IMPERFECTIONS IN THE ATOMIC AND IONIC

ARRANGEMENTS

The arrangement of atoms or ions in the engineered materials contains imperfections or defects that often have a profound effects on the properties of materials.

The three basic types of imperfections are point defects, line defects (dislocations) and surface defects. The presence of such defects is useful.

Point Defects

Point defects are localized disruptions in the otherwise perfect atomic or ionic arrangements in a crystal structure. The disruption affects a region involving several atoms or ions.

These imperfections may be introduced by movement of the atoms or ions when they gain energy by heating, during processing of material, by introduction of impurities (elements or compounds that are present from raw materials) or doping (elements or compounds that are added with an intended beneficial effect on properties).

The effect of impurities is harmful, whereas the effect of dopants on the properties of materials is useful.

Point defects typically involves one atom or ion, or a pair of atoms or ions.

Point defects: vacancy, interstitial atom, substitutional atom (small/large), Frenkel defect, and Schottky defect. All of these defects disrupt the perfect arrangement of the surrounding atoms.

1.Vacancy

It is produced when an atom or ion is missing from its normal site in the crystal structure.

It is introduced into metals and alloys during solidification, at high temperatures, or as a consequence of radiation damage.

It plays an important role in determining

the rate at which atoms or ions can move

around or diffuse in a solid material.

2.Interstitial Defect

It is formed when an extra atom or ion is inserted into the crystal structure at a normally unoccupied location. Interstitial atoms/ions, although smaller than the atoms/ions located at the lattice points, are still larger

The number of interstitial atoms/ions in the structure remains nearly constant even when the temperature is changed.

than the interstitial sites that they occupy; consequently, the surrounding crystal region is compressed and distorted.

Interstitialcy is created when an atom identical to those at the normal lattice point is in an interstitial position. These defects are most likely to be found in crystal structures having a low packing factor.

3.Substitutional Defect

It occurs when an atom/ion is replaced by a different type of atom/ion.Substitutional atoms/ions occupy the normal lattice site.Substitutional atoms/ions may either be larger than the normal atoms/ions in the crystal structure, in which case the surrounding interatomic spacings are reduced, or smaller causing the surrounding atoms/ions to have larger interatomic spacing.The number of defects is relatively independent of temperature.

Small substitutional atom/ion

Large substitutional atom/ion

4.Frenkel Defect

It is shown by ionic solids.It is a vacancy-interstitial pair formed when the smaller ion (usually the cation) is displaced from its lattice position to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location.

 It occurs due to thermal vibrations.

5.Schottky Defect

It is unique to ionic materials and is commonly found in many ceramic materials.The defect forms when oppositely charged ions leave their lattice sites, creating vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid. The vacancies are then free to move about as their own entities.

DISLOCATIONS

Dislocations are line imperfections in an otherwise perfect crystal.

They are introduced typically into the crystal during solidification of the material or when the material is deformed permanently.

Although dislocations are present in all materials, including ceramics and polymers, they are particularly useful in explaining deformation and strengthening in metallic materials.

We can identify three types of dislocations: the Screw Dislocation, the Edge Dislocation and the Mixed Dislocation.

1. Screw dislocation

The screw dislocation can be illustrated by cutting partway through a perfect crystal, then skewing the crystal one atom spacing.

The motion of a screw dislocation is also a result of shear stress.

Motion is perpendicular to direction of stress, rather than parallel (edge).

Screw dislocation line moves perpendicular to applied stress

2. Edge dislocation

An edge dislocation can be illustrated by slicing partway through a perfect crystal, spreading the crystal apart and partly filling the cut with an extra plane of atom.

These atoms also define the dislocation line.

Edge dislocations move in response to shear stress applied perpendicular to the dislocation line.

Edge dislocation line moves parallel to applied stress

As the dislocation moves, the extra half plane will break its existing bonds and form new bonds with its neighbor opposite of the dislocation motion.

This step is repeated in many discreet steps until the dislocation has moved entirely through the lattice.

After all deformation, the extra half plane forms an edge that is one unit step wide.

3. Mixed dislocation

Have both edge and screw components, with a transition region between them.

Makes up most of the dislocations encountered in real life.

Significance of Dislocations

Dislocations are most significant in metals and alloys since they provide a mechanism for plastic deformation, which is the cumulative effect of slip of numerous dislocations.

Plastic deformation is the irreversible deformation or change in shape that occurs when the force or stress that caused it is removed which is caused by dislocation motion.

Slip is the movement of dislocations. Slip provides ductility and in metals. If no dislocations were present, an iron

bar will be brittle and the metal could not be shaped by metalworking process.

We control the mechanical properties of a metal or alloy with the movement of dislocations. Thus, the presence of dislocations helps strengthen metallic materials.

Dislocation Density the total dislocation length per unit volume or the number of dislocations intersecting a unit area.

Dislocations also influence electronic properties of materials.

For example, the resistance of pure copper increases with increasing dislocation density.

Similarly, the speed with which charge carriers such as electrons can move on silicon depends on the dislocation density.

Surface Defects

Surface defects - Imperfections, such as grain boundaries, that form a two-dimensional plane within the crystal.

Hall Petch equation - The relationship between yield strength and grain size in a metallic material—that is,

Small angle grain boundary - An array of dislocations causing a small misorientation of the crystal across the surface of the imperfection.

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Figure 4.15 (a) The atoms near the boundaries of the three grains do not have an equilibrium spacing or arrangement. (b) Grains and grain boundaries in a stainless steel sample. (Courtesy Dr. A. Deardo.)

Figure 4.15 (a) The atoms near the boundaries of the three grains do not have an equilibrium spacing or arrangement. (b) Grains and grain boundaries in a stainless steel sample. (Courtesy Dr. A. Deardo.)

(c) 2003 Brooks/C

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Tilt boundary – small angle boundaries formed by edge dislocations

Twist boundary - small angle grain boundary composed of an array of screw dislocations.

Stacking fault – a surface defect in FCC metals caused by the improper stacking sequence of close packed planes.

-Normally stacking sequence of ABCABCABC is produced in a perfect FCC crystal.

ABC ABAB CABC

This small region, which has HCP stacking sequence instead of FCC stacking sequence, represents a stacking fault. Stacking faults interfere with the slip process.

(c) 2003 Brooks/C

ole Publishing / Thom

son Learning

Figure 4.19 Application of a stress to the perfect crystal (a) may cause a displacement of the atoms, (b) causing the formation of a twin. Note that the crystal has deformed as a result of twinning.

(c) 2003 Brooks/C

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(c)A micrograph of twins within a grain of brass (x250). (d) Domains in ferroelectric barium titanate. (Courtesy of Dr. Rodney Roseman, University of Cincinnati.) Similar domain structures occur in magnetic materials.

Twin boundary – a surface defect across which there is a mirror image misorientation of the crystal structure . Twin boundaries can also move and cause deformation of the material.

Ferroelectric – a dielectric material that develops a spontaneous and reversible electric polarization.

Domain – a small region of a ferroelectric material in which the direction of dielectric polarization or magnetization are remains the same.

Importance of defects

Effect on Mechanical Properties via Control of the Slip Process

Strain Hardening Solid - Solution Strengthening Grain – Size Strengthening Effects on Electrical , Optical and

Magnetic Properties

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Figure 4.20 If the dislocation at point A moves to the left, it is blocked by the point defect. If the dislocation moves to the right, it interacts with the disturbed lattice near the second dislocation at point B. If the dislocation moves farther to the right, it is blocked by a grain boundary.