CHE217 - 2.1 Defects in Crystalline Materials

Point Defects

Irregular or deviations from ideal arrangement around a point/atom in a crystalline substance.

• Vacancy

• Impurity

Vacancy

• atom missing from an atomic site

• occurs due to imperfect packing during crystallization

• DECREASE in density

Interstitial

• Addition of an extra atom within a crystal structure

• Causes atomic distortion

• INCREASE in density

Impurity Defects

• Extrinsic defects

Less than 15%

Size difference between the solute and solvent must be:

Electronegativity

Two atomic species’ __ must be comparable.

Valence

Two species’ __ must be similar.

Crystal structures

Two species' __ __ must be the same.

• Solidification

• Added impurities

• Heating

• Stress

Origin of point defects

Line defects

• Irregularities or deviation from ideal arrangement

• Due to misalignment of atoms

• Also known as dislocations

Edge dislocation

• Caused by termination of a plane of atoms in the MIDDLE of a crystal

• Increase in density

• Plane bend around the edge of terminating plane

Screw Dislocation

• Defect in which atoms are not arranged in a line but in a form of a screw thread

Mixed Dislocation

• Has both edge and screw character

• Free surface

• Boundaries

• Another dislocation

Dislocations can terminate at:

Planar Defects

• Associated with boundaries that are separate regions of the materials

• Have different crystal structure

• 2D

• Due to change in orientation of the atomic planes

Grain Boundaries

• Boundaries between crystals

• Produced by the solidification process

• Change in crystal orientation across them

• Impede dislocation motion

Surfaces

• Atoms lack the required coordination number.

Twin defects

• Change in crystal orientation which lead to the formation of a mirror image of a crystal

Stacking Fault

• Refers to anomalies in the stacking sequence of crystal planes that disrupt the continuity of a lattice

Volume Defects

• These include pores, voids, cracks, and foreign inclusions.

Pores and Voids

• Trapped gases

• Decreases mechanical strength

Cracks

• Local discontinuity produced by a fracture

Foreign inclusions

• Undesirable foreign particles