Materials Science: Lattice Defects

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Questions and Answers

There is only one type of dislocation, which is the edge dislocation.

False

The Burgers vector is parallel to the dislocation line for edge dislocations.

False

Dislocations are linear defects in a crystal lattice.

True

The slip direction is the same as the direction of the Burgers vector for screw dislocations.

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Mixed dislocations are rare in crystalline materials.

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Dislocations are not involved in the plastic deformation of metals.

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Twinning is a type of point defect.

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Dislocations are not helpful in understanding the mechanical behavior of materials.

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Point defects are defects that occur at or around a single lattice point and are extended in space in one dimension.

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Interstitial defects are formed when an atom is missing from a normal site.

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Substitutional defects are introduced as an impurity to create an alloy.

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Frenkel defects are a pair of vacancies created in an ionic solid.

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Line defects are defects that occur at or around a single lattice point.

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Surface defects are defects that occur on the surface of a material.

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Vacancy defects are introduced into the crystal during solidification at low temperatures.

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Schottky defects are a vacancy-interstitial pair formed in an ionic solid.

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C stalline solids have a random arrangement of atoms.

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The position of any point along a line in a space lattice can be obtained by a simple translation, ru = ua.

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The atomic arrangement in aluminum provides good strength.

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About 80% of elemental metals crystallize into three densely packed crystal structures.

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Crystalline solids always have a perfect periodic crystal structure.

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The positions of atoms or molecules in crystalline solids are not fixed.

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Defects in crystalline solids can be found in the form of vacancy defects.

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Crystalline solids never have a perfect crystal structure due to the presence of defects.

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Study Notes

Defects in Crystalline Solids

Crystalline solids are not perfect, and they have three types of defects: point, line, and surface defects.
These defects affect the physical and mechanical properties of metals.

Point Defects

Point defects occur at or around a single lattice point and are not extended in space.
They involve few extra or missing atoms in the lattice.
Point defects can be caused by heating, processing, impurities, or intentionally during alloying.

Types of Point Defects

Vacancy Defect: occurs when an atom is missing from a normal site.
Interstitial Defect: occurs when an extra atom is inserted into the lattice structure at a normally unoccupied position.
Substitutional Defect: occurs when an atom is replaced by a different type of atom.
Frenkel Defect: a vacancy-interstitial pair formed when an ion jumps from a normal lattice point to an interstitial site.
Schottky Defect: a pair of vacancies created in an ionic solid, where an anion and a cation may be missing from the lattice to maintain electrical neutrality.

Dislocations

Dislocations are linear defects where some of the atoms of the crystal lattice are misaligned.
There are two basic types of dislocations: edge dislocation and screw dislocation.
Mixed dislocations, combining aspects of both types, are also common.

Edge Dislocation

Results from a mismatch in the rows of atoms, as if an extra plane of atoms was inserted.
The Burgers vector, b, represents how far an atom needs to be moved to bring it back into registry.
The Burgers vector, b, is perpendicular to the dislocation line.

Screw Dislocation

Results from shearing in the crystal.
The Burgers vector, b, is parallel to the slip plane.

Significance of Dislocations

Dislocations help understand the mechanical behavior of materials.
They explain why the strength of metals is much lower than the value predicted from metallic bonds.
Dislocations provide ductility in metals.
Controlling the movement of dislocations allows us to control the mechanical properties of a metal or alloy.

Plastic Deformation

There are two basic modes of plastic deformation: slip and twinning.

Crystalline Structures

Crystalline solids have a periodic arrangement of atoms, which can be described by a network of points in space called a lattice.
The periodicity of atoms in crystalline solids can be described by a three-dimensional array of points, each of which has identical surroundings.

Principal Metallic Crystal Structures

Most elemental metals (about 90%) crystallize into three densely packed crystal structures: face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP).
These structures are formed because energy is released as the atoms come closer together and bond more tightly with each other.