LATTICE DEFECTS

The term defect or imperfection is generally used to define any deviation from an orderly array of lattice points When the deviation from periodic arrangement of lattice is localized to the vicinity of only few atoms it is called point defect or imperfection If it extends through microscopic region of crystals it is called lattice imperfections lattice imperfections divided further into line defects and surface or plane defectsPOINT DEFECTS a vacancy or vacant lattice site exists when an atom is missing from normal lattice position in pure metals small number of vacancies are created by thermally excitation and these are stable at temperatures greater than absolute zero At equilibrium the fraction of lattices that are vacant at given temperature is given by Where n is no of vacant sites in N sites and Es is energy required to move atom from interior to surface Higher than equilibrium concentration of vacancies can also be produced by extensive plastic deformation or as result of bombardment with high energy nuclear particles When density of vacancies become extensively large it is possible for them to cluster together to form voids An atom that is trapped inside a crystal at a point intermediate between normal lattice position is called an interstitial atom Occurs in pure metals as a result of bombardment with high energy nucleus particles but does not occur frequently due to thermal damage The presence of impurity atom at lattice or interstitial position results in local disturbance of periodicity of latticeLINE DEFECTS DISLOCATION The dislocation is defect responsible for phenomena of slip by which most metals deform plastically Its a localized lattice disturbance separating slipped and unslipped region of a crystal All the atoms above area c have been displaced one atomic distance in slip direction while d has not been AB is boundary between slipped and unslipped regions As the dislocation occurs the slip moves in area over which it moves, dislocation can move easily on only a small load applied This helps explain why real crystal deform easily than would be expected for crystal with perfect lattice Slip has occurred in direction of slip vector abcd Line ad has not slipped yet Parts of crystal above slip plane have been displaced in direction of slip The amount of displacement is equal to burger vector B of the dislocation Burger vector always perpendicular to dislocation line The plane corresponds to a (100) plane is simple cubic lattice The atomic arrangement results in comprsssive stress above the planes and tensile stress below the plane A pure edge dislocation canglide or slip in direction perpendicular to its slip However it can move vertically by process known as climb if diffusion or vacancies of atoms can take place at an appreciable rate The second type of dislocation is screw The upper part ad has moved relative to lower part in direction of slip No slip has taken place to left of ad therefore ad Is dislocation line Dislocation line is parallel to burger vector Screw dislocation does not has a preferred slip plane whereas an edge dislocation has in screw dislocation moment by climb is not possibleDEFORMATION BY SLIP The usual method of plastic deformation in metals is by sliding of blocks over one another along definite crystallographic planes called slip planes Slip occurs when shear stress exceeds a critical value The atoms move an integral number of atomic distances along the slip plane and a step is produced in the polished surface When viewed from the polished surface from the above the step shows up as a line called slip line if the surface is repolished the step line will disappear Because of translational symmetry of crystal lattice the structure is perfectly restored after the slip has taken place provided its uniform Slip lines are due to changes in surface elevation and surface must be suitably prepared for the microscopic viewing The figure below shows slip lines in copper Slip occurs most readily in certain specified crystallographic planes Slip plane is the plane of the greatest atomic density and the slip direction is closest packed direction within slip plane Since plane of the greatest atomic density is most widely spaced plane the resistance to plane is generally less to these planes than for any other set of planes

The slip plane together with slip direction establishes the slip system

In hexagonal closed packed metals, the only plane with high atomic density is basal plane. The axes are the closed packed directions

for zinc, cobalt , magnesium and cadmium slip occurs In (0001) plane and the direction

The hcp structure possess three slip systems

The limited number of slip system is the reason for extreme orientation dependence and low ductility of hcp crystals

In the fcc the {111} octahedral planes and the (110) directions are closed packed systems there are eight {111} planes in the fcc unit cell

However the planes at the opposite face of octahedron are parallel to each other so that there are only four sets of octahedral planes each {111} plane contains three (110) direction, therefore fcc has 12 possible slip systems

The bcc structure is not a closed pack structure like hcp or fcc, there is no plane of predominant density Slip lines in bcc model have a wavy appearance , this is e to the fact that slip occurs in several planes {110}, {112} , {113} but always in closed packed (111) direction Dislocation can easily move from one plane to other by cross sip, giving rise to irregular wavy slip bands Aluminum deforms on the {110} plane at elevated temperature, while in magnesium the {1011} pyramidal plane plays an important role, however the slip direction always remain same