Notes

Outline

Crystal Defects The Structure of  Matter Edward Goo University of Southern California

Perfect Crystal A perfect crystal with every atom in the correct position does not exist. “Crystals are like people: it is the defects in them which tend to make them interesting!” - Colin Humphreys Most materials properties are determined by the crystal defects present.

Processing, Microstructure and Properties Processing is the manner in which the material is made. The processing determines the defects that are present. Since materials properties are determined by crystal defects there is a correlation between the processing of the materials and the materials properties.

Materials Processing Much of the early work on materials in the 1800’s and early 1900’s was on materials processing. A large amount of empirical information was acquired on the effect of materials processing on materials properties.

Microstructure, Properties and Processing

Materials Science The academic discipline of Materials Science developed in the 1950’s to study the processing-microstructure and microstructure-properties correlations. The Atomic Energy Commission played an important role in developing the field of Materials Science because of the materials problems facing nuclear reactors.

Radiation Swelling Radiation swelling was the phenomena that metal pipes in a nuclear reactor would swell when exposed to radiation in a nuclear reactor. This would pose a serious problem especially for moving parts in the reactor.

Formation of Vacancy Loop

Interstitial and Vacancy Loops

Image of Interstitial Loop High resolution transmission electron micrograph of an interstitial loop in CdTe

Voids Due to Neutron Irradiation Transmission electron micrograph showing voids created in stainless steel irradiated with high energy neutrons.

Types of Crystal Defects Point defects Line defects - dislocations Planar defects Bulk defects

Point Defects Vacancies Interstitials Impurity atoms substitutional interstitial

Vacancies and Interstitials

Impurity Atoms

Point Defects in Ionic Crystals Ionic crystals unlike other solids are made up of charged ions. Point defects in ionic crystal are charged. Charge neutrality must always be maintained.

Forces between Charges The attractive force between unlike charges and the repulsive force between like charges are called coulombic forces. If one took 45 pounds of protons in a ball one foot in diameter and placed it in the center of the moon, the protons, which repel each other, would fly apart with such force that it would blow apart the moon.

Defect Complexes FD - Frenkel defect - cation vacancy and cation interstitial. SD - Schottky defect - anion and cation vacancies.

Dislocations In the early 1900’s engineers and scientist noticed that metals would bend at forces very much less than that predicted based on the theoretical strength of the forces holding the metal together.

Deformation of Solids

Edge Dislocation

Dislocation Motion

Motion of Edge Dislocation

Motion of Corn Borer

Edge vs. Screw Dislocations An edge dislocation is shown on the left and the screw dislocation on the right.

Motion of a Screw Dislocation

Planar Defects Stacking faults The dislocation was proposed as the defect in the crystal that allowed metals to deform at much lower forces than predicted for a perfect crystal. Grain boundaries

Stacking Faults

Grain Boundaries

Formation of Grains

Image of Grain Boundary

Straws

Defects in the Straws

Formation of a Twin

Twinning

Image of Twins in NiTi

Shape-Memory Alloys Shape-memory alloys will return back to its original shape after deformation if heated to a higher temperature. Shape-memory alloys are twinned and when deformed they untwin. Upon heating the alloy returns back to the original twin configuration and hence goes back to the original shape.

Twinning in Shape-Memory Alloys

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