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CRYSTAL DEFECTS (IMPERFECTIONS)

INTRODUCTION :

Most Materials When solidified consist of many crystals and grains. Single crystals can be manufactured by withdrawing a crystal fragment under carefully controlled conditions just above freezing point. In case if single crystal approach a nearly perfect lattice structure it is possible to calculate theoretical strength of a metal by the force required to separate bonds between adjoining atoms. But due to defects in crystals ,  This theoretical value of strength  obtained from single crystal  would be 1000 times smaller than original crystal structure. Hence it is important to study the defects in crystals to know how they affect properties of materials

 

TYPES OF DEFECTS

Defect in a crystal can be defined as a deviation from its ideal structure.

Defects in crystals can be broadly classified into 3 types based on their dimensionality

·      Zero dimensional defects or Point defects

·      One dimensional defects or Line defects

·      Two dimensional defects or Interfacial defects or Surface defects

 

1)    POINT DEFECTS

Point defects are the zero dimensional crystal defects where an  atom or ion is missing or in an irregular position in otherwise a perfect crystal structure. Point defects can be categorized into 2 types namely stoichiometric and non stoichiometric defects.

 

STOICHIOMETRIC DEECTS

A)   VACANCIES

Vacancies are simply empty atom sites in otherwise a perfect lattice. Due to this density of metal decreases. Atoms surrounding a vacancy tend to be closer distorting lattice planes.

 B) INTERSTITIALS

If the impurity atoms are very small , they will occupy non lattice positions of the  lattice which lead to defect in crystal structure. Interstitials tend to push the surrounding atoms farther apart and also profile distortion of lattice planes

         C) SUBSTITUTIONAL DEFECTS

If the impurity atoms are large so that they cannot fit into interstitials then the impurity atoms substitute the atoms present in lattice position which leads to defect in otherwise perfect crystal structure. Atoms surrounding the substitutional atom may be closer or pushed away according to its size.

     D) FRENKEL DEFECTS

FRENKEL DEFECTS are mostly exhibited in ionic solids. It is also termed as dislocation defect in which the smaller ion , generally cation displaces from lattice site and occupies interstitial position leaving vacancy at original lattice position.

 

    E) SCHOTTKY DEFECTS

Schottky defects are only observed in ionic solids in which ions from the lattice sites move to the surface in such a way that electrical neutrality is maintained and leaving behind vacancies in the crystal structure. Schottky defects leads to decrease in density of the ionic solid.

NON STOICHIOMETRIC DEFECTS

 

         A)METAL EXCESS DEFECT

This defect may occur in two ways….

1.    This defect is formed when  a negative ion is missing from a lattice site leaving behind a hole, which is occupied by one or more pair of electrons. These are called as f-centers. Electrons in such vacancy tend to absorb light in visible spectrum and gives color to the crystal.

2.    This defect may also be observed when extra cations occupy interstitials sites. Electrical neutrality is maintained by electrons that are present in other interstitial sites.

 

       B) METAL DEFICIENCY DEFECT

When some cations are missing from original lattice, To maintain electrical neutrality other atoms of same crystal increases their valences. For example in FeO if some Fe2+ atoms are missing…Then loss of charge can be neutralized by increasing Valency of existing Iron from Fe2+ to Fe3+ .

 

2)    LINE DEFECTS ( DISLOCATIONS )

Dislocations are one dimensional defects around which some atoms are misaligned. The movement of these dislocations allows the atoms to slide or slip. Two vectors are defined to describe the direction of dislocation and direction of slip which are tangent vector and burger vector respectively.

A)   EDGE DISLOCATION

Edge Dislocation is a one dimensional defect in which An extra portion of a plane of atoms, or half-plane, the edge of which terminates within the crystal. It is a linear defect that centers on the line that is defined along the end of the extra half-plane of atoms. This is sometimes termed the dislocation line. Atoms experience compressive stress below dislocation line and tensile stress above dislocation line. Movement of Dislocation(slip) and dislocation line are perpendicular to each other i.e. burger vector is perpendicular to tangent vector.


B ) SCREW DISLOCATION

This is a kind of dislocation in which the Slip plane and dislocation line are parallel to each other i.e. burger vector is parallel to tangent vector. Screw dislocation can be thought of as shifting upper front of crystal by one atom distance relative to bottom portion by application of shear stress.

  

C)   MIXED DISLOCATIONS

This is a general case of Dislocation in which dislocation line and movement of dislocation are neither parallel nor perpendicular to each other i.e burger vector is at an angle to the tangential vector.

3)    INTERFACIAL / SURFACE DEFECTS

Interfacial defects are boundaries that have two dimensions that separate regions of the materials that have different crystal structures and/or crystallographic orientations. These imperfections include grain boundaries, phase boundaries, twin boundaries, and stacking faults.

 

A)   GRAIN BOUNDARIES

Boundaries between two adjacent grains are called as grain boundaries. Grains are the regions in which the  atomic lattices and their orientations are continuous. These orientations can be understood as rotation of grains on one side of grain boundary with respect to other . This rotation can be  represented by rotation with respect to grain boundary plane and angle of rotation. Small angle grain boundaries and large angle grain boundaries comes under rotation by an angle, Tilt boundary and twist boundary comes under rotation w.r.t grain boundary plane.

B)   TWIN BOUNDARIES

These are the boundaries in the grains at which atomic arrangement on one side of the grain is the mirror image of atoms in the other side of grain. Twins result from atomic displacements that are produced from applied mechanical shear forces (mechanical twins), and also during annealing heat treatments. The region between pair of boundaries is called twinned region.

C) PHASE BOUNDARIES

Phase boundaries exist in multiple phase Materials in which there exists different constituent phases on each side of grain boundary. Phase boundaries effect mechanical characteristics of multiphase metal alloys

 


D)  STACKING FAULTS

Stacking faults are observed in FCC crystals when there is an interruption in ABCABCABCABC… Stacking sequence of closely packed planes.

 

E)    EXTERNAL SURFACES

Atoms present in the surface possess high energy than the atoms that are in their interior positions. This is because, Surface atoms are not bonded to maximum number of nearest neighbor . As the bonds of these surface atoms not satisfied, they give rise to surface energy. To reduce this energy materials tend to minimize the surface area. For example  Liquids droplets are spherical as the sphere has the minimum surface area. However this cannot be observed in solids as they are rigid.

 

By

Abhilash

Metallurgical and materials engineering, VNIT Nagpur

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