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# Structure of Metals and Alloys

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It's estimated that $62 million worth of pennies are thrown out or lost each year. While that is a lot in total, losing 2 or 3 cents here or there doesn't matter to most people. In fact, people are trying to stop the mint from producing them, since we lose$70 million a year producing them. The material used in pennies is worth more than the penny itself!

Originally, pennies were made of just copper but were later made with a copper-zinc alloy due to rising copper costs. In this article, we will be learning about the structure of metals and alloys!

• First, we will look at the basics of metals and define alloys
• Next, we will look at the basic structure and properties of metals and alloys
• Then we will look into the composition and structure of alloys
• We will also look at the atomic structures of each
• Lastly, we will look at the three types of crystalline structures that can be formed and how they make the grain structure.

## Introduction to Metals and Alloys

Before diving into the structure of metals and alloys, let's elaborate on the types of elements found on the periodic table. There are three types of elements:

• metalloids

• metals

• non-metals

Periodic table sorted by element type. Wikimedia commons.

As you'll see in pink, there are a lot of metals, which are grouped based on the properties they share (which we will discuss later).

Metals can form alloys.

An alloy is a mixture of two or more metals or a mixture of a metal and a non-metal.

There are two types of alloys: 1. Substitutional alloys

2. Interstitial alloysIn a substitutional alloy, some of a metal's atoms are substituted by another element's atoms of a similar size. In an interstitial alloy, the smaller atoms of another element fill in the "gaps" in a metal's structure.

## Structure and Properties of Metals and Alloys

Now, let's talk about the structure and properties of metals and alloys. Metals are set apart by their unique characteristics. These include:

• High melting point

• Good conductors of heat and electricity

• Malleable (can be bent/shaped easily)

• Ductile (can be stretched easily without breakage)

• High density

While you might assume that alloys share the characteristics of the individual metals they were made from, you would be incorrect. We typically create alloys as a way to "maximize" certain characteristics.

The difference between metals and alloys are:

• Alloys are harder than component metals

• Alloys are more resistant to corrosion than pure metals

• Alloys have a lower melting point than component metals

• Alloys are more ductile than component metals

• Alloys are more durable than component metals

• Alloys are less conductive than component metals

These properties make alloys more useful than pure metals. For example, steel (iron + carbon) is a common alloy used in building materials. This makes sense since it can hold more weight, is less likely to corrode, and can be more easily shaped than iron.

## Composition and Structure of Metal Alloys

The overall structure of a metal alloy is dependent on its composition. Alloys can have different ratios of metals and can have several metals within them. Here is a chart with some common alloys and their compositions.

 Name of Alloy Composition Example of use Amalgam Mercury (45-55%), remaining (45-55%): silver, tin, copper, and zinc Dental filings Brass Copper (65-90%), zinc (10-35%) Door knobs and locks Bronze Copper (78-95%), tin (5-22%), remaining %: manganese, phosphorus, aluminum, or silicon Statues Cast iron Iron (96-98%), carbon (2-4%), remaining %: silicon Cookware Gunmetal Copper (80-90%), tin (3-10%), zinc (2-3%), remaining %: phosphorus Guns Pewter Tin (80-99%), remaining %: copper, lead, antimony Decorative items Stainless steel Iron (>50%), chromium (10-30%), remaining %: carbon, nickel, manganese, molybdenum, and other metals. Jewelry Sterling silver Silver (92.5%), copper (7.5%) Medical tools

## Atomic structure of metal and alloys

The atomic structure of a metal is pretty simple:

Structure of a pure metal. StudySmarter Original.

The atoms are neatly aligned and are all the same size. They don't necessarily need to be in a rectangle shape but are always evenly spaced and relatively close together.

Alloys are different. The atomic structure is dependent on the type of alloy: substitutional or interstitial.

Here is what a substitutional alloy looks like:

Structure of a substitutional alloy. StudySmarter Original

As the name suggests, one metal's atoms are being replaced with another's. These new atoms are similar in size to the other metal's atoms.

Then there are interstitial alloys:

Structure of an interstitial alloy. StudySmarter Original.

In an interstitial alloy, the second metal's atoms are much smaller than those of the pure, original metal. These smaller atoms fit in the "holes" of the original structure.

These types of alloys can be combined, so an alloy can have a structure that is a combination of the two shown above.

## Crystal structure of metals and alloys

Metals and alloys typically have a crystalline structure. There are three main structures that a crystal can have:

1. Body-centered cubic (BCC)

2. Hexagonal closed packed (HCP)

3. Cubic closed packed (CCP)/face-centered cubic (FCC)

When we look at these structures, we often refer to the unit cell.

The unit cell is the smallest section of the lattice that shows the 3D pattern of the whole crystal.

Essentially, a crystal is just the same unit cell repeated several times. Metals and alloys form these structures since they fill space the most efficiently.

The first type of crystal is the body-centered cubic (BCC). Its structure is shown below:

Body-centered cubic unit cell and whole structure. StudySmarter Original.

The general shape is a cube, with an atom at each corner. There is also another atom at the center of the "body", hence the name.

Next, we have the hexagonal closed packed (HCP) structure:

Hexagonal closed-packed structure and unit cell. StudySmarter Original.

The unit cell for this type is much more complex. The top and bottom faces of the structure are hexagons, with an atom on each point and in the center. In the center of the cell is a triangle shape, with an atom on each point.

Lastly, we have the cubic closed packed (CCP)/face-centered cubic (FCC) structure:

Cubic closed-packed/face-centered cubic unit cell and structure. StudySmarter Original.

Like with the BCC structure, the basic shape is a cube. There is an atom on each corner and one centered on each face.

