Lever Rule - feaweb.aub.edu.lb
Transcript of Lever Rule - feaweb.aub.edu.lb
Solute RedistributionTopic 6
M.S DarwishMECH 636: Solidification Modelling
dCL
d!s
kCo
Cs=kCo
CL
Introduction• Local Equilibrium
– Solidus and liquidus curves
– Equilibrium distribution coefficient, k0
– Tie–line “jump”
• Macrosegregation at Planar Interfaces
– Gulliver–Scheil equations
» Assumptions
» Mass balance
» Limitations
– Transport–limited segregation
» Initial Transient
» Steady–state solute distribution
» Terminal Transient
• Effects of Stirring & Convection
– Burton–Prim–Slichter Theory, kBPS
Lever Rule
Introduction
CS
CL
Distance
Composition
solid liquid
kCo Co Co/k
T
Cmax Ceut
TLT
TS
CSCL k=CS/CL
C
A B
To
Composition
Temperature
?
In practice equilibrium is maintained at the solid liquid interface where composition agrees with the phase diagram
? �
k = Cs*
Cl*
Equilibrium at S/L
C
liquidus
solidus
T
Tie line “jump”
CBS L
Molecular scale
Continuum scale
Com
posi
tion
Distance0
CS = Co
End of Solidification
*
Com
posi
tion
Co
kCo
Distance0 L
CL = CL! Co
Start of Solidification
*
Equilibrium Solidification
Com
posi
tion
Co
CL
CSkCo
Distance0 L
At Temperature T
�
χ sCs + χ lCl = Co
Complete Diffusion in Solid and Liquid Respectively
kCo Co Co/k
T
Csm Ceut
TLT
TS
CSCL k=CS/CL
C
A B
To
Composition
Temperature
Lever Rule
Schiel Rule
Gulliver–Scheil Differential Mass Balance
Although local interfacial equilibrium generally holds during solidification, the system cannot remain in “global” equilibrium because diffusion rates in the solid are too slow.
G. Gulliver (1921) and E. Scheil (1942) both assumed:
Perfect mixing in liquid
No diffusion in solid
Local equilibrium at S/L interface
�
Cs* −Cl
*( )d x = L − x( )dCl
�
Cl* Cs
*
Cl* −1
⎛
⎝ ⎜
⎞
⎠ ⎟ d x = L − x( )dCl
�
Cl = Cl*( )
�
Cl* k −1( )dx = L − x( )dCl
�
k −1( ) d ′ x L − ′ x ( )0
x
∫ = d ′ C l′ C lC0
Cl
∫
�
χ s = xLnote that
solid liquid
dxL0
x
Gulliver–Scheil Segregation Equation
• The Gulliver–Scheil mass balanceeventually leads to non–physicalpredictions as χs→1.
• The phase diagram provides limits to how high the concentrations, Cl or Cs can rise due to segregation. Secondphase formation (generally from eutecticreaction) then occurs.
Gulliver–Scheil is independent of the shape of the S/L interface!
Where
�
χ s = xL�
1− χ s( ) k0−1( ) = Cl
C0
No Solid Diffusion(Non-Equilibrium Lever Rule)
Com
posi
tion
Co
CL
CSkCo
Distance0 L
At Temperature T
*
Com
posi
tion
Co
kCo
Distance0 L
CL=CL! Co
Start of Solidification
*
kCo
Co
Csm
CE
Com
posi
tion
Distance0 L
End of Solidification
kCo Co Co/k
T
Csm Ceut
TLT
TS
CSCL k=CS/CL
C
A B
To
Composition
Temperature
�
Cl −Cs*( )dχ s = 1− χ s( )dCl
solidification of
�
dχ s
�
Cs* = kCo 1− χ s( )k−1
�
Cl = Co 1− χ s( )k−1�
dχ1− χ0
χ s
∫ = dCl
Cl 1− k( )Co
Cl
∫
Scheil relation
dCL
d!s
kCo
Cs=kCo
CLrejected solute from
solidsolute increase in
liquid
0
1
2
3
4
5
0 0.2 0.4 0.6 0.8 1
C l/Co
Fraction Solid, fs
0.1
k0=0.01
0.20.30.40.50.60.7 0.8
k0<1
Gulliver–Scheil Segregation Equation
�
1− χ s( ) k0−1( ) = Cl
C0
0
0.5
1
1.5
2
0 0.2 0.4 0.6 0.8 1
Cs/C
0
Fraction Solid, fs
k0=0.03
0.10.20.30.40.50.60.7
0.8
k0<1
Gulliver–Scheil Segregation Equation
�
k0 1− χ s( ) k0−1( ) = Cs
C0
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
C l/C0
Fraction Solid, fs
k0=10
5
3
2
1.2
1.5
1.05
k0>0
Gulliver–Scheil Segregation Equation
1�
1− χ s( ) k0−1( ) = Cl
C0
0
1
2
3
4
5
0 0.2 0.4 0.6 0.8 1
Cs/C
0
Fraction Solid, fs
k0=10
5
3
2
1.5
1.21.05
k0>1
Gulliver–Scheil Segregation Equation
�
k0 1− χ s( ) k0−1( ) = Cs
C0
Maximum solubilityof Cu in Al is 2.5 at.% or 4.5 wt%.
Al-Cu Phase Diagram
Application of Gulliver–Scheil Segregation
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10
Dendritic Microsegregation(Al-4.5 wt% Cu)
Conc
.ent
ratio
n, C
u (a
t.%)
Distance, X (microns)
Segregation limit set by solubility limit.
Al–2.5 at% Cu
Plane Front Solidification
Moving S/L
liquidus
solidus
T
C
xsolid liquid
CB
vC∞
Cs(x)
Cl(x)
Solute boundary layerdevelops ahead of the advancing S/L interface.
�
Cs* T( )
�
Cl* T( )
�
Cl* T( )
�
Cl* T( )
Steady State Solute Boundary Layer
!
Cl
*
!
Cs
*
!
Csx( )
!
Clx( )
!
C0
Solid Liquid
Solid LiquidJB
v
Computing the Steady-State Solute Profile
• Consider Geometry
• Apply Fick’s second law
• Find the general SS solution
• Apply boundary conditions
• Find solute gradient at the interface
v
v
Apply Fick’s Law and SS Condition
In 1-D
�
dCdt
= ∇ ⋅D∇C ≈ D∇2C
�
dCdt
= D ∂2C∂z
�
dCdt
= D ∂2C∂z
= 0
Apply a Flux balance at Interface
dz
v
Cs
Cl
dtAJ =1
J1 J2
02
=
−=zdz
dcDJJ1=J2
dCdz
�
= Cl −Cs*( ) dzdt
�
= v Cl −Cs*( )
�
v = − DCl −Cs
*( )dCdz z= 0
Example 1
Cα
Cγ
Z
z
Z
vtZz −=
vdtdz −=
dttcdZ
zcdc
∂∂+
∂∂=
tc
dtdZ
zc
dtdc
∂∂+
∂∂=
tcv
zc
dzcD
∂∂+
∂∂−=∂
2
2
zc
Dv
dzc
tc
D ∂∂+∂=
∂∂
2
21
02
2
=∂∂+∂zc
Dv
dzc
( ) ⎟⎠⎞⎜
⎝⎛−+= z
Dvaazc exp10
( ) ( )( )tzctZcc == ,
Cα
Cγ
Z
z
Z
vtZz −=
vdtdz −=
( ) ⎟⎠⎞⎜
⎝⎛−+= z
Dvaazc exp10
Apply Boundary conditions to evaluate the constants:
( ) 0Czc =∞= ( ) γCzc == 0
00 Ca = 01 CCa −= γ
( ) ( ) ⎟⎠⎞⎜
⎝⎛−−+= z
DvCCCzc exp00 γ
( )00
CCDv
zc
z−−=
∂∂
=γ
( )0=
⎟⎠⎞⎜
⎝⎛∂∂−=−
zzcDCCv αγ
( ) ( )⎥⎦⎤
⎢⎣⎡ −−−=− 0CC
DvDCCv γαγ
( )( ) 10 =
−−
αγ
γ
CCCC
Ω=
Flux Balance at SS:
Evaluate dc/dz:
(Supersaturation)
Must equal 1 for SS!
Cα
Cγ
Z
C0
Destabilization requires a certain growth distance.
