The elements

They give us what we have to work with on an engineering scale

The elements - structural engineering coloured

Looking at Aluminium (yellow) - some common alloying elements (green)

Contrasting to a significant competitor:
Ni has been highlighted (cyan). You can very much convert the "primary" nature of Fe with about 10m%Ni: BCC Fe -> FCC alloy (tough)

Iron - characteristics (cheap & cheap & cheap!!!)

Some remarkable features of Fe which lower its cost of application massively:

• has allotropic phase transformation -> all sorts of processing tricks
• oxide melting point is lower than metal melting point
• v.v.v.cheap to extract
• v.easy to fusion-weld

However:

• "defective" crystal structure -> inherently impaired toughness
• OTOH it machines very easily and as associated with allotropy, couples to turn hardness on-or-off as easily as a switch

Aluminium - "primary" characteristics

• no allotropic phase changes
• F.C.C., so inherently tends to toughness
• High S.F.E. (stacking-fault energy) so not easy to harden
• low elastic modulus c.f. Fe, which is "blessed" in this regard
• low density (~2.5g/cm^3) (vs ~7.9g/cm^3 for steel)
• refractory adherent oxide -> corrosion resistance, adhesive bonding

Where does this lead in the applications of Aluminium?

• low density - 1/3rd density of steel and typically > 1/3rd of strength, so application when looking to light weight
• (in mass-critical applications, rigidity propto d^4 tends to overwhelm Elastic Mod. issue)
• tendency to toughness

Whenever weight causes a constant addition to cost-in-service you are likely to look to Aluminium

Now let us look to processing routes, how they fit with the properties, the economics and where this gets us with applications

Pictures presentation - casting