DIFFUSION IN SOLIDS DIFFUSION IN SOLIDS FICK’S LAWS KIRKENDALL EFFECT ATOMIC MECHANISMS Diffusion in Solids P.G. Shewmon McGraw-Hill, New York (1963) MATERIALS SCIENCE MATERIALS SCIENCE & & ENGINEERING ENGINEERING Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email: [email protected], URL: home.iitk.ac.in/~anandh AN INTRODUCTORY E-BOOK AN INTRODUCTORY E-BOOK Part of http://home.iitk.ac.in/~anandh/E-book.htm A Learner’s Guide A Learner’s Guide
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DIFFUSION IN SOLIDS FICK’S LAWS KIRKENDALL EFFECT ATOMIC MECHANISMS Diffusion in Solids P.G. Shewmon McGraw-Hill, New York (1963) MATERIALS SCIENCE.
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DIFFUSION IN SOLIDSDIFFUSION IN SOLIDS FICK’S LAWS
A & B welded together and heated to high temperature (kept constant → T0)
Flux
f(x)|t
f(t)|x
Non-steadystate
If D = f(c) c(+x,t) c(x,t)
i.e. asymmetry about y-axis
C(+x, 0) = C1
C(x, 0) = C2
C1
C2
A = (C1 + C2)/2
B = (C2 – C1)/2
Dt
xerfBAtxc
2 ),(
AB = C1
A+B = C2
1 2 2 1( , ) 2 2 2
C C C C xc x t erf
Dt
kT
Q
eDD 0
Temperature dependence of diffusivity
Arrhenius type
Diffusivity depends exponentially on temperature.
This dependence has important consequences with regard to material behaviour at elevated temperatures. Processes like precipitate coarsening, oxidation, creep etc. occur at very high rates at elevated temperatures.
ATOMIC MODELS OF DIFFUSION
1. Interstitial Mechanism
Usually the solubility of interstitial atoms (e.g. carbon in steel) is small. This implies that most of the interstitial sites are vacant. Hence, if an interstitial species wants to jump, ‘most likely’ the neighbouring site will be vacant and jump of the atomic species can take place.
Light interstitial atoms like hydrogen can diffuse very fast. For a correct description of diffusion of hydrogen anharmonic and quantum (under barrier) effects may be very important (especially at low temperatures).
The diffusion of two important types of species needs to be distinguished: (i) species ‘sitting’ in a lattice site(ii) species in a interstitial void
2. Vacancy Mechanism
For an atom in a lattice site (and often we are interested in substitutional atoms) jump to a neighbouring lattice site can take place if it is vacant. Hence, vacancy concentration plays an important role in the diffusion of species at lattice sites via the vacancy mechanism.
Vacancy clusters and defect complexes can alter this simple picture of diffusion involving vacancies
Interstitial Diffusion
1 2
1 2
Hm
At T > 0 K vibration of the atoms provides the energy to overcome the energybarrier Hm (enthalpy of motion)
→ frequency of vibrations, ’ → number of successful jumps / time
kT
Hm
e '
Substitutional Diffusion
Probability for a jump (probability that the site is vacant).(probability that the atom has sufficient energy)
Hm → enthalpy of motion of atom
’ → frequency of successful jumps
kT
HkT
Hmf
ee '
kT
HH mf
e '
kT
HH mf
eD 2Where, is the jump distance
Interstitial Diffusion
kT
Hm
eD 2
Substitutional Diffusion
kT
HH mf
eD 2
D (C in FCC Fe at 1000ºC) = 3 1011 m2/s
D (Ni in FCC Fe at 1000ºC) = 2 1016 m2/s
0 f mH H
kTD D e
of the form
0 mH
kTD D e
of the form
Diffusion Paths with Lesser Resistance
Qsurface < Qgrain boundary < Qpipe < Qlattice
Experimentally determined activation energies for diffusion
Core of dislocation lines offer paths of lower resistance → PIPE DIFFUSION
Lower activation energy automatically implies higher diffusivity
Diffusivity for a given path along with the available cross-section forthe path will determine the diffusion rate for that path
Comparison of Diffusivity for self-diffusion of Ag → single crystal vs polycrystal
1/T →
Log
(D
) →
Schematic
Polycrystal
Singlecrystal
← Increasing Temperature
Qgrain boundary = 110 kJ /mole
QLattice = 192 kJ /mole
Applications based on Fick’s II law Carburization of steel
Surface is often the most important part of the component, which is prone to degradation.
Surface hardening of steel components like gears is done by carburizing or nitriding.
Pack carburizing → solid carbon powder used as C source. Gas carburizing → Methane gas CH4 (g) → 2H2 (g) + C (diffuses into steel).
C(+x, 0) = C1
C(0, t) = CS
A = CS
B = CS – C1
SolvedExample
A 0.2% carbon steel needs to be surface carburized such that the concentration of carbon at 0.2mm depth is 1%. The carburizing medium imposes a surface concentration of carbon of 1.4% and the process is carried out at 900C (where, Fe is in FCC form).
Data: 4 20D (C in -Fe) 0.7 10 m / s 157 /Q kJ mole
The solution to the Fick’ second law: ( , ) 2
xC x t A B erf
Dt
The constants A & B are determined from boundary and initial conditions:
(0, ) 0.014SC t A C , 0( , ) 0.002C t A B C or 0( ,0) 0.002C x A B C
S 0B C C 0.012 , ( , ) 0.014 0.012 2
xC x t erf
Dt
-44
1
1
2 10(2 10 , ) 0.01 0.014 0.012
2C m t erf
Dt
S S 0( , ) C (C -C ) 2
xC x t erf
Dt
0
( , )=
2S
S
C x t C xerf
C C Dt
(2)
(1)
-4
1
1 2 10
3 2erf
Dt
x (in mm from surface)
% C
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.2
t =0
t = t1 = 14580s
t = 1000st = 7000s
t
0.4 0.6 0.8 1.0 1.2 1.4
x (in mm from surface)
% C
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.2
t =0
t = t1 = 14580s
t = 1000st = 7000s
t
0.4 0.6 0.8 1.0 1.2 1.4
-4
1
121
2 10(0.3333) 0.309
2 7.14 10erf
t
24
1 12
1 1014580
0.337.14 10t s
From equation (2)
-4
1
1 2 10
3 2erf
Dt
Approximate formula for depth of penetrationDtx
0
0
( , )1
2S
c x t C xerf
C C Dt
Let the distance at which [(C(x,t)C0)/(CSC0)] = ½ be called x1/2 (which is an ‘effective penetration depth’)
1 211
2 2
xerf
Dt
1 2 1
22
xerf
Dt
1 1
2 2erf
1 2 1
22
x
Dt
penetrationx Dt
The depth at which C(x) is nearly C0 is (i.e. the distance beyond which is ‘un’-penetrated):
0 12
xerf
Dt
Erf(u) ~ 1 when u ~ 2
22
x
Dt
4x Dt
0
( , )=
2S
S
C x t C xerf
C C Dt
End
Another solution to the Fick’s II law
A thin film of material (fixed quantity of mass M) is deposited on the surface of another material (e.g. dopant on the surface of a semi-conductor). The system is heated to allow diffusion of the film material into the substrate.
For these boundary conditions we get a exponential solution.
2
( , ) exp4
M xc x t
DtDt
Boundary and Initial conditions
C(+x, 0) = 0
0cdx M
Ionic materials are not close packed Ionic crystals may contain connected void pathways for rapid diffusion These pathways could include ions in a sublattice (which could get disordered)
and hence the transport is very selective alumina compounds show cationic conduction Fluorite like oxides are anionic conductors
Due to high diffusivity of ions in these materials they are called superionic conductors. They are characterized by: High value of D along with small temperature dependence of D Small values of D0
Order disorder transition in conducting sublattice has been cited as one of the mechanisms for this behaviour
Diffusion in ionic materials
Element Hf Hm Hf + Hm Q
Au 97 80 177 174
Ag 95 79 174 184
Calculated and experimental activation energies for vacancy Diffusion
1 2
Vacant site
c = atoms / volume c = 1 / 3
concentration gradient dc/dx = (1 / 3)/ = 1 / 4 Flux = No of atoms / area / time = ’ / area = ’ / 2
242
''
)/(
dxdc
JD
kT
Hm
eD 2
20 D
kT
Q
eDD 0
On comparisonwith
3. Interstitialcy Mechanism
Exchange of interstitial atom with a regular host atom (ejected from its regular site and occupies an interstitial site)
Requires comparatively low activation energies and can provide a pathway for fast diffusion
Interstitial halogen centres in alkali halides and silver interstitials in silver halides