Q. What is crystal imperfection? Briefly discuss all types of point defects in crystals.
Ans.The various type of crystal imperfection are as follows:
(1) Point defects: In a crystal lattice point defect in one which is completely local in its effect. The possible point defects are:
(a) Vacancies: Vacancy is an atom site from which an atom is missing. The main causes of this defect are:
(1) Due to thermal vibration at high temperature.
(2)Imperfect packing during crystallization.
(3)Due to different process like annealing, precipitation, sintering, surface hardening, creep etc.
Atoms which is surrounding vacancy tend to be closed and compressed and thereby destroying the lattice plane.
(2)Interstitially: If one atom is extra within a crystal, then it is known as interstitially, thus it is just opposite to vacancy. This is due to Low atomic packing factor and Adding of an alloy.
(3)Frenkel defect: If an ion is displaced from a regular site to an interstitial site is called frenkel defect.This doe not affect the electrical neutrality.
(4)Schottky defect: In ionic crystal, point defects are more complex due to necessity to maintain electrical neutrality. Thus if an ionic crystal there is vacancy in a neighboring negative ion site, such a pair of vacant site is called schottky defect.
(2)Line defects: The most important two dimensio0nal or line defect is the dislocation. The two basic types of dislocation are:
(a)Edge dislocation: An edge dislocation is produced due to an extra row of atoms either above or below the slip plane. Before discussing the edge dislocation first we will discuss about the perfect crystal. The magnitude and direction of lattice distortion associated with a dislocation is expressed in terms of burger vector, denoted by a b.
For perfect crystal let a point A in the crystal drawn a line through a line through centre of 'n'atoms horizontally and N number of atoms vertically. In perfect crystal the ending point will exactly concide with the starting point 'A'.This kind of circuit is known as perfect burger circuit.
If the same procedure adopt for the burger circuit is imperfect crystal with edge dislocation. The end of circuit is not concide with the starting point 'A' instead of this end point of circuit B.
For the perfect crystal vertical planes are parallel to one another and to the side faces. If the one of these vertical planes will short within the crystal, a dislocation present in the perfect crystal, the atoms are in equilibrium position and all the bond length is equal. But in Imperfect crystal, just above the edge of the incomplete plane, the atoms are squeezed and are in state of compression. Now the bond lengths are smaller.
Now assume that shear stress is τ and burger vector of dislocation is b then force produced by dislocation is given by
F=τb
And this force is responsible for movement of dislocation in the crystal. the movement of dislocation is continue till the slip on crystal surface propagates one atomic distance.
If the extra half plane is above the slip plane it is known as positive dislocation and below the slip plane known as negative dislocation. The positive and negative dislocation is represented by symbol - and T.
(b)Screw dislocation: In screw dislocation the distortion of crystal structure follows a helical or screw path either right handed or left handed. In the screw dislocation upper front region of crystal is shifted one atomic distance to the right relative to the bottom portion.
Screw dislocation lies parallel to its burger vector but movement of slip plane will be perpendicular to burger's vector.
(3)SURFACE DEFECTS: It include grain boundaries ,tilt boundaries ,twin boundaries ,stacking faults ,ferromagnetic domain walls etc.Grain boundaries are those planer imperfect ions in polycrystalline material that separate crystal of different orientations. Tilt boundary is another surface imperfection and it may be regarded as an array of edge dislocations. Twist boundaries, the second class of low angle boundaries result from a set of screen dislocations.
A twin boundary separates two parts of crystal having the same orientations and they look like mirror image of each other.
(4)VOLUME DEFECTS: Volume defects such as cracks or stacking faults may arise where there is only small dissimilarity between the stacking sequences of closed-packed planes in F.C.C. and H.C.P. metals.