  This chapter begins with pure aluminium and a discussion of the form of the
crystal structure and different unit cells that can be used to describe the
crystal structure. Measurements of the face-centred cubic lattice parameter and
thermal expansion coefficient in pure aluminium are reviewed and
parametrisations given that allow the reader to evaluate them across the full
range of temperatures where aluminium is a solid. A new concept called the
vacancy triangle is introduced and demonstrated as an effective means for
determining vacancy concentrations near the melting point of aluminium. The
Debye-Waller factor, quantifying the thermal vibration of aluminium atoms in
pure aluminium, is reviewed and parametrised over the full range of
temperatures where aluminium is a solid. The nature of interatomic bonding and
the history of its characterisation in pure aluminium is reviewed with the
unequivocal conclusion that it is purely tetrahedral in nature. The
crystallography of aluminium alloys is then discussed in terms of all of the
concepts covered for pure aluminium, using prominent alloy examples. The
electron density domain theory of solid-state nucleation and precipitate growth
is introduced and discussed as a new means of rationalising phase
transformations in alloys from a crystallographic point of view.
