Imperfections

IMPERFECTIONS IN CRYSTALS • Crystal- solid composed of atoms arranged , similar in three directions • IDEAL CRYSTAL- PERFECT • REAL CRYSTALS -NOT PERFECT • Lattice distortion, imperfections, irregularities, defects present • Computed yield stress and real yield stress different • Many physical and mechanical properties affected by imperfections

CLASSIFICATION OF IMPERFECTIONS/ DEFECTS FOUR MAIN DIVISIONS POINT LINE SURFACE DEFECTS DEFECTS (PLANAR,

INTERFACIAL, GRAIN

VOLUME DEFECTS

BOUNDARIES)

DEFECTS •Vacancies •Interstitials •Impurities •Electronic

•Edge •Screw

•Grain Boundaries •Tilt Boundaries •Twin Boundaries

•Cracks •Stacking faults

POINT DEFECTS • • •

ONE WHICH IS COMPLETELY LOCAL IN ITS EFFECTS eg: VACANT LATTICE SITE POINT DEFECT IF INTRODUCED- INCREASES INTERNAL ENERGY No. of defects (nd ) at equilibrium at a certain temperature

nd = N (e –Ed/kT) • • • •

N = total no. of atomic sites/cubic metre or per mole E d = Energy of activation necessary to form the defect k = Boltzmann’s constant = 1.38 X 10-23 J/atom-K (8.62 X 10-5eV/atom-K) T = absolute temperature

•

Possible point defects are: » Vacancies » Interstitials » Impurities » Electronic defects

VACANCIES An unoccupied atom position within a crystal lattice - empty atom sites Lattice vacancies are a stable feature of metals at all temperatures above absolute zero Heat treatment processes involve transport of atoms through lattice with the help of vacancies Vacancies- lead to increase in randomness of structure Vacancies- due to improper packing /thermal vibrations at high temps.

Self-interstitial Frankel defect

Vacancy Schottky defect

Interstitial impurity atom

Substitutional impurity atom

IMPURITY POINT DEFECTS Two types:

Interstitial eg: C-Fe Fills the voids/interstices •High APF, positions small •Hence, small diameter impurity than host atoms

Substitutional eg: Cu-Ni Solute atoms replace/ substitute host atoms Depends on: Atomic Size factor (± 15%) Crystal Structure (same) Electronegativity (extremes- intermetallic compound; otherwise, Sub.Solid Solns)

Valances (high-low: tendency to dissolve)

ELECTRONIC DEFECTS • Result of errors in charge distribution in solids • These defects free to move in crystal under the influence of electric field • Vacancy or interstitial impurity may produce excess/deficit of ve /_ve charges • Eg: ZnO- excess of interstitial zinc ions

LINE DEFECTS 2D defects Dislocation: A disturbed region between two substantially perfect parts of a crystal

Two types: Edge Screw

LINE DEFECTS 2D defects Dislocation: A disturbed region between two substantially perfect parts of a crystal

Two types: Edge Screw

Edge dislocation

Screw dislocation

MIXED

BURGER’S VECTOR

Angle of misalignment

High angle grain boundary Small angle grain boundary

Angle of misalignment

Small & High angle grain Boundaries and adjacent atom positions

Tilt boundary having an angle of mis-orientation Θ results from an alignment of edge dislocations

• In addition to slip, plastic deformation in metallic materials occur by the formation of mechanical twins, or twinning.

• The concept of a twin A shear force can produce atomic displacements such that on one side of a plane (the twin boundary), atoms are located in mirror-image positions of atoms on the other side.

Open circles represent atoms that did not move, and dashed and solid circles represent original and final positions, respectively The displacement magnitude within the twin region (indicated by arrows) is proportional to the distance from the twin plane

Demonstration in Figures (a) &(b)

Slip and twinning deformations as compared for a single crystal subjected to a shear stress .

• eg: for BCC metals, the twin plane and direction are (112) and [111], respectively. • Twinning occurs on a definite crystallographic plane and in a specific direction that depend on crystal structure • For twinning, the shear deformation is homogeneous • For slip, the crystallographic orientation above and below the slip plane is the same both before and after the deformation; • for twinning, there will be a reorientation across the twin plane. • In addition, slip occurs in distinct atomic spacing multiples, whereas the atomic displacement for twinning is less than the interatomic separation.

• Mechanical twinning occurs in metals that have BCC and HCP crystal structures, at low temperatures, and at high rates of loading (shock loading) • There are few operable slip systems. • The amount of bulk plastic deformation from twinning is normally small relative to that resulting from slip. • Annealing twins are found in FCC struture metals • Mechanical twins in BCC & HCP metals