Introduction
The properties of materials depend on their structure. Over the course of this section, we will look at the structure of materials from their smallest building blocks – atoms. Be the end of this section you should understand the basic structure of engineering materials and how this affects their properties and will also understand the structure of an atom and the bonds that are found in engineering materials. You will also be aware that metals are crystalline materials.
Again, these notes have been adapted supplied by Dr Gareth Bradley at Perth College UHI, his hard work is gratefully acknowledged.
Atomic Structure
As mentioned, the properties of materials depend on their structure. All materials are made up of atoms and the resultant properties depend on the type of atoms, and the bonds between them.
All Atoms consist of a nucleus, which is made up of protons and (almost always) neutrons, surrounded by a cloud of electrons. Neutrons do not have an electrical charge, protons have a positive charge, and electrons have negative charge.
A visualisation of an atom is shown in the diagram – though in reality it is almost impossible to see an atom, and certainly not to this level of detail.
Gareth Bradley, Perth College UHI
The photograph below does show a single atom trapped in an electric field, look closely!
Figure 1 (C) David Nadlinger - University of Oxford
Of these sub-atomic particles, electrons have the strongest influence on bonding in many ways.
As shown in the diagram, electrons orbit the nucleus, but only specific orbits exist and these are known as shells. Each shell can only contain a maximum number of electrons, although shells can be divided into subshells. Atoms want the shells to contain the maximum number of electrons and can donate, receive or share electrons to achieve this. If the shell is not full, the atom will be chemically unstable and will undergo chemical reactions – the level reactivity changes between the elements, and the reason why is beyond the scope of this course but it can be easily found with some brief research.
The first 3 shells are shown in the diagram below. The 1st or K shell can contain 2 electrons; the 2nd or L shell can contain 8 electrons consisting of two subshells; and the 3rd or M shell can contain 18 electrons, consisting of three subshells. There are further shells and subshells, but they are rarely encountered and again are beyond the scope of the course.
Atomic Bonding
Atomic bonding is – literally – what holds everything together at a molecular level. It can be divided into two types – primary and secondary. Primary – or intramolecular – bonds are relatively strong and occur between atoms in the same molecule or same structure. They can be further divided into 3 subroups: ionic, covalent or metallic. Secondary – or intermolecular – bonds on the other hand are relatively weak and are what hold adjoining molecules together. Again they can be subdivided into Van der Waals (instantaneous dipole bonds), hydrogen bonding and others.
Ionic Bonding
The animation below shows an example of Ionic bonding. An atom (in this case sodium) donates an electron to another atom (in this case fluorine). The atoms become ions (in this case Na+ and F-) with complete outer shells of electrons.
Wikimedia/CC BY-SA 3.0,
Materials with ionic bonding tend to have high strength, high elastic modulus, high melting point and poor electrical conductivity in their solid state – though they will conduct when liquid. Examples of materials that have ionic bonding include sodium chloride (NaCl) (table salt), magnesia (MgO), alumina (Al2O3) and cement.
Covalent Bonding
In covalent bonding, Atoms share electrons to give them complete outer shells. In the example shown in the diagram four hydrogen atoms each share one electron with a single carbon atom to give CH4, methane.
Covalently bound materials are commonly found in nature. Materials with covalent bonding tend to have, very high elastic modulus, high (inherent) strength, high melting point and low electrical conductivity.
Examples include diamond, silicate ceramics and polymers (between the carbon atoms in the backbone and the cross links in thermosetting polymers).
Most naturally occurring gases also show covalent bonding – such as methane as shown above, and carbon dioxide as shown below.
Gareth Bradley, Perth College UHI
Metallic Bonding
Not surprisingly, metallic bonding is found only in metals. Materials with metallic bonds “donate” electrons to a common pool and these are shared by all the atoms as shown in the animation.
Metallic Bonding, Dr Gareth Bradley
As you can probably imagine from thinking about metals such as iron, Materials with metallic bonding tend to have High elastic modulus, High strength, Good electrical and thermal conductivity, and Good ductility. Examples would include iron, aluminium, titanium, or any other pure metal.
Van der Waals bonds
These weak intermolecular forces are caused by a dipole between two atoms. An example would be the forces between N2 (nitrogen) molecules. Polythene polymer chains are bonded to each other through van der Waals bonding. The two diagrams help to visualise what is actually happening between molecules when Van der Waals bonding occurs.
Gareth Bradley, Perth College UHI
Gareth Bradley, Perth College UHI
Crystallographic Structure
The crystallographic structure of a material is at a larger scale than the atomic level, though in most cases it is still at a microscopic level. Metals and ceramics, but not glasses, are made up of small crystals or grains that exhibit a regular, repeating structure.
The crystallographic structure of most metals is one of three types:
- Body centred cubic (BCC)
- Face centred cubic (FCC)
- Close packed hexagonal (CPH)
The diagram shows a theoretical image of this, whereas the false colour image – which is a micrograph – shows what this actually looks like when viewed under a microscope.
Gareth Bradley, Perth College UHI
Body Centred Cubic
In a body centred cubic crystal, a central atom is surrounded by several others as shown in the diagrams. They are often found in metals which tend to be harder and less ductile and include iron, sodium, chromium and tungsten
Gareth Bradley, Perth College UHI
Face Centred Cubic
Here the crystal has an “open space” in the centre of the crystal. For this reason Metals tend to be soft and ductile and include aluminium, copper and nickel.
Gareth Bradley, Perth College UHI
Hexagonal Close Packed
Here the shape of the crystal has changed from a cube, to a hexagonal shape as shown in the diagram. Metals tend to be harder and less ductile and include magnesium, zinc and titanium.
Unit cells and Lattice Structures
All of the crystallographic structures mentioned above - BCC, FCC and HCP - are known as unit cells. Individual unit cells combine to form a lattice structure which consists of a regular repeating structure of unit cells as shown in the image below.
Image: A. Siber, with Permission
Grain Formation and Growth
When a large volume of liquid metal cools below its freezing point solidification will occur at numerous points, rather than spreading out from a single point.
As cooling continues further solidification occurs and the original solidifications grows in size. Each solid particle having a regular repeating lattice structure known as a dendrite. These dendrites have a fir tree shape and are shown in the micrograph.
As further cooling occurs the dendrites will continue to grow in size until they collide with each other. Where the dendrites meet is known as a grain boundary. Each grain has a regular repeating structure, but the direction differs between each grain. The image tries to give an indication of this, showing the regular structure within the grains and the random structure between them.