Like all metals, aluminium has a crystalline structure: the atoms are arranged in a three-dimensional crystal lattice with a spacing of about a hundred-millionth of a centimetre (10-8 cm). With a great amount of effort it is possible, however, to produce a piece of metal with a completely intact lattice as a "single crystal". Parts made by casting pure metals or alloys are always polycrystals comprising monocrystals, the crystal grains, between a thousandth of a millimetre and several millimetres in size with irregular grain boundaries.
The most important processes for examining the grain structure in research and quality assurance, which are also carried out on castings and semi-finished products, are:
- visual inspection, with the naked eye, of fracture surfaces, looking for pores or precipitation of oxides at ground boundaries
- optical microscopy of etched sections
- microhardness testing by indentation of a diamond pyramid at various points in a grain
- electron microscopy for the highest magnifications
- X-ray analysis of the composition of a microstructure or the distribution of elements within a grain.