Structure, Composition & Properties of Metals and Alloys

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.

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.

Atomic structure of metal and alloys

The atomic structure of a metal is pretty simple:

Structure of a pure metal. Vaia 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. Vaia 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. Vaia 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.

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. Vaia 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. Vaia 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. Vaia 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.