a crystalline material, and the grid the atoms are arranged in is called the
crystal lattice. Not all materials have a regular structure
like this. In glass for example the atoms are arranged
randomly, so it’s an amorphous material, not a crystalline one. We can think of the crystal lattice as a repeating
number of identical units, that we call the unit cell. There are several different ways the atoms
of a metal can pack together, which means that there are several different types of
unit cell. At room temperature, copper atoms for example
pack together as shown here, where there is an atom at the corner of each unit cell and
one at the centre of each face. We can see this better if we shrink the size
of the atoms and display the bonds between them. This is called the face-centred cubic structure,
or FCC. But iron atoms prefer to pack together in
a structure where the atoms at the centre of each face are replaced by a single atom
in the middle of the unit cell. This is the body-centred cubic structure,
or BCC. And titanium atoms prefer to pack together
in what’s called the hexagonal close-packed structure. These are the three most common crystal structures
in metals. Both the FCC and the HCP structures have a
packing factor of 74%, meaning that the atoms occupy 74% of the total volume of the unit
cell. The BCC structure is slightly less closely
packed, with a packing factor of 68%. The close packing of the atoms is one of the
reasons metals have much higher densities than most other materials. In reality lattices aren't perfect like the
one shown here, but contain numerous defects, of which there are several different types. A vacancy defect occurs when an atom is missing
from the lattice. An interstitial defect occurs when an atom
squeezes into the gap between existing atoms in the lattice. This is a self-interstitial defect, since
the extra atom is of the same element as the lattice, but interstitial defects can also
be created by impurity atoms of a different element. And then we have substitutional defects, where
certain atoms in the lattice are completely replaced by impurity atoms. These are all point defects, because they
affect a single location within the lattice. Lattices also contain linear defects, called
dislocations, where a number of atoms are offset from their usual position in the lattice. The first type of dislocation is an edge dislocation,
where the lattice contains an extra half plane of atoms. Let’s shrink the atom size so that we can
show the atomic bonds. This is a stable configuration, but when a
stress is applied to the lattice, the atomic bonds break and re-form, allowing the extra
half plane of atoms to glide through the lattice. Another type of dislocation is the screw dislocation,
where an entire block of atoms is shifted out of alignment with the perfect lattice
structure. It gets its name because if you follow a path
of atoms around the dislocation, it will spiral down through the lattice like the thread of
a screw. Again when a shear stress is applied the atoms
rearrange into a new stable configuration. Most real dislocations will actually be a
combination of edge and screw dislocations. Because dislocations move through the lattice
by the breaking and re-forming of atomic bonds, the process is irreversible - a dislocation
doesn’t return to its original position when the applied shear stress is removed. This is the underlying mechanism behind plastic
deformation in metals - it’s essentially the motion of a large number of dislocations
at the atomic level. Elastic deformation is caused by the stretching
of atomic bonds. Unlike the motion of dislocations, this stretching
is completely reversed when the load is removed. This graph shows how a material’s yield
strength changes with dislocation density. Materials that contain a large number of dislocations
have improved strength, because dislocations can get tangled, preventing each other from
moving through the lattice. The motion of dislocations through the lattice
is also affected by how the atoms pack together. It's easiest for dislocations to move along
planes where the atoms are closest to each other, since it’s easier for those bonds
to break and re-form. This corresponds to different planes depending
on the structure of the unit cell. In reality even pure metals don’t maintain
a regular crystalline structure over long distances. Let’s zoom in to some molten metal and see
how it solidifies. As the metal cools down, atoms group together
and a lattice structure begins to form in several different locations at the same time. Each of these lattices has its own orientation,
and as the metal cools down the lattices continue to grow until it has completely solidified. We end up not with one continuous lattice,
but with multiple lattices that are oriented in different directions. This creates what we call grains within the
metal’s structure, and materials made up of a collection of these grains are said to
be polycrystalline. The grains are separated by grain boundaries. Since each grain has specific planes along
which it’s easier for slip to occur, the presence of grains impedes the motion of dislocations,
and so polycrystalline materials tend to be stronger than materials made up of a single
uniform crystal. The smaller the grain size, the stronger the
material will be. This is captured in the Hall-Petch equation. We can use this information to intentionally
strengthen metals, by controlling the size of the grains that form as the metal is cooled. Impurities called inoculants can intentionally
be added to the molten metal so that crystal nucleation occurs at more sites than it otherwise
would have, leading to smaller grain sizes. Another way we can do this is by controlling
how fast the metal is cooled. If a metal is cooled very rapidly, nucleation
occurs at more locations and the crystals don’t have much time to grow, so the metal
will end up with a finer grain structure, and will be stronger as a result. Controlling grain size to strengthen a metal
is called grain boundary strengthening. This is just one of many strengthening techniques. We can also strengthen a metal by plastically
deforming it, using techniques like cold rolling or forging. This increases the number of dislocations,
and so increases the strength of the material, at the cost of reducing its ductility. This is called work hardening. One very useful quality of metals is that
they can be mixed with small quantities of other metallic and non-metallic elements to
improve the properties of the base metal in some way. Metals that are created by combining different
elements in this way are what we call alloys. We typically split metals and their alloys
into ferrous and non-ferrous categories, depending on whether or not the base metal of the alloy
is iron. Brass for example is a non-ferrous alloy of
copper and zinc. It typically contains 65% copper and 35% zinc,
although other alloying elements are sometimes added. It is used for its attractive appearance and
the ease with which it can be machined. Aluminum alloys are important in engineering
and are often used for the good strength properties they provide at a light weight and reasonable
cost. Common alloying elements are Copper, Manganese,
Silicon, Zinc and Magnesium. Aluminum alloys are classified according to
