Defects in Engineering Materials
Vacancies, dislocations, boundaries and pores govern the behaviour of real materials.
Perfect crystals are a model
Engineering solids contain point, line, surface and volume defects. Their populations respond to temperature, composition, deformation and processing history.
Point defects
Vacancies enable substitutional diffusion. Interstitial atoms can diffuse rapidly and strongly distort lattices. Solute atoms may strengthen a metal, stabilise a phase or alter electrical behaviour.
Dislocations
Plastic deformation in crystals occurs mainly through dislocation motion. The theoretical shear strength of a perfect crystal is very high; dislocations allow slip at far lower stresses. Strengthening methods work largely by obstructing that motion.
Interfaces and volume defects
Grain boundaries can strengthen a material by interrupting slip, but they also accelerate diffusion and may promote corrosion or creep. Pores, inclusions and cracks concentrate stress and often control fatigue or fracture.
Takeaway
Performance is governed less by ideal atomic order than by the type, density, distribution and mobility of defects.