In general, there are eight major classifications of materials according to Ashby. These are foams, elastomers, natural materials, polymers, non-technical ceramics, technical ceramics, composites and metals. Each group of materials has certain properties linked to the microstructure and the atomic arrangement/bonds of the materials.
When making a jet engines, some materials are less likely to be useful due to a lack of structural integrity in extreme conditions. Considering this, we can rule out a number of materials as inappropriate or unlikely to feature in a jet engine:
- Foams: These are considered as either a rigid polymers like polystyrene, or a flexible polymers like the sole of running shoes.
- Elastomers: These are materials like rubber, latex, neoprene, and polyurethane.
- Natural materials: This include animal or plant products such as wood and leather.
- Polymers: Although having advanced a long way, don’t yet stand up to the extreme conditions inside the engine. Some examples include PET and epoxies.
- Non-technical Ceramics: These are ceramics not designed for technical application, for example marble and sandstone.
Looking at what is left, we see there are three types of materials that are capable of handling some of the extreme conditions in a jet engine.
- Technical Ceramics: These include Alumina, Zirconia and Preovskites and are designed specifically for high temperature applications. Ceramics have a high modulus and are stiff, whilst also being corrosive and oxidation resistant.
- Composites: These materials are essentially a combination of two other materials on a micro or atomic scale. Examples include mortar, reinforced plastic, but of specific interest composites that use metals and ceramics.
- Metals: Arguably the most well known group of materials, metals come in the form of pure metals, alloys which are a combination of pure metals, and more recently superalloys.
Looking deeper into the problem of selecting a material for a component, it becomes more complex as materials of the same type come in different forms. For example, when we look at metals they can come with very different microstructures. When you think about it the microstructure can be thought of like eggs Benedict, an omelette and a souffle. These three dishes have the same base ingredients but are very different, which is the same for materials of the same composition but different microstructures. The two most basic forms of a material are single crystal and polycrystal.
A single crystal is in theory a block of material with regular, uniform and perfect atomic bonds. In a material, atoms arrange themselves in a regular pattern depending on a number of factors. This pattern is called the crystal structure and describes the atomic arrangement of a material i.e. what atoms go where. There are three major ways atoms arrange themselves called Body Centered Cubic (BCC), Face Centered Cubic (FCC) and Hexagonal Close Packed (HCP). If you take some tennis balls and try to arrange them in as many ways as possible you will come up with only these three arrangements (TRY IT!). A single crystal is essentially these arrangements repeated over and over and over and over and over… This means to break a single crystal you need to break the inter-atomic bonds. Now thinking about how small atoms are, you would think this is easy to do, but remember if you jump off a building it is not the fall/gravity that kills you, its the interatomic bonds in the concrete that do.
The other form materials come in is polycrystalline. This means basically we have a lot of single crystals or grains facing in random directions that grow and eventually run into each other. Grain boundaries are where the single crystals connect to each other, and because they are all facing different directions,the atoms at the edges bond with their neighbours however they can. This means the regular atomic bonds of the single crystals becomes a mess of atoms joining to each other anyway possible at the grain boundaries. This means we have many single crystals joined together and so it is called a polycrystal. If we think about the atomic bonds, the grain boundaries are essentially a network of chaos, but this chaos isn’t always bad when it comes to material properties.