Material Defects

7/29/2019 Material Defects

1/15

Engineering Materials & Design

7/29/2019 Material Defects

2/15

2nd Semester

Imperfections in Materials

Phase Diagrams

Metals, Polymers, Ceramics, Semiconductors

Mechanical, Thermal & Electrical Properties

Strengthening Mechanisms

7/29/2019 Material Defects

3/15

Material Defects

7/29/2019 Material Defects

4/15

Lecture 7

Point Defects

Dislocations

Solid Solutions

7/29/2019 Material Defects

5/15

Imperfections in Solids

There is no such a thing as a perfect crystal!

In general, a defectsimply refers to a disruption in the crystalline order of an

otherwise periodic material.

Properties of a material are significantly affected by defects.Defect does not necessarily imply a bad thing.

Every material has vacancies and impurities, to some extent.

Types of defects:

1.Point defectsVacancy atoms

Interstitial atoms

Substitutional atoms

2. Line defectsDislocations

3. Surface/Area defects

7/29/2019 Material Defects

6/15

Point Defectsvacancy= a lattice site that is missing an atom

All crystals contain some vacancies.

self-interstitial= an atom from the crystal that crowds its

way into an otherwise empty void between atoms

Self-interstitials are far less common than vacancies because of

the relatively large energy required to squeeze an atom into the

small voids between existing sites.

impurity= addition of an atom of a different species

substitution impurity or interstitial impurity

alloys = other types of atoms are deliberately added to

give the material certain properties

7/29/2019 Material Defects

7/15

Example

(b) Pure material

(c) Interstitial alloy

(a) & (d) Substitutional alloys

7/29/2019 Material Defects

8/15

Line Defects

DISLOCATIONS

Dislocations result from solidification of metal, mechanical or thermal processing, etc.

It is very difficult to prepare a dislocation-free crystal!!!

2 Types of dislocations:

Edge dislocations

Screw dislocations

Dislocations make metals weaker

BUT also allow metals to be deformed.

Edge dislocation is an extra half-plane of atoms inserted in a crystal.

That extra half plane causes misalignment of other atomic planes.

Dislocations can move through the material!

7/29/2019 Material Defects

9/15

Example

Introduction to dislocations

7/29/2019 Material Defects

10/15

DiffusionDiffusion is the mass transport through atomic motion at high temperatures.

Self-diffusion: In an elemental solid, atoms also migrate.

A

B

C

D

7/29/2019 Material Defects

11/15

Inter - DiffusionInterdiffusion occurs spontaneously!

Thermal energy supplied (heat) causes atoms to vibrate.

In solids when vibrations are sufficient, bonds rupture and the atom is free to diffuse.

Rate of diffusion depends on: concentration & temperature !

7/29/2019 Material Defects

12/15

DiffusionThe presence of vacancies allows substitutional diffusion = atoms have empty space to

move to.

In interstitial diffusion atoms need energy to squeeze past fixed atoms.

Much faster: more empty sites, no vacancies needed

7/29/2019 Material Defects

13/15

Solutions

DEPENDS ON CONCENTRATION

Liquid (Standard) Solution Solid Solution

Type Liquid + Solid Liquid + Liquid Solid + Solid

Example

Limitations

Separation?

Liquid (Standard) Solution Solid Solution

Type Liquid + Solid Liquid + Liquid Solid + Solid

Example Water + sugar/salt Alloys

Limitations

Separation?

Liquid (Standard) Solution Solid Solution

Type Liquid + Solid Liquid + Liquid Solid + Solid

Example Water + sugar/salt Alloys

Limitations Yes No YES!

Separation?

ALLOY = a solid solution of one or more elements within a metallic element

Solute + Solvent = Solution

Solute the substance that dissolves to form a solution

Solvent the substance in which a solute dissolves

Solution a mixture of one or more solutes dissolved in a solvent

7/29/2019 Material Defects

14/15

Solid Solution

Solid Solution = a homogeneous distribution of two or more elements.

Solventis the host material (compound present in the greatest amount)

Solute is the minor phase, added to the solvent

Phase is a region of uniform composition or crystal structure

solute atoms dont alter the crystal structure of the solvent = same phase! = solid solubility!

Solid solution is a particular type of alloy : substitutional and interstitial

IN THE

SOLID

STATE!

7/29/2019 Material Defects

15/15

Diffusion in Solid Solutions

Properties of diffusing species (solute) and host material (solvent) are important.

smaller atoms can squeeze in between host atoms more easily (interstitial)

in lower packing density host material (less neighbouring atoms) it is easier foratoms to migrate because there are fewer bonds to be ruptured

solids with lower melting points normally contain weaker bonds (easier to push

apart)

Temperature!