## Grain Structure of Metals and Alloys

The individual crystal structures group together to form grains. These grains combine to form the grain structure, which can be viewed through a microscope. The image below is the grain structure for stainless steel.

Grain structure of stainless steel under a microscope. Wikimedia commons.

The size and orientation of the grains are dependent on:

• Composition (alloy)
• Chemical influences (ex. corrosion)
• Physical influences (ex. heat)
• Mechanical influences (due to the forming process, ex. forging)

The grains themselves are formed when the molten material solidifies. The grain structure is adapted for the application of the metal alloy. For example, cupro-nickel's grain structure is designed so that the metal can be pressed to make nickels and dimes.

Looking at the grain structure (also called the microstructure) can tell you the properties of the material such as strength, hardness, and ductility.

## Structure of Metals and Alloys - Key takeaways

• An alloy is a mixture of two or more metals or a mixture of a metal and a non-metal.
• There are two types of alloys: 1. Substitutional alloys 2. Interstitial alloys
• The difference between metals and alloys are: ~Harder than component metals ~More resistant to corrosion than pure metals ~Lower melting point than component metals ~More ductile than component metals ~More durable than component metals ~Less conductive than component metals
• In a substitutional alloy, atoms from one metal take the place of some atoms in another. In an interstitial alloy, atoms from one metal fit into the gaps of a metal's structure.
• There are three main structures that a crystal can have:
1. Body-centered cubic (Bcc)
2. Hexagonal closed-packed (Hcp)
3. Cubic closed-packed (ccp)/face-centered cubic (fcc)
• The unit cell is the smallest section of the lattice that shows the 3D pattern of the whole crystal.
• The crystal structures combine to form grains, which form the grain structure. This structure is altered based on the use of the metal or alloy.

Alloys are created to "maximize" the properties of metals. Alloys can be either substitutional (atom(s) of one metal substitute in for an atom(s) of a pure metal or interstitial (a metal's atoms fill in the "gaps" in the pure metal's structure).

In a pure metal, atoms are closely packed together and are neatly organized to maximize space.

Alloys are harder, more durable, more ductile, and more resistant to corrosion than pure metals. However, they are also less conductive and have lower melting points.

There are three common metal/alloy crystal structures. These are: body-centered cubic (bcc), hexagonal closed packed (hcp), and cubic closed packed (ccp)/face centered cubic (fcc).

Some properties of metals are:

1. High melting point
2. Good conductors of heat and electricity
3. Malleable
4. Ductile
5. High density

## Structure of Metals and Alloys Quiz - Teste dein Wissen

Question

A substitutional alloy forms between elements with ____ size atoms, and an interstitial alloy forms between elements with ___ size atoms.

different, similar

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Question

True or False. Interstitial alloys usually form from a mixture of exclusively metals.

True

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Question

True or false. Substitutional alloys form when elements have similar sized atoms

True

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Question

Which alloy results in a more rigid structure? Why?

Interstitial Alloys; because the atoms are different sizes, it's harder for the atoms to easily move around.

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Question

If Ti3+ and Ca2+ were mixed, what type of alloy would form?

Substitutional alloy because they have similar atomic radii

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Question

Explain why bronze, made up of copper and tin,  is a substitutional alloy.

Copper and Tin atoms have a similar atomic radius. This allows tin atoms to replace copper atoms in its crystal structure which forms a substitutional alloy

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Question

How can you tell if an alloy is interstitial?

the interstitial atoms (smaller atoms) make the alloy less ductile and malleable

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Question

What are some of the benefits of substitutional alloys?

They are harder, stronger, and more durable than pure metals.

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Question

Substitutional alloys have a _____ melting point, and interstitial alloys have a ______melting point compared to pure metals.

higher, higher

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Question

Why do interstitial alloys tend to be less malleable and ductile?

The different sized atom inside make the the crystal lattice structure more rigid which in turn means it is harder for the atoms to move around.

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Question

Why are alloys created?

They make metals softer and easier to work with

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Question

Name an example of a substitutional alloy and an interstitial alloy.

Substitutional: sterling silver, bronze, brass

Interstitial alloy: steel, cast iron

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Question

What is an alloy?

An alloy is a mixture of two or more metals or a mixture of a metal and a non-metal.

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Question

Which of these is NOT a characteristic of metals?

Low melting point

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Question

Fill in the blanks: Alloys have a ___ melting point and are ___ durable than their component metals

lower, more

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Question

Which of the following is NOT an alloy?

Tin

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Question

What are the two types of alloys?

1. Substitutional alloys
2. Interstitial alloys

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Question

What does the atomic structure of a substitutional alloy look like?

The atoms of one metal "substitute" in for some atoms in a pure metal

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Question

What does the atomic structure of an interstitial alloy look like?

The atoms of one metal fill in the "gaps" in the structure of another.

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Question

What are the three types of crystalline structure?

1. Body-centered cubic (Bcc)
2. Hexagonal closed packed (Hcp)
3. Cubic closed packed (ccp)/face centered cubic (fcc)

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Question

What is a unit cell?

The unit cell is the smallest section of the lattice that shows the 3D pattern of the whole crystal.

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Question

True or False: A unit cell where there is an atom on each point of a cube and one atom in the centered is called body-centered cubic.

True

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Question

What is a grain?

individual crystal structures group together to form grains

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Question

What factors is the grain structure size and orientation dependent on?

• Composition (alloy)
• Chemical influences (ex. corrosion)
• Physical influences (ex. heat)
• Mechanical influences (due to the forming process, ex. forging)

Show question

Question

True or False: Looking at grain structure can tell you the properties of a material

True

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