metastable intermetallics in aluminum alloys available...
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Outlines
Metastable intermetallics in aluminum alloys
Available approaches
Our approach
Results
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Ternary Al-Fe-Si Alloys
Skejerpe, [1987]
α
Griger
et al. [1989]
β
Al13
Fe4
Westiengen
[1982]
Fe=0.3 wt% Si=0.15 wt%
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Al-Fe intermetallics
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Phases named α
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Phases named β
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Most of researches done in this area can be divided into two categories
Transition from one phase to another do not necessarily occur at
one unique cooling rate or solidification velocity
Cooling Rate(Competitive nucleation)
Growth Rate(Competitive growth)
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Simultaneous formation of phases
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Al6
Fe can nucleate
Alm
Fe can nucleate
Alm
Fe can grow
Al6
Fe can grow
Conditions for both nucleation
and
growth
should be satisfied
S. B
ruse
thau
g, D
. Por
ter
and
O.V
orre
n, 1
987,
Hyd
ro a
lum
inum
, Suu
ndal
Ver
k. In
: ILM
T. 4
72
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Our approach
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α
β
G
X0 1
Liquid
c
Liquid α
βG
X0 1
c
Driving force forattainment of full equilibrium
Driving force foronset of precipitation
Two
approaches
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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
LL
T2 < T1 x0Gib
bs e
nerg
y of
pha
se, k
J/m
ole
x
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Global undercooling
Aluminum alloy with 0.3
wt% Fe and 0.45
wt% Si
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1.
Accumulation of FCC phase occurs according to Scheil formalism
2.
Rejection of solutes by growing α-Al dendrites into the remaining liquid is accompanied by decrease of temperature (positive driving forces for onset of nucleation)
Modified model
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Coupling the driving forces concept and Scheil solidification mode
Aluminum alloy with 0.3
wt% Fe and 0.45
wt% Si
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• Since all heterogeneous nucleation sites have already been consumed by FCC nuclei
• Surface of rapidly growing FCC dendrites can not work as new sites for the nucleation of intermetallics
• Composition of intermetallic phases is usually quite different from that of the parent liquid phase
• Intermetallic phases are usually faceted
Homogeneous nucleation
Still we need high degrees of supercooling. How is this justified ?
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Calculations for different aluminum alloys
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17 RPM = 400≈
300 μm
RPM = 930≈
150 μm
RPM = 1520≈
70 μmGraphite
Copper
Water-Cooled Copper
Experiment
Composition
Alloy #1
Low Si
Intermediate Fe
Alloy #2
Intermediate Si
Intermediate Fe
Alloy #3
High Si
Intermediate Fe
Alloy #4
Intermediate Si
High Fe
Si
wt% 0.05 ± 0.01 0.15 ± 0.01 0.45 ± 0.01 0.20 ± 0.01
Fe
wt% 0.30 ± 0.01 0.30 ± 0.01 0.30 ± 0.01 0.50 ± 0.01
Experiments
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C
Cs‐1Alloy #1
(Fe=0.3 Si=0.05)Alloy #2
(Fe=0.3 Si=0.15)Alloy #3
(Fe=0.3 Si=0.45)Alloy #4
(Fe=0.5 Si=0.2)
Graphite Crucible
Alm
FeAlX
FeAl13
Fe4α
Al13
Fe4Alm
Feα
α Al13
Fe4Al9
Fe2
CopperAlm
FeAlX
Feα α α
Water‐cooled copper
Alm
FeAlX
FeAl6
Feα
αAlX
Feα
αAlX
FeAl6
FeAlm
FeAl13
Fe4
Ribbon 300 μmα
AlX
Feα α
αAlX
FeAlm
Fe
Ribbon 150 μmα
AlX
Feα
AlX
Feα α
Ribbon 70 μmα
AlX
Fe‐
αAlX
Fe
αAlX
FeAlm
Fe
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Formation of intra-cellular globular particles
P. Liu, Key Engineering Materials Vols. 44 & 45 (1990) pp.69-86
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Formation of inter-dendritic particles
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Available theories for the formation of globular particles
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C
Cs‐1Alloy #1
(Fe=0.3 Si=0.05)Alloy #2
(Fe=0.3 Si=0.15)Alloy #3
(Fe=0.3 Si=0.45)Alloy #4
(Fe=0.5 Si=0.2)
Graphite Crucible
Alm
FeAlX
FeAl13
Fe4α
Al13
Fe4Alm
Feα
α Al13
Fe4Al9
Fe2
CopperAlm
FeAlX
Feα α α
Water‐cooled copper
Alm
FeAlX
FeAl6
Feα
αAlX
Feα
αAlX
FeAl6
FeAlm
FeAl13
Fe4
Ribbon 300 μmα
AlX
Feα α
αAlX
FeAlm
Fe
Ribbon 150 μmα
AlX
Feα
AlX
Feα α
Ribbon 70 μmα
AlX
Fe‐
αAlX
Fe
αAlX
FeAlm
Fe
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Alloy #1(Fe=0.3 Si=0.05)
Alloy #2(Fe=0.3 Si=0.15)
Alloy #3(Fe=0.3 Si=0.45)
Alloy #4(Fe=0.5 Si=0.2)
Ribbon 150 μmα
AlX
Feα
AlX
Feα α
Ribbon 150 μmAlm
Feα
AlX
Fe
αAlX
FeAlm
Fe
α α
Increasing the casting temperature from ≈770 ˚C to ≈1000 ˚C
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Conclusions
• Stable intermetallic, Al13
Fe4
, forms only at vey low cooling rates.
• By increasing both silicon content and cooling rate, formation of α-AlFeSi
is promoted.
• Fe-rich binary metastable intermetallics are stabilized by increasing iron content.
• It seems that concept of the driving forces for the beginning of
precipitation from the liquid phase during rapid solidification provides a sensible conformity with the experimental findings.
• A greater deal of accuracy and reliability of the model can be attained in the surface energies
are taken into account.
• A complete database are needed. It is likely that first-principle calculations should be employed, because of a virtual impossibility to accumulate an amount of such phases (not in a mixture with other
intermetallics) suitable for a calorimetric investigation.
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THE END