Solute buildup increases with time(or growth distance).
Sequential Crystallization Concepts
• First step of crystallization:
• r th step of crystallization:
Where
At the interface
Limited Liquid DiffusionC
om
posi
tion
Co
Co/k
Distance0 L
Start of Solidification
kCo Co Co/k
T
Csm Ceut
TLT
TS
CSCL k=CS/CL
C
A B
To
Composition
Temperature
�
dCdt
= D d2Cdx 2
�
dCdx
dxdt
= v dCdx
�
−D dCdx
⎛ ⎝ ⎜
⎞ ⎠ ⎟ ∗
= v Cl∗ −Cs
∗( )
�
dCdx
⎛ ⎝ ⎜
⎞ ⎠ ⎟ ∗
= − vDCl
∗ 1− k( )
far from the interface
�
Cl = Co
�
Cl = Co /k
�
Cl = Co 1+ 1− kk
e− xD / v
⎛
⎝ ⎜
⎞
⎠ ⎟
Characteristic Distance
�
v dCdt
= D d2Cdx 2
�
D / v
InitialTransient
Com
posi
tion
Co
CEut
Distance0 L
Steady State
Csm
FinalTransient
After Solidification
InitialTransient
Com
posi
tion
Co
CEut
Distance0 L
Steady State
Csm
FinalTransient
After Solidification
�
Cl = Co 1+ 1− kk
e− xD / v
⎛
⎝ ⎜
⎞
⎠ ⎟
�
Cs∗ = Co 1+ 1− k( )e
−k xD / v
⎛
⎝ ⎜
⎞
⎠ ⎟
abfacdb
L
L
abf
L
T1ab
A B
c dT2e
f T3
Solute Boundary Layer
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5 3
[Cl(x
)-C0]/[
C 0/k0-C
0]
ζ=-(vX)/Dl
Steady-State Dimensionless Solute Field in Melt
1/e
1/e2
1/e3
Note the characteristicdiffusion length, ζ,
Diffusion Limited Steady State Segregation
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5 3
[Cl(x
)-C0]/[
C0/k
0-C0]
ζ=-(vX)/Dl
Steady-State Dimensionless Solute Field in Melt
1/e
1/e2
1/e3
• Diffusion is negligible beyond 3 e-folding lengths from the S/L interface.• The linear boundary layer approximation is equal to 1 e-folding, ζ=1.
Terminal Solidification Transient
Terminal freezing was studied by V.G. Smith, W.A. Tiller, and J.W. Rutter, Canadian Journal of Physics, 33, p. 723, (1955). They obtained the following infinite series solution. Here Δx is the distance to the end of the ingot:
0
5
10
15
20
25
00.20.40.60.81
Scal
ed S
olid
Con
cent
ratio
n, C
s/C0
Dimensionless Distance from End, Δx/(Dl/v)
k0=0.05
0.2
0.5
3 1
Terminal Transient
0.1
• As a practical note, terminaltransients are seldom ofinterest in real crystal growthprocesses. The growth processis usually halted before thistransient begins.
• Note, little if any interaction occurs when the S/L interfaceis more than a distance Dl/v from the end of the crucible.
Example 2: SS Spherical Growth
r
R
For spherical symmetry: 02
2
=∂∂=
∂∂=
∂∂
θθφccc
⎥⎦⎤
⎢⎣⎡
⎟⎠⎞⎜
⎝⎛
∂∂
∂∂=∇=
rcr
rrc
dtdc 2
22 1
For steady-state: 02 =⎟⎠⎞⎜
⎝⎛
∂∂
∂∂
rcr
rthus a
rcr −=∂∂2
and ( ) brarc +=
where a and b are constants.
Fick’s 2nd law: cDcDdtdc 2∇≈∇•∇=
⎥⎦
⎤⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛∂∂+⎟⎟⎠
⎞⎜⎜⎝
⎛∂∂
∂∂+⎟
⎠⎞⎜
⎝⎛
∂∂
∂∂=∇ 2
2
22
22
sin1sin
sin11
θφφφ
φφcc
rcr
rrc
In spherical coordinates:
r
R where a and b are constants.
