Engineering - The Structure of Materials

There is a close relationship between the structure of the atoms within a material and the properties that the material possesses.

The Structure of An Atom

Atoms are very small and an Electromagnetic Microscope is used to view them. An atom is made up of a Nucleus and Electrons flying around it. The Nucleus is made up of Protons, which are positively charged, and other particles. In relative terms, the Electrons, which are negatively charged, fly around the Nucleus at a very large distance. The number of Electrons is usually equal to the number of Protons for each type of atom. Atoms which do not have the same number of Electrons and Protons are called Ions. Because the charges of the Protons and the Electrons are opposite they are attracted to eachother, and this is very important.

Have you read about Atomic Sturcture in the Junior Cert. Metalwork section ?


A magnified view of a Hydrogen Atom.

A view of the Neculeus of an Atom.

Electrostatic Forces

Electrostatic Forces work in much the same way as magnetic forces. Like forces repel and unlike forces attract.

Electrostatic Induction is what occurs in cases such as when you rub a balloon and then it can stick to your jumper. What happens is that the negative charge on the balloon, caused by the rubbing, repels the negative charges in your jumper causing the positive charges to be attracted to the balloon. Thus they stick.

Have you read about Electrostatic Forces in the Junior Cert. Metalwork section ?

The Periodic Table of the Elements

The Periodic Table of the Elements has been developed over the centuries, and is a 'list' defining all of the elements known to man. As a result of the work of many scientists, a lot of information can be recovered from the Periodic Table, such as the number of Protons in a certain atom, the mass of the atom, and much, much more.

Goto the Interactive Periodic Table if you wish to learn more about who developed it, or about particular atoms.

How Atoms Bond Together

Atoms generally bond together in 3 different ways. They are :

Whichever way the atoms bond depends on both the number of Electrons the atom has in it's outer shell and the atom it is bonding with. Goto Chemical Bonding to learn more about this topic in total.

General Properties

All materials have properties, and as was stated earlier these properties are dependant on the atomic make-up of the material, both the atoms themselves and the manner in which they are bonded. Goto the Properties page for a more detailed description of the common properties of materials.

Here are some basic properties of Metals and Non-Metals :

Metals : Good conductors of heat and electricity
Malleable and Ductile to some extent
Electron donors, (Cations), which form oxides
High densities
High Tensile Strengths
Solid at room temperature except for mercury
Can be given a shine
Non-Metals : Brittle
No shine
Electron acceptors, (Anions)
Bad conductors of heat and electricity
Semi-Metals : Some properties of both
(e.g. Silicon, (Si), is used in semi-conductors for the computer industry.)

States of Matter

There are 3 States of Matter. They are :

  • Solid : Definite shape and volumn.
  • Liquid : Definite volumn but takes the shape of the container.
  • Gas : No definite volumn and no definate shape.

The State of Matter depends on the temperature and the pressure. Goto States of Matter to learn more about this topic.

Solidifying Metal And Dendretic Growth

The cooling of a metal from a liquid to a solid is extremely important. As a metal reaches its cooling point small particles cool first. Solidification takes place in a pattern. This pattern is called Dendretic Growth and looks like the branches of a tree. Each small particle grows to form a crystal or grain. Crystals grow together to form a solid.

The solidification starts at one point and spreads out like the branches of a tree. Eventually the Dendrite branches meet and form grains as in the final diagram.

Crystal Patterns and Unit Cells

In metals the atoms bond together in patterns that repeat.
3 common patterns are :

  • Body Centred Cubic (BCC)
  • Face Centred Cubic (FCC)
  • Close Packed Hexagonal (CPH)

A single pattern is called a Unit Cell. A group of patterns is called a Lattice.

The Unit Cells join together in three dimensions to form the Lattice, through Dendritic Growth, finally resulting in a Crystal or Grain. The formation of the grain follows the sequence shown below.

Unit Cell
Lattice
Dendritic Growth
Crystal / Grain


Basic Cubic

Body Centered Cubic

Face Centered Cubic

Close Packed Hexagonal

 

The images above are only representations. They are the basic unit structures that form the grain.

Slip In BCC & FCC Metals

Atoms in a BCC structure are not closely packed and so a large force is required to cause them to slip. As a result shearing is less likely to occur. Therefore brittle metals have a BCC atomic structure.

Atoms in a FCC structure are more closely packed together. Therefore slip occurs more easily. Therefore shear is more likely to occur.

Here are some examples of what metals have which atomic structure.

Unit Structure Typical Metals
BCC Iron, Chromium, Manganese, Cobalt, Tungsten, Titanium, Sodium, Molybdenum, Potassium
FCC Iron, Aluminium, Copper, Nickel, Calcium, Gold, Silver, Platinum

Atomic Imperfections in Metals

Crystal Defects are any imperfections within the crystal structure.

Here, to the left we can see a Line Defect. The three diagrams to the left represent a 2-dimensional view of a lattice structure. The lines numbered 1 to 9 represent columns of atoms. You can see in the top diagram that half a line is missing from line 2. This is the Line Defect. When a Shearing force, (represented by the arrows), is applied the Line Defect 'moves' along to the next line, as you can see below.

This defect continues to move along the columns of atoms, until it reaches the end of the material, or the shearing force stops. Because the crystal structure of metals is not perfect slip occurs. This defect gives metals their Ductile property.

Another type of defect commonly found in metal crystalyne structures is called a Point Defect. There are four basic types :

In a perfectly formed crystal, the atoms are arranged in a regular pattern within the space lattice. ( This is a perfect structure and is very unlikely to occur in reality ).

If there is an atom missing from the lattice, then the whole lattice is distorted as the others atoms are forced into the vacant space. This is known as a Vacant Site Defect.

In this case a much larger atom has been substituted into the lattice and this also distorts the structure. This is known as a Substitute Defect. Sometimes the substitute atom is smaller than the parent atom.

This diagram shows an Interstitial Crystal Defect where a foreign atom has moved into the space between the atoms of the lattice. Again the substitute atom can be larger than the parent atoms.