whether they’re designed to be used for casting, or to be worked, and are designated
using specific numbering systems. But steel is probably the most important engineering
alloy of all. Pure iron is too soft for it to be used for
structural purposes, but it can be combined with small amounts of carbon and in some cases
other elements to produce steels that have incredibly useful properties. Steels are separated into a few different
categories, depending on the amount of carbon and other alloying elements. Low-carbon or “mild” steel contains up
to 0.25% carbon. It doesn't have particularly high strength,
but is ductile and relatively low-cost. Medium-carbon steel contains between 0.25
and 0.6% carbon, and high-carbon steel contains between 0.6% and 2% carbon. Since these steels contain a larger amount
of carbon, they are stronger and can be more easily strengthened using different heat treatment
methods like quenching and tempering. Between 2% and 4% carbon we obtain cast iron. It has good fluidity and the additional carbon
lowers the melting point of the alloy, making it good for casting, although it tends to
be brittle. We can add additional elements to the iron-carbon
mix to obtain specific properties. Stainless steel for example incorporates chromium
to provide resistance to corrosion, the most common being type 304 stainless steel, that
contains 18% Chromium and 8% Nickel. Alloys are created by melting the base metal
and various alloying elements together. They can either be substitutional or interstitial,
depending on the relative size of the atoms. Steel is an interstitial alloy, because the
atomic radius of carbon is much smaller than the atomic radius of iron. The presence of alloying elements distorts
the crystal lattice, which tends to impede the motion of dislocations, and so has a strengthening
effect. This is called solid solution strengthening. But the alloying elements aren’t always
able to fully dissolve into the base metal's lattice. If an alloying element is added beyond a certain
saturation point, it can separate out and produce a distinct homogeneous phase within
the metal’s microstructure that has a different composition. There are several different ways the particles
making up the second phase can be incorporated into the material and, unsurprisingly, they
can significantly affect the properties of the material. Like grain boundaries, the boundaries between
phases impede the motion of dislocations, and increase a material’s strength. Using heat treatment to intentionally produce
a phase of uniformly dispersed particles with the goal of strengthening a material is called
precipitation hardening. Pure iron goes through several phase transformations
with changes in temperature. Below 912 degrees celsius it’s in BCC form,
which is called ferrite. Above 912 degrees it changes from BCC to FCC,
which is called austenite. It then changes back to BCC at 1394 degrees,
and the melting point is at 1538 degrees, so above that it’s a liquid. The different solid phases are called allotropes
of iron, and for convenience a Greek letter is assigned to each one. We can extend this diagram to show how the
phases within the material change with the presence of different amounts of carbon. This is what is called the phase diagram for
the iron-carbon alloy. Because of the nature of the BCC structure,
ferrite can only hold a very small amount of interstitial carbon. When the solubility of ferrite is exceeded,
the extra carbon atoms have to go somewhere, and so a new phase called cementite forms
alongside the ferrite. Cementite is a hard, brittle compound made
up of one carbon atom for every three iron atoms, which corresponds to 6.7% carbon by
weight. A two-phase ferrite-cementite material looks
something like this. The exact way in which the two phases combine
together within the material will depend on the amount of carbon and other factors like
how fast the material has been cooled. Because of its FCC structure, austenite can
hold a much larger amount of interstitial carbon than the BCC structure of ferrite. But in the same way, if more carbon is added
we obtain a two-phase material with austenite and cementite phases. There are several other possible phase combinations
depending on the temperature and the amount of carbon present. The presence of a cementite phase can have
a significant strengthening effect, which is part of the reason steel is much stronger
than pure iron. If you’d like to learn more, you can check
out the extended version of this video over on Nebula, where I've covered phase diagrams
in a bit more detail, including how two techniques, the tie-line method and the lever rule, can
be used to figure out the composition and proportion of each of the different phases,
and how it’s possible to obtain phases like martensite that don’t appear on the phase
diagram. If you’re a regular viewer you’ll know
that Nebula is a streaming platform I’ve been building with a group of educational
creators, including Real Engineering, Practical Engineering, Mustard and many others. Nebula’s packed full of thoughtful, entertaining
and educational content from independent creators, so it’s only natural that we’ve teamed
up with CuriosityStream, the best place on the internet to watch high-quality big-budget
documentaries, with quite literally thousands to choose from. So after you’ve watched my extended videos
on Nebula, why not check out Ancient Engineering on CuriosityStream - a fascinating 10 episode
show that explores how modern engineering is built on breakthroughs made centuries ago
by ancient engineers, from genius solutions to irrigation and flooding problems to the
innovative designs used in ships, castles and temples. Best of all, if you sign up to CuriosityStream
using this link, you’ll get 26% off the annual plan, and you’ll get full access
to Nebula, for free, for as long as you have CuriosityStream. It’s the best deal in streaming, and signing
up is a great way to support this channel. So head over to curiositystream.com/efficientengineer
to get started! And that’s it for this look at metals and
alloying. Thanks for watching!
Super interesting. It’s like every subtopic they address and move on from has to have so much information around it that could be expanded upon for so long.
It was also cool that things were introduced and brought back in for later applications and configurations to tie it all together/show how it all relates.
I like these kinds of videos because you learn just enough to sort of understand it, but also enough to really realize how much you don’t know.
The visualizations were also really easy to follow.
It really makes you look around the room and see that every discreet object is made up of groups of materials that prefer to hang out with each other, through human intervention, and arranged in a way that both makes it functional and nice for our eyes to look at.
I understand my charging cable well enough, but I’m increasingly lost as we go from that to the charging adaptor, to the city of neatly arranged materials that makes up the iPad or laptop.
At least I can look at my wedding ring and be like, “I finally sort of understand you, bro.”
Very interesting and high quality video. Especially the animations fitting perfectly to the narration!