124 Prakt. Met. Sonderband 30 (1999) 
(iv) the anomalous eutectic morphology can evolve from the metastable phase dendrites by trans- 
formations in the semisolid or solid state on slow cooling of bulk samples. With increasing un- 
dercooling the volume fraction of anomalous eutectic microstructure is raised. 
The in situ observation of the recalescence process combined with microstructure investigations of 
substrate-quenched samples with controlled undercooling prior to solidification provides some new 
insight into microstructure evolution processes. It supplied useful information about the intermedi- 
ate non-equilibrium phase precursors which can encounter the solidification process of undercooled 
Ni-Si melts. A mechanism is predicted involving post-solidification transformation of metastable 
phases towards the anomalous eutectic microstructure found in slowly cooled samples with high 
undercooling levels. An analogous mechanism was found in the undercooled eutectic Nb-Al alloy 
(8). Some other eutectic alloy systems, which are currently investigated, exhibit similar phenomena. 
Acknowledgement 
We would like to thank Mrs. I. Stange, B. Arnold, U. Kiihn, Dr. S. Oswald and I. Bächer for ex- 
perimental assistance. Prof. L. Schultz and for valuable discussions. The authors express their 
gratitude for financial support by Deutsche Forschungsgemeinschaft Contract-No. Lo 548/2-2. 
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