Understanding Metals

Summary

This video explores the fundamentals of engineering materials, focusing on metals and their properties. It discusses microstructures, alloying, and heat treatment techniques used to enhance metal performance. Key metals like iron, aluminum, and titanium are highlighted for their engineering applications. The video delves into crystal structures, dislocations, grain boundaries, and strengthening techniques such as work hardening and grain boundary strengthening. Additionally, it covers alloys, phase diagrams, and heat treatments that improve material strength. Viewers can learn more about metals and their applications through educational platforms like Nebula and CuriosityStream.

Takeaways

• 🔧 Engineering materials are typically divided into four categories: metals, polymers, ceramics, and composites.
• ⚙️ Metals are critical in engineering, with iron being the most important due to its role in creating steel.
• 🛩️ Aluminum and titanium are prized for their high strength-to-weight ratios, with titanium being especially valuable for high-temperature aerospace applications.
• 🔬 Metals have a crystalline structure where atoms are arranged in a regular lattice, but these lattices often have defects like vacancies or interstitial atoms.
• 🧩 Dislocations, or atomic misalignments, are responsible for plastic deformation in metals. Their movement causes permanent changes in structure when stress is applied.
• 🏗️ Strengthening metals can be achieved through methods like grain boundary strengthening, work hardening, and alloying with other elements.
• 🥇 Alloys are classified as ferrous (iron-based) or non-ferrous, with steel being the most significant ferrous alloy due to its various carbon content-based properties.
• 🔥 Heat treatment processes like quenching and tempering can further alter the properties of steels, enhancing their strength.
• 🧱 The arrangement of atoms in metals can follow structures like face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal close-packed (HCP), affecting their density and strength.
• 🌐 Phase diagrams, such as the iron-carbon diagram, illustrate how metals change phases at different temperatures and carbon levels, helping engineers predict material behavior.

Q & A

What are the four categories of engineering materials?

The four categories of engineering materials are metals, polymers, ceramics, and composites.

Why is iron considered the most important metal for engineering?

Iron is considered the most important metal for engineering because it is used to create steel, which is a high-strength material with a wide range of engineering applications.

What makes aluminum a popular choice for engineering applications?

Aluminum is popular for its high strength-to-weight ratios, relatively low melting temperature which facilitates casting, and its relatively low cost.

Why is titanium commonly used in aerospace applications?

Titanium is commonly used in aerospace applications due to its excellent strength-to-weight properties, high strength, and suitability for high-temperature applications, despite its higher cost and difficulty in processing.

What is the difference between crystalline and amorphous materials?

Crystalline materials, like metals, have atoms arranged in a regular, repeating grid called a crystal lattice. Amorphous materials, such as glass, have atoms arranged randomly without a long-range order.

What are the three most common crystal structures in metals?

The three most common crystal structures in metals are face-centred cubic (FCC), body-centred cubic (BCC), and hexagonal close-packed (HCP).

What is a vacancy defect in a crystal lattice?

A vacancy defect occurs when an atom is missing from its normal position in the crystal lattice.

How does the presence of dislocations affect the strength of a material?

Dislocations can impede each other's motion through the lattice, which increases the material's strength. Materials with a higher dislocation density tend to be stronger.

What is the significance of grain size in polycrystalline materials?

The presence of grains in polycrystalline materials impedes the motion of dislocations, making the material stronger. Smaller grain sizes result in stronger materials, as described by the Hall-Petch equation.

What is an alloy and how does it differ from a pure metal?

An alloy is a mixture of metals or metals with non-metals created to improve the properties of the base metal. Alloys differ from pure metals in that they have added elements that can enhance properties such as strength, corrosion resistance, or ductility.

What is the role of heat treatment in improving the properties of steel?

Heat treatment, such as quenching and tempering, can alter the microstructure of steel, leading to improved strength and toughness by controlling the formation of different phases like martensite.

Outlines

00:00

🔧 Introduction to Engineering Materials and Metals

This section introduces the four main categories of engineering materials: metals, polymers, ceramics, and composites. It focuses on metals, their microstructure, and techniques like alloying and heat treatment to improve their properties. Iron and steel are highlighted as key materials, while aluminum and titanium are noted for their strength-to-weight ratios, with aluminum being easier to process and titanium preferred in aerospace for its higher strength and melting point.

05:06

🧱 Atomic Structures and Crystal Lattices in Metals

The focus here is on the atomic arrangement in metals, which have a regular, crystalline structure. Copper, iron, and titanium are used as examples of different crystal lattice types—face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP). The concept of unit cells is explained, and it covers the packing factor of these structures and how the density of metals is influenced by this arrangement. Common lattice defects like vacancies, interstitial, and substitutional defects are also introduced, along with dislocations, which impact a metal's mechanical properties.

10:06

🔩 Dislocations and Metal Deformation

This paragraph explains two main types of dislocations in metals—edge and screw dislocations—both of which contribute to plastic deformation when subjected to stress. The process is irreversible, unlike elastic deformation caused by atomic bond stretching. Dislocations interact and can tangle, which strengthens the metal. The role of crystal structure and dislocations in determining material strength and deformation is discussed. The paragraph also touches on how grains form during solidification and how smaller grain sizes contribute to stronger materials.

