Point Defects Flashcards
Name the four types of defects in materials?
Point defects 0D
Linear Defects 1D
Planar defects 2D
Bulk or volume defects 3D
Explain point defects
Vacancies are always present
Self interstitials that introduce large distortion.
Can predict vacancies using boltzman distribution
Interstitial defects that are small and sit in between the gaps
Substitutional impurity atoms are different from the host
Explain Edge dislocation
When a force is applied slip may occur
Smaller grain size have larger grain boundaries which reduce slip and strengthen.
Dislocations create small elastic deformation of the lattice
Bonds broken from left to right but remade in quick succession.
Burgers vector represents magnitude and direction, at right angles to dislocation.
Less brittle more ductile
Explain screw dislocations
Spiral planar ramp resulting from shear deformation.
Burgers vector parallel to dislocation.
Extra half plane of atoms.
Less brittle more ductile.
Name the two types of linear defects?
Edge and screw dislocations
Name the two type of planar defects?
External surfaces and grain boundaries
Explain external surfaces
Surface atoms have unsatisfied atomic bonds and higher energies.
Minimisation of surface area reduces energy.
Solid surfaces can reconstruct or grow to satisfy atomic bonds at surface.
These bond with impurities and lead to internal distortion due to diffusion
Explain grain boundaries defects
Smaller grain angles don’t block dislocations
Large grain angles do block dislocations and strengthen material.
Disordered and lower in density
High mobility
High diffusivity and mobility
Grains grow in size at the expense of smaller grains to minimise energy.
Volume defects
Pores
Cracks
Foreign inclusions
Can effect mechanical electrical and thermal properties
What is optical microscopy?
Allows the analysis of the surface giving info about the grain size and orientation.
Polishing removes surface features such as scratches and etching changes reflectance.
Grain boundaries are imperfections and are more susceptible to etching
What is a solid solution?
Solid with an impurity.
Host (solvent)
Impurity (solute)
What are the four Hume Rothery rules?
Atomic size- more than 15% unlikely to be soluble
Crystal structure
Electronegativity
Valancy
What is diffusion?
Atoms tend to move from an area of a high conc to an area of low conc
Random
How do solids diffuse?
Vacancy diffusion and interstitial diffusion
What is Vacancy diffusion?
Where an atom moves to a vacancy. Can only move if there’s sufficient energy from thermal energy of atomic vibrations.
Slow process
Less common but greater bonding
