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1.15 Scales of material structure

Exactly what influences the properties of a component can depend on many things: We've already mentioned the importance of materials properties and the component geometry, for example.

The component geometry is an example of structure on a macroscopic scale. Look at Figure 12(a), which shows the second Severn Crossing. The bridge has the structure it does because it was built to achieve the task of providing a path for vehicles across the estuary, at an acceptable cost and with complete safety during construction and during use. The central portion is an example of a cable-stay bridge, where the deck of the bridge is hung from the supporting cables. This structure was chosen, presumably, as being the best solution.

Figure 12(a)
Figure 12(a) The second Severn Crossing

If we look at the structure of a support cable for such a bridge (Figure 12(b)), we see that it is not a solid bar of material, but is 'woven' from many thinner strands of wire. This structure (still a macrostructure) is chosen for several reasons, including safety, as with a reasonable safety factor, it shouldn't matter if a flaw causes the failure of one wire strand, as there are multiple paths for the load that the cable is supporting. In addition there are some beneficial properties that cable structures have compared to large single strands, such as flexibility.

Figure 12(b)
Figure 12(b) A steel cable

The structure story doesn't stop with the material for one strand, though. I've already indicated that steel is a mixture of iron with carbon, and how the carbon affects the structure of the iron on a microscopic scale depends on the amount of carbon in the iron, and the heat treatment that the iron has had. Figure 12(c) shows the micro structure of a typical steel (so-called because we are looking at the steel on a microscopic scale). This shows that as we look in closer detail, we begin to see that what we thought was quite a smooth, plain metal surface has a lot of underlying structure to it. Once we've zoomed in so that we can see features as small as 10 micrometres, it becomes clear that the metal is composed of small individual 'grains'. This structure in turn determines the mechanical properties, like strength and toughness, of the steel. We can change the structure of the iron: through alloying and heat treatment the grain size and structure can be altered, so tailoring the properties of the material that we make.

Figure 12(c)
Figure 12(c) Optical micrograph of steel

Figure 12(d) zooms in still further, showing us more of the structure within the grains themselves. Influencing things at this level is more complicated, but it can be done, and again can help to tailor the material properties.

Figure 12(d)
Figure 12(d) Transmission electron microscope (TEM) micrograph of steel

Finally, we can zoom down to the level of the atomic structure (Figure 12(e)). In this case, we're looking at carbon, one of the elements in steel. The bonding between the atoms, and the structure they take up, critically influences the material properties, but there's nothing we can do to change it! Some materials are more useful than others because they have the right sort of atomic bonding and atomic structure, and a microstructure that we can do useful things with. In Figure 12(e), you can see that each carbon atom is surrounded by six others in an hexagonal pattern. This is simply the way that carbon atoms arrange themselves in this instance (carbon is versatile in that it can adopt several atomic arrangements).

Figure 12(e)
Figure 12(e) Scanning tunnelling microscope image of carbon atoms

We will refer to microstructure frequently in this section. It is a key factor in determining mechanical properties, and it can be greatly affected by the choice of manufacturing process for a material.