Imperfections in Crystalline Solids

Imperfections in Crystalline SolidsChapter 4

M.V.K.Chaitanya

09831A0307

GNITMech

WHAT YOU NOW KNOW

Metals,many ceramics and some polymers are crystalline.

The atoms re-arranged in ordered arrays extending in all directions.

Characterised by a ‘UNIT CELL’,the repeat unit of the array.

Metals have fcc,hcp or bcc crystal structures. Directions and planes in the crystal are described

by Miller or Miller-Bravais(hcp only) indices.

IMPERFECTIONS IN CRYSTALLINE SOLIDS

There is no such thing as perfect crystal. Real type of crystals contain various types of

imperfections. We know the fact that many engineering

materials are polycrystals. Many of the important properties of materials

are due to the presence of these imperfections.

TYPES OF IMPERFECTIONS

Point Defects Vacancies Interstitials Impurities

Line Defects Edge Dislocations Screw Dislocations

Planar Defects Grain Boundaries Tilt Boundaries Twin Boundaries Micro Cracks

POINT DEFECTS

A Point Defect involves a single atom change to the normal crystal array.

There are three major types of point defects: vacancies,interstitials and impurities

They may be built-in with the original crystal growth or activated by heat.

They may be the result of radiation or electric current etc.,

VACANCIES:

A vacancy is the absecnce of an atom from a site normally occupied in the lattice.

INTERSTITIALS:

An interstitial is an atom on a non-lattice site. There’s need to be enough room for it, so this

type of defect occurs in open covalent structures, or mettalic structures with large atoms.

IMPURITIES:

An impurity is the substitution of regular lattice atom with an atom that does not normally occupy that site.

The atom may come from within the crystal,(e.g. a chlorine atom on a sodium site in a Nacl crystal) or from the addition of impurities.

IMPORTANCE OF POINT DEFECTS

The concentration of point defects in a crystal is typically between 0.1% and 1% of the atomic sites.

The concentration and movement of point defects in a solid is very important in controlling colour and deformation.

Control solid state diffusion. Influence mechanical properties• Plasticity• Creep

LINE DEFECTS (DISLOCATIONS)

Dislocations are one-dimensional defects around which atoms are misaligned, characterized by a vector in the lattice called as Burger’s vector b, a crystal vector, [u,v,w,], which is the measure of the distortion of the lattice around the defect.

Edge Dislocation: b ┴ to dislocation line

Screw Dislocation: b ║ to dislocation line

BURGERS VECTOR

Burgers vector b is defined by ‘burgers circuit’.

Looking along the positive direction of the dislocation of the line, make a clockwise circuit along the dislocation core of an equal number of steps to the right, down, to the left and up. The vector required to bring you back to your starting point is the Burgers Vector.

The plane defined by the Burgers Vector and the line direction is the ‘glide plane’.

Both the line direction and the burgers vector can be represented by miller indices .

EDGE DISLOCATION

An edge disloaction in a metal may be regarded as insertion or removal of an extra half plane of atoms in the crystal structure.

The only severe disruption to the crystal structure occurs along the dislocation line (perpendicular to the page).

Note that the perpendicular to the page, the line may step up or down.These steps are known as ‘jogs’.

SCREW DISLOCATION

Screw dislocation changes the character of the atom planes.

The atom planes no longer exist separately from eachother.

They form a single surface, like screw thread, which spirals from one end of the crystal to other.

It is actually a helical structure because it winds up in 3D, not like a spiral that is flat.

In the average crystal structure, there are ~1012 m of dislocation lines per m3 of crystal

PLANAR DEFECTS

A planar defect is a discontinuity of the perfect crystal structure across a plane.

There are mainly four types of planar defects: Grain Boundaries Tilt Boundaries Twin Boundaries Micro Cracks

GRAIN BOUNDARIES

A Grain Boundary is a general planar defect that separates regions of different crystalline orientation (i.e. grains) within a polycrystalline solid.

The atoms in the grain boundary will not be in perfect crystalline arrangement.

Grain boundaries are usually the result of uneven growth when the solid is crystallising.

Grain sizes vary from 1 µm to 1 mm.

TILT BOUNDARIES

A Tilt Boundary, between two slightly mis-aligned grains appears as an array of edge dislocations.

TWIN BOUNDARIES

A Twin Boundary happens when the crystals on either side of a plane are mirror images of each other.

The boundary between the twinned crystals will be a single plane of atoms.

There is no region of disorder and the boundary atoms can be viewed as belonging to the crystal structures of both twins.

Twins are either grown-in during crystallisation, or the result of mechanical or thermal work. 

MICRO CRACKS A Micro crack occurs where internal broken bonds create new

surfaces. They are about 10 µm in size and there is a tendancy to

form on the surface of a solid rather than in the bulk. They also form at grain boundaries and other regions of

disorder. The region across which the bonds are broken is known

as the separation plane.  Micro cracks are formed when there is abrasion (or

impacts) with dust particles. They are important in determining how, and where, a

solid may fracture.  When a crystal has more than one type of atom, there

will be Chemical as well as Physical disorder in the grain-boundaries.

VOLUME DEFECTS

Volume defects are Voids, i.e. the absence of a number of atoms to form internal surfaces in the crystal.

They have similar properties to micro cracks because of the broken bonds at the surface.

AMORPHOUS MATERIALS AND POLYMERS

Since they are not just distortions from a perfect crystal structure, Micro cracks and Voids can exist in both Crystalline and Amorphous solids.

Polymers can have partly crystalline regions so may also have all of these kinds of defects.

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