Morphological instabilities of lamellar eutectics
- Northeastern Univ., Boston, MA (United States). Physics Dept.
The authors present the results of a numerical study based on the boundary integral technique of interfacial pattern formation in directional solidification of thin-film lamellar eutectics at low velocity. Microstructure selection maps that identify the stability domains of various steady-state and nonsteady-state growth morphologies in the spacing-composition ({lambda} {minus} C{sub 0}) plane are constructed for the transparent organic alloy CBr{sub 4}-C{sub 2}Cl{sub 6} and for a model eutectic alloy with two solid phases of identical physical properties. In CBr{sub 4}-C{sub 2}Cl{sub 6}, the basic set of instabilities that limit steