1

Outlines

Metastable intermetallics in aluminum alloys

Available approaches

Our approach

Results

2

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%

3

Al-Fe intermetallics

4

Phases named α

5

Phases named β

6

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)

7

Simultaneous formation of phases

8

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

9

Our approach

10

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

11

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

12

Global undercooling

Aluminum alloy with 0.3

wt% Fe and 0.45

wt% Si

13

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

14

Coupling the driving forces concept and Scheil solidification mode

Aluminum alloy with 0.3

wt% Fe and 0.45

wt% Si

15

• 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 ?

16

Calculations for different aluminum alloys

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

18

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

19

Formation of intra-cellular globular particles

P. Liu, Key Engineering Materials Vols. 44 & 45 (1990) pp.69-86

20

Formation of inter-dendritic particles

21

Available theories for the formation of globular particles

22

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

23

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

24

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.

25

THE END