15:08

🔬 Strengthening Techniques in Metals

Several methods to strengthen metals are covered here. Grain boundary strengthening is explained, where smaller grains result in stronger metals. Rapid cooling and the addition of inoculants are techniques for achieving smaller grain sizes. The section also covers work hardening, which increases dislocation density, making metals stronger but less ductile. Alloying is introduced, along with ferrous and non-ferrous classifications, and the importance of alloys like brass and aluminum alloys. The section concludes by discussing the significance of steel as a versatile engineering material and how its properties are adjusted by varying carbon content.

⚙️ Phases and Heat Treatments in Steel

This final section dives into the phase transformations in metals, particularly steel. It explains how different phases, like ferrite and austenite, emerge at specific temperatures, as shown in phase diagrams. The addition of carbon forms cementite, which strengthens steel. Alloying elements are covered in more depth, explaining solid solution strengthening and how heat treatments like quenching and tempering are used to control phases and improve metal properties. The paragraph concludes with an introduction to precipitation hardening and other advanced heat treatment techniques used to strengthen metals.

Keywords

💡Metals

Metals are a category of engineering materials that are characterized by their high electrical conductivity, malleability, and strength. In the video, metals are highlighted for their unique crystal structures and properties, which make them essential in engineering applications. Key metals discussed include iron, aluminum, and titanium, each chosen for specific characteristics like strength-to-weight ratio or melting temperature.

💡Crystal Lattice

The crystal lattice is the orderly arrangement of atoms in a metal, forming a repetitive structure that defines its physical properties. The video describes several lattice structures, including the face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP) structures, explaining how the arrangement influences the metal’s density, strength, and other traits.

💡Alloy

An alloy is a metal made by combining two or more elements, often to improve the base metal's properties. For example, the video explains how steel, an iron-carbon alloy, gains enhanced strength through alloying. Other alloys discussed include brass and stainless steel, showcasing how different metals and non-metals can be added to achieve desired characteristics.

💡Heat Treatment

Heat treatment is a process that involves heating and cooling metals to alter their microstructure and improve properties like strength and ductility. The video mentions techniques like quenching and tempering, which are used on steel to enhance its toughness and hardness. Heat treatment is crucial in optimizing a metal's performance for specific applications.

💡Grain Structure

Grain structure refers to the small crystals or 'grains' that form as a metal solidifies. The video explains that these grains have boundaries that impact the metal's strength, with smaller grains generally leading to stronger materials. The grain size can be controlled during cooling, a concept referred to as grain boundary strengthening.

💡Dislocation

Dislocations are irregularities within the crystal lattice that play a key role in the plastic deformation of metals. The video describes edge and screw dislocations, explaining how they move through the lattice under stress. Controlling dislocations is essential for manipulating a metal’s mechanical properties, such as its yield strength.

💡Plastic Deformation

Plastic deformation occurs when a metal permanently changes shape under stress, due to the movement of dislocations. The video contrasts this with elastic deformation, where atomic bonds stretch but return to their original positions when the stress is removed. Understanding plastic deformation helps in designing metals that can withstand high loads without breaking.

💡Phase Diagram

A phase diagram is a graphical representation showing the phases of a metal at various temperatures and compositions. The video presents the iron-carbon phase diagram to explain how different forms of iron and steel can be achieved. Phases like austenite and ferrite are determined by factors such as carbon content and temperature.

💡Work Hardening

Work hardening, or strain hardening, is a process where a metal's strength increases due to plastic deformation. In the video, work hardening is demonstrated through techniques like cold rolling, which increases dislocation density and thereby enhances the metal's strength. However, this comes at the cost of reducing the material's ductility.

💡Solid Solution Strengthening

Solid solution strengthening is a method where alloying elements are added to a metal to improve its strength by distorting the crystal lattice. The video discusses how this impedes dislocation movement, enhancing strength. For example, carbon in steel creates an interstitial alloy, making the lattice less uniform and thus stronger.

Highlights

Engineering materials are typically categorized into four groups: metals, polymers, ceramics, and composites.

Metals are crystalline materials, meaning their atoms are arranged in a regular grid, known as a crystal lattice.

Iron, used to create steel, is one of the most important engineering metals due to its high strength and versatility.

Aluminum alloys are known for their high strength-to-weight ratio and are widely used in engineering applications.

Titanium, with its excellent strength-to-weight ratio and high melting point, is ideal for aerospace applications.

Metals have various crystal structures like face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP), which influence their properties.

The presence of defects in a metal's crystal lattice, such as vacancy and dislocation defects, significantly affects its mechanical properties.

Plastic deformation in metals occurs due to the motion of dislocations, whereas elastic deformation is reversible and involves atomic bond stretching.

Polycrystalline materials, composed of multiple grains, are typically stronger than single-crystal structures due to the grain boundaries impeding dislocation motion.

Grain boundary strengthening and work hardening are key techniques for increasing the strength of metals.

Alloying is the process of mixing metals with other elements to improve their properties, such as in the creation of steel from iron and carbon.

Stainless steel contains chromium, which enhances its corrosion resistance, making it a versatile engineering material.

Solid solution strengthening occurs when alloying elements distort the crystal lattice, impeding dislocation movement and increasing material strength.

Phase diagrams, like the iron-carbon diagram, are used to predict the different phases and properties of alloys based on their composition and temperature.

Precipitation hardening strengthens metals by creating uniformly dispersed particles within the metal's microstructure.

Transcripts

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.

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And that’s it for this look at metals and alloying.

Thanks for watching!