Boundary conditions:
General solution:
Particular solution:�
C r >> R( ) = C0 = ar
+ b
�
C r = R( ) = Cl* = a
R+ b
�
a = R Cl* −C0( )
�
C r( ) = C0 + RrCl* −C0( )
r
R Controlling solute gradient:
Flux balance: J1=J2
Substituting yields: or
NOTE: diffusivity
shape factorsupersaturation
�
dCdr
= − Rr2
Cl* −C0( )
�
dCdr r=R
= −Cl* −C0( )R�
C r( ) = C0 + RrCl* −C0( )
�
v = − DCl −Cs
*( )dCdz r=R
�
v = D 1R
⎛ ⎝ ⎜
⎞ ⎠ ⎟ Cl −C0( )Cl −Cs
*( )
�
RvD
=Cl −C0( )Cl −Cs
*( )
Example 3: SS Needle growth
Needle with hemispherical cap
r
Rv
Flux balance
A Ah
�
dCdr r=R
= −Cl* −C0( )R
�
A Cl* −C0( ) dzdt = −AhD
dCdr r=R
�
v = D Ah
AR⎛ ⎝ ⎜
⎞ ⎠ ⎟ Cl* −C0( )
Cl* −Cs
*( )
�
v = D 2R
⎛ ⎝ ⎜
⎞ ⎠ ⎟ Cl* −C0( )
Cl* −Cs
*( )
�
Ah
A= 2πR
2
πR2= 2
v
Left for homework…
The Shape Factor
Plane Sphere Plate Needle
Effect of Convection
Effect of Convection and Stirring
During crystal growth and plane–front solidification stirring occurs, which affects the segregation process. This was analyzed by Burton, Prim and Slichter (BPS Theory). They suggested three important zones: Solid zone (no diffusion) A “static” layer, δ, (diffusion transport) Liquid zone (complete mixing)
..
diffusionregion convective mixing region
0 x
melt
δ
C0
Cl^
solid
no diffusionCs^
Cs(x)
BPS Segregation Theory
BPS approximated the fluid dynamic momentum boundary layer as a “static zone,” and bulk liquid as fully mixed.They derived segregation curves (“inner” & “outer”) consistent with these approximations in the fluid mechanics.
Solid Liquid
δ
�
Cl x( ) = Cs* + Cl
* −Cs*( )e
− vDl
x⎛
⎝ ⎜
⎞
⎠ ⎟ , (x ≤ δ)
�
Cl x( ) = C0, (x > δ)
�
Cs*
�
Cl*
�
kBPS ≡Cs*
C0
BPS results (ko<1)
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
K BPS
δ/(Dl/v)
k0=0.005
0.020.01
0.30.5
0.15
0.05
Slower stirringRapid stirring
k0<1
0
20
40
60
80
100
0 2 4 6 8 10
k BPS/k o
δ/(Dl/v)
k0=0.01
0.0133
0.02
0.05
0.10.3
0.03
Slower stirringRapid stirring
BPS results (ko>1)
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4 5
k BPS/k 0
δ/(Dl/v)
k0=5
3
2
1.5
1.2
1.1
k0>1
Slower stirringRapid stirring
BPS Limits
i. no mixing: δ→∞ No natural
convectionCs C0
C0k0
δ→∞solid melt
Cs
Cs → C0
kBPS → 1
⎧⎨⎪
⎩⎪
solidCs → k0C0
kBPS → k0
⎧⎨⎪
⎩⎪
melt
Cl
CsCs
←δ ~0
C0ii. perfect mixing: δ→0 Very rapid stirring
BPS Summary
As summarized below, stirring provides kinetic control on the amount of macrosegregation segregation during steady–state plane front growth.
Faster stirring δ kBPS Cs
0 k0 k0 C0
0<δ<∞ k0< kBPS<1 k0 C0<Cs<0
∞ 1 C0
Key Points
• Equilibrium (lever-law) at S/L interfaces requires:
– Tie-line “jumps”
– Solidus & liquidus slopes (ms & ml)
– Segregation coefficient, k0
• Motion of the interface causes mass transport
– Diffusion boundary layer
– Solute rejection
• Gulliver-Scheil behavior leads to macrosegregation.
• Transients arise during solidification:
– Initial
– Steady-state
– Terminal
• Bouyancy convection treated with Burton-Prim-Schlichter theory