Diffusion in Solids-Lecture-1.ppt

to enhance the knowledge about phase transformation, and clear the basics of diffusion.

1. Course MM-501:
Phase Transformations in Solids

2. MM 501Ashraf Ali NED University
Diffusion in Solids
ISSUES TO ADDRESS...
• How does diffusion occur?
• Why is it an important part of processing?
• How can the rate of diffusion be predicted for
some simple cases?
• How does diffusion depend on structure
and temperature?
• What is Diffusion?

3. MM 501Ashraf Ali NED University
•Often think of diffusion in a medium where atoms are relatively
free to move around such as liquids and gases…
e.g. making chocolate milk. Really need to stir the milk to
get that viscous chocolate syrup evenly mixed!
•Even in solids, atom is not fixed at its position but constantly
moves around. The degree of movement depends on its
temperature (extrinsically) and structure (intrinsically).
Mechanisms
•Gases & Liquids –random (Brownian) motion
•Solids –vacancy diffusion or interstitial diffusion
•But all movements are random motion in nature!
What is diffusion?
•Phenomenon of material transport by atomic motion.

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Diffusion in Solids
Diffusion plays an important role in…
• Alloying
• Strengthening and Heat Treatment Processes
• High Temperature Mechanical Behavior
• Phase Transformations
• Environmental Degradation

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Why do we care about diffusion in solids?
e.g.,Materials processing
1.Chemical reactions/treatments require mass transfer (i.e.
diffusion).
e.g. alloying, strengthening of materials via case
hardening etc…
2.Many materials are heat treated to achieve necessary
properties: prefer to develop methods that will allow relatively
high diffusion rates to achieve an efficient/cost-effectively
process,Homogenisationfor example.

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1.Hard Facing(Carburizing of Steels)
Tough Tools and Parts.
Wear Facing of Gears, Wheels and Rails
2.Chemical Tempering of Glass and Ceramics
Toughened Ceramics (Corel Ware)
Shard resistant safety glass
3.Thin Film Electronics(CMOS and Bipolar Transistors)
Doping of Semiconductors
Applications of Diffusion in Solids
(besides nucleation and growth)

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4.Diffusion Bonding--(Adhesives and Cements -
ceramic, metallic and polymer)
a. Portland Cement as Bonding for Construction
b. Solvent Cements for PVC Polymeric Piping
c. Thermocouple Junctions
5.Corrosion Protection
Galvanizing, Electroplating, Anodizing, Inhibiting
6.Gas (Chemical) Separation Processes-
Diffusion membranes
Applications of Diffusion in Solids….

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PROCESSING USING
DIFFUSION(1)
• Case Hardening:
--Diffuse carbon atoms
into the host iron atoms
at the surface.
--Example of interstitial
diffusion is a case
hardened gear.
• Result: The "Case" is
--hard to deform: C atoms
"lock" planes from shearing.
--hard to crack: C atoms put
the surface in compression.
Fig. 5.0,
Callister 6e.
(Fig. 5.0 is
courtesy of
Surface
Division,
Midland-
Ross.)
From Callister 6e resource CD.

9. MM 501Ashraf Ali NED University
Diffusion Bonding
Diffusion bonding is a method of creating a joint between similar or
dissimilar metals, alloys, and nonmetals.
Two materials are pressed together (typically in a vacuum) at a specific
bonding pressure with a bonding temperature for a specific holding time.
PROCESSING USING DIFFUSION (2)

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• “Solid state” joining process
• Apply heat & pressure across
interface to be joined
• Local plastic deformation +
diffusion = joint!
• Useful for joining difficult to
weld metals, dissimilar materials,
metals and ceramics
B-18
Diffusion Bonding…

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• Dopingof Silicon with Pfor n-typesemiconductors:
• Process:
1. Deposit Prich
layers on surface.
2. Heat it.
3. Result: Doped
semiconductor
regions.silicon silicon
Fig. 18.0,
Callister 6e.
From Callister 6e resource CD.
PROCESSING USING DIFFUSION (3)
Thin Film Electronics(CMOS and Bipolar Transistors)

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DIFFUSION: THE
PHENOMENON
• Interdiffusion: Atoms of one material diffusing into another and vice versa.
e.g. In an alloy, atoms tend to migrate from regions of large concentration.
Initially
After some time 100%
Concentration Profiles
0
Adapted from
Figs. 5.1 and
5.2, Callister 6e.
From Callister 6e resource CD.
Consider a concentration gradientthen atoms move from high conc. to low conc..

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DIFFUSION: THE
PHENOMENON• Self-diffusion:Atoms within one material exchanging
positions. (i.e. In an elemental solid, atoms also migrate).
Label some atoms After some timeA
B
C
D
From Callister 6e resource CD.

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Diffusion mechanisms
How do atoms move in a crystalline solid?
For diffusion to occur:
1.Adjacent site needs to be empty (vacancy or
interstitial).
2.Sufficient energy must be available to break
bonds and overcome lattice distortion.
There are many possible mechanisms but let’s consider
the simple cases:
1.Vacancy diffusion.
2.Interstitial diffusion.

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Vacancy diffusion
-An atom adjacent to a vacant lattice site moves into it.
Essentially looks like
an interstitial atom:
lattice distortion
First, bonds with the neighboring
atoms need to be broken
From Callister 6e resource CD.

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Interstitial Diffusion
-Migration from one interstitial site to another (mostly for small atoms
that can be interstitial impurities: e.g. H, C, N, and O).
Typically more rapid than vacancy diffusion.
Callister fig. 5.3
Carbon atom in Austenite

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Interstitial Diffusion-
Animation

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MODELING DIFFUSION: FLUX
Consider atoms, M, going through a plane of area, A, in time, t:
t = 0
t = t’
2 atoms passed from left to right (+ direction)
1 atom passed from right to left (-direction)
Net result:'
1
tarea
atom

or'At
M

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MODELING DIFFUSION: FLUX
• Flux (J):
• Directional Quantity
• Flux can be measured for:
--vacancies
--host (A) atoms
--impurity (B) atoms
In general: diffusion flux may or may not be the same over time

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MODELING DIFFUSION
What causes net flow of atoms?
• Concentration Profile, C(x): [kg/m
3
]
• Fick's First Law:Concentration
of Cu [kg/m
3
]
Concentration
of Ni [kg/m
3
]
Position, x
Cu fluxNi flux
• The steeper the concentration profile,
the greater the flux!
Adapted from
Fig. 5.2(c),
Callister 6e.

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• Steady State:the concentration profile doesn't
change with time.
Steady-state diffusion
• Apply Fick's First Law:
• Result: the slope, dC/dx, must be constant
(i.e., slope doesn't vary with position)!
J
x
D
dC
dx
dC
dx






left

dC
dx






right
• If Jx)left= Jx)right, then
From Callister 6e resource CD.

22. MM 501Ashraf Ali NED University

23. MM 501Ashraf Ali NED University
Thanks!
Any questions?
You can find me at
●@hamzaahmed
●[email protected]