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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:
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:
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:
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:
Body-centered cubic (BCC)
Hexagonal closed packed (HCP)
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:
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:
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:
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.
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:
- Body-centered cubic (Bcc)
- Hexagonal closed-packed (Hcp)
- 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.
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Frequently Asked Questions about Structure, Composition & Properties of Metals and Alloys
How can alloying modify the structure and properties of metals?
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).
What is the structure of a metal?
In a pure metal, atoms are closely packed together and are neatly organized to maximize space.
What are the differences between metals and alloys?
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.
What are the common structures formed when metals and alloys crystallize?
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).
What are 5 physical properties of metals?
Some properties of metals are:
- High melting point
- Good conductors of heat and electricity
- Malleable
- Ductile
- High density
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