1. Solidification of Peritectic Cu-Ge Alloys in Strong Magnetic Field J
Solidification of Peritectic Cu-Ge Alloys in Strong Magnetic Field   J. Gao, J. Fan, Y.K. Zhang, J.C. He Key Lab of Electromagnetic Processing of Materials, Northeastern University, Shenyang , China S. Reutzel, D.M. Herlach Institute of Space Simulation, German Aerospace Center, Cologne, Germany

2. Review on Effects of Static Magnetic Fields in Alloy Solidification
Lorentz force
Suppression of melt convection
Magnetization force
a. Texturing of materials
b. Phase separation
c. Shift of phase equilibrium

3. Solidification of Undercooled Melts
Stable
solid
Liquid
Metastable
GV (J/m3)
T (K)
TLMS
TLSS TN DT
T (K)
t (s) TL DT
Like rapid cooling, large undercooling can lead to the
formation of a variety of metastable microstructure.

4. Question
Both strong magnetic field and undercooling are attractive for fabrication of advanced materials by solidification.
If we apply a strong static magnetic field to solidification processing, how will it affect or interact with liquid undercooling?

5. Undercooling in Magnetic Fields
Hasegawa (1994): copper in 0.5 T
― Increase of maximum undercooling
― More regular change of undercooling during repeated solidification
Tagami (1999): water in 17.9 T
― Containerless crystallization by magnetic levitation: DT=10 K
Aleksandrov(2000): water in 0.5 T
― Decrease of undercooling with increasing field
― Neglegible undercooling above 0.5 Tesla
Gaucherand (2001, 2004): cobalt alloys in 3T
― Co-Sn : DT= 26 K, aligned primary Co
― Co-B: DT= 20 K, primary ferromagnetic Co
Asai (2005): bismuth in SC magnetic field
― Remarkable recalescence for DT= 21 K

6. Motivation Phase selection in peritectic alloys is of great
technical interest as introduced in my first talk.
If a static strong magnetic field influences liquid undercooling, it will also influence phase selection.
In present work, we did undercooling experiments on peritectic Cu-Ge alloys using the glass fluxing method in a 10 T magnetic field to check this point.

7. Experimental Set-Up
Cu-Ge
in B2O3
Small crucible
Bmax=12 Tesla
T max=1200°C
Magnet
Strong Magnetic Field Facility
Big crucible
Undercooling experiments were alse carried out in the absence of a magnetic field for comparision.

8. Experimental Procedures
alloy composition
melting / solidification
1050°C×2h
T (°C)
14.4
300°C/h
B=10 T
1200°C/h
B2O3: softening
at 580°C
Cu-Ge
alloy
Aluminia
crucible
t (h)
Ge wt% 

9. Microstructure of samples solidified in the 10 T magnetic field
Low Magnification
High Magnification
All three samples were solidified into a single-phase microstructure.

10. Compositional Analysis
EDX anylasis
Cu-14.4Ge
Element wt.%
Cu K
Ge L
Ge wt% 

11. X-ray Diffraction Analysis
(Cu): fcc
Ge wt% 
Not all diffractions are from CuSS!

12. Results of Comparision Exp.
Ge wt% 
A two-phase microstructure with
primary Cu for DT up to 120 K
Implication: Magnetic Field promotes liquid undercooling!

13. Possible Mechanisms
Ren (2004):
Possible mechanisms for the promotion of liquid undercooling:
1) shift of phase equilibrium
2) enhanced purification
3) increased liquid viscosity
4) reduced nucleation barrier
for peritectic phase by
modification of liquid/solid
interfacial energy
Ge wt% 
Spaepen (1975):
To verify them requires delicated experiments including measurements of liquid undercooling and susceptibility.

14. Conclusions and Outlook
Peritectic Cu-Ge alloys were solidified into a single- phase microstructure by glass fluxing in a strong magnetic field.
The results imply the promotion of liquid undercooling by the strong magnetic field.
Several possible mechanisms have been proposed, and further investigations will be done in cooperation with partners from DLR, Cologne.

15. Acknowledgements
Thanks to E.G. Wang, Q. Wang, L. Zhang, F. Li, and Z.M. Zhou for useful discussions and help in experimental work. Thanks to the Alexander von Humboldt Foundation and the Institute of Safety Research, FZ-Rossendorf for kind support to the present presentation.