In this notes sheet:
There are two types of deformation of crystalline materials: elastic and plastic. The former is fully reversible, whereas in the latter, atomic planes slip over one another to form a new, different formation. This kind of deformation is irreversible.
Note that the elastic deformation graph could be non-linear (e.g. rubber)
Slip & Slip Systems
The slipping of atomic planes (slip planes) happens in whatever direction requires the least energy. It is brought about by the application of an external force, resulting in shear stresses within the crystal structure.
In a perfect sample, the slip would occur over the whole plane at once, however imperfections in the crystal structure prevent this: slip is a gradual process.
Close-packed planes are the most susceptible to slip. This is because more energy is required for a non-close-packed plane to slip, as there is a greater distance for each displaced atom to move.
The number of slip systems a crystal lattice has reflects how likely slip in the structure is.
The more slip systems present, the more susceptible to slip.
This is determined as the product of close-packed directions and non-parallel close-packed planes:
Lattices with few slip systems are said to show brittle deformation, while those that have many undergo ductile deformation.
HCP Slip Systems
There are three close-packed directions per close-packed plane, however all close-packed planes are parallel. Therefore, the number of slip systems is 3, so the deformation is brittle: