Structural Imperfections - 2013

7/28/2019 Structural Imperfections - 2013

1/23

STRUCTURALIMPERFECTIONS

(DEFECTS)IN CRYSTALLINE SOLIDS


2/23

Real Crystalline solids are almost neverperfect. These imperfections can be classified

according to their dimensionality:

1. Point defects (0-Dimension)

2. Line defects (1-D)

3. Interfacial defects (2-D)

4. Bulk defects (3-D)


3/23

Relative Size Ranges of Defects

10-610-8 10210110010-210-410-12

Electronicpointdefect

Atomicpointdefect

Line defect Bulk defect

Interfacial defect

cm


4/23

Solidification- result of casting of molten material 2 steps

Nuclei form

Nuclei grow to form crystalsgrain structure

Start with a molten material

all liquid

Imperfections in Solids

Adapted from Fig.4.14 (b), Callister 7e.

Crystals grow until they meet each other

nuclei crystals growing grain structureliquid


5/23

Imperfections in Solids

There is no such thing as a perfect crystal.

What are these imperfections?

Why are they important?

Many of the important properties of materials

are due to the presence of imperfections.


6/23

1. POINT DEFECTS

These are defects of atomic dimensions thatusually result from:

1. The presence of an impurity atom Substitutional larger atoms

Interstitial smaller atoms

2. The absence of a matrix atom (vacancy)3. The presence of a matrix atom in a wrong

place (self-interstitial)


7/23

Vacancies:-vacant atomic sites in a structure.

Self-Interstitials:-"extra" atoms positioned between atomic sites.

Point Defects

Vacancy

distortion

of planes

self-interstitial

distortionof planes


8/23

Presence of an impurity atom:-"extra" atoms positioned between atomic sites.

Substitutional

Interstitial


9/23

The point defects discussed so far occur in metallicstructures. Those in ionic structures differ because ofthe charge neutrally requirement.

An anion

and a cation

is missing

An anion or acation is at an

insterstital site


10/23

are line defects, slip between crystal planes result when dislocations move,

produce permanent (plastic) deformation.

Dislocations:

Schematic of Zinc:

before deformation after tensile elongation

slip steps

2. Line Defects (Dislocations)


11/23

Linear Defects (Dislocations)

Are one-dimensional defects around which atoms

are misaligned Edge dislocation:

extra half-plane of atoms inserted in a crystalstructure

Screw dislocation:

spiral planar ramp resulting from shear deformation


12/23

Edge Dislocations


13/23

Screw Dislocations


14/23


15/23

Dislocations are simply slide or slip of oneportion of crystal system over another.

When dislocations pass through the wholesystem, the system permanently deforms.

Dislocations are on boundary between theregions where slip has occurred and where ithas not.

On either side of the dislocation crystallinesystem is essentially perfect.


16/23

3.INTERFACIAL DEFECTS

(BOUNDARIES)

1. Free surfaces: Interfaces between liquids andgases.

2. Grain boundaries: Interfaces between crystalsystems having different orientation.

In each crystal system the atoms arearranged orderly. However, at the boundarythere is a transition zone which is not alingedwith either of the crystal systems.


17/23


18/23

Grain Boundaries

Tilt boundary: Result of a

set of edge dislocations.


19/23


20/23


21/23

O TA C O CT O S


22/23

IMPORTANCE OF IMPERFECTIONS

Most of the properties of materials are affected

by imperfections: Small amount ofimpurity atoms may increase the electrical

conductivity of semi-conductors.

Dislocations are responsible for ductility. Strength of

materials can be increased to a large extent by themechanism strain-hardening which produces line defectsthat act as a barrier to control the growth of otherimperfections.

Presence ofbulk defects such as cracks, notches, holescauses brittle materials, which break at very low stresseswithout showing large deformations.


23/23

Point, Line, and Area defects exist in solids.

The number and type of defects can be varied

and controlled (e.g.,T

controls vacancy concentration)

Defects affect material properties (e.g., grain

boundaries control crystal slip).

Defects may be desirable or undesirable(e.g., dislocations may be good or bad, depending

on whether plastic deformation is desirable or not.)

Summary

Structural Imperfections - 2013

Documents

Transcript of Structural Imperfections - 2013