Imperfections in Solids

Point Defects

Vacancies and Self-Interstitials

A vacancy is a lattice position that is vacant because the atom is missing. It is createdwhen the solid is formed. There are other ways of making a vacancy, but they also occur naturally as a result of thermal vibrations.

An interstitial is an atom that occupies a place outside the normal lattice position. It maybe the same type of atom as the others (self interstitial) or an impurity atom. In the case of vacancies and interstitials, there is a change in the coordination of atoms around the defect. This means that the forces are not balanced in the same way as for other atoms in the solid, which results in lattice distortion around the defect A hightemperature is needed to have a high thermal concentration of vacancies.

Frenkel-defect is a vacancy-interstitial pair of cations

Schottky-defect is a pair of nearby cation and anion vacancies

Theoretical yield point

Theoretical yield is the maximum quantity of a product that could be formed in a chemical reaction if all the limiting reactant reacted to form products (distinguished from actual yield).

Dislocations—Linear Defects

Dislocations are abrupt changes in the regular ordering of atoms, along a line (dislocation line) in the solid. They occur in high density and are very important in mechanical properties of material. They are characterized by the Burgers vector, found by doing a loop around the dislocation line and noticing the extra inter atomic spacing needed to close the loop. The Burgers vector in metals points in a close packed direction.

Line Defects

•   Line defects or Dislocations are abrupt change in atomic order along a line.

•    They occur if an incomplete plane inserted between perfect planes of atoms or when vacancies are aligned in a line.

•    A dislocation is the defect responsible for the phenomenon of slip, by which most metals deform plastically.

•  Dislocations occur in high densities (108-1010 m-2  ), and are intimately connected to almost all mechanical properties which are in fact structure-sensitive.

• Dislocation form during plastic deformation, solidification or due to thermal stresses arising from rapid cooling.

Line Defects – Burger’s Vector

•   A dislocation in characterized by Burger’s vector, b.

• It  is  unique  to  a  dislocation,  and  usually  has  the  direction  of  close  packed  lattice direction. It is also the slip direction of a dislocation.

• It represents the magnitude and direction of distortion associated with that particular dislocation.

•    Two limiting cases of dislocations, edge and screw, are characterized by Burger’s vector perpendicular to the dislocation line (t) and Burger’s vector parallel to the dislocation line respectively. Ordinary dislocation is of mixed character of edge and screw type.

Edge dislocations occur when an extra plane is inserted. The dislocation line is at the end of the plane. In an edge dislocation, the Burgers vector is perpendicular to the dislocation line.

Line Defects – Edge Dislocation

• It is also called as Taylor-Orowan dislocation.

• It will have regions of compressive and tensile stresses on either side of the plane containing dislocation.

Screw dislocations result when displacing planes relative to each other through shear. In this case, the Burgers vector is parallel to the dislocation line.

Line Defects – Screw   Dislocation

•  It is also called as Burger’s dislocation.

•  It will have regions of shear stress around the dislocation line

• For positive screw dislocation, dislocation line direction is parallel to Burger’s vector, and vice versa.

Interfacial Defects:

The environment of an atom at a surface differs from that of an atom in the bulk, in that the number of neighbors (coordination) decreases. This introduces unbalanced forces which result in relaxation (the lattice spacing is decreased) or reconstruction (the crystal structure changes). The density of atoms in the region including the grain boundary is smaller than the bulk value, since void space occurs in the interface. Surfaces and interfaces are very reactive and it is usual that impurities segregate there. Since energy is required to form a surface, grains tend to grow in size atthe expense of smaller grains to minimize energy. This occurs by diffusion, which is accelerated at high temperatures.

Interfacial Defects

• An interfacial defect is a 2-D imperfection in crystalline solids, and have different crystallographic orientations on either side of it.

•   Region of distortion is about few atomic distances.

•  They usually arise from clustering of line defects into a plane.

•  These imperfections are not thermodynamically stable, but meta-stable in nature.

•    E.g.: External surface, Grain boundaries, Stacking faults, Twin boundaries, Phase boundaries.

Bulk or Volume Defects :

Other defects exist in all solid materials that are much larger than those heretofore discussed. A typical volume defect is porosity, often introduced in the solid during processing. A common example is snow, which is highly porous ice.

Bulk or Volume Defects

Volume defects are three-dimensional in nature.These defects are introduced, usually, during processing and fabrication operations like casting, forming etc.E.g.: Pores, Cracks, Foreign particles These defects act like stress raisers, thus deleterious to mechanical properties of parent solids.In some instances, foreign particles are added to strengthen the solid – dispersion hardening. Particles added are hindrances to movement of dislocations which have to cut through or bypass the particles thus increasing the strength.