Callister Chapter 4: Imperfections in Solids

Candidacy season is upon us, so this chapter summary will just be a very quick one. In this post, I will run through the different kinds of point defects, dislocations, and interfacial defects. Please refer to the textbook for additional information on this topic, such as calculating the number of vacancies, imaging defects, and determining grain size.

Point defects
-this type of defect is associated with one or two atomic positions
-vacancies occur when atoms are missing from their lattice positions
-interstitials occur when atoms are found in lattice sites that are normally unoccupied for a given crystal structure
-impurities are foreign atoms that either take the place of lattice atoms or squeeze into interstitial sites
-a solid solution may form when the original structure of a crystal is maintained despite the addition of impurity atoms
-substitutional solid solutions form when an impurity atoms substitutes a host atom (this happens when their atomic radii and electronegatives are similar and they have the same crystal structure)
-interstitial solid solutions form when the impurity atom is small and occupies interstitial sites

Dislocations
-this type of defect is linear/one-dimensional
-types of dislocations include edge, screw, or a combination of the two
-in an edge dislocation, the Burgers vector is perpendicular to the dislocation line
-in a screw dislocation, the Burgers vector is parallel to the dislocation line
-the following figure depicts these types of defects (point A is pure screw and point B is pure edge)

Callister, William D. Materials Science and Engineering: An Introduction. New York: John Wiley & Sons, 2007. Print.

Interfacial defects

-this type of defect is two-dimensional

-external surfaces are considered interfacial defects because surface atoms don't bond to surrounding atoms the same way as atoms throughout the crystal do (also, surface atoms have higher energy)

-grains boundaries are boundaries separating grains that are oriented in different crystallographic directions

-twin boundaries are grain boundaries that have mirror symmetry

-stacking faults occur when there is an interruption in stacking sequence

-phase boundaries occur when there are changes in physical/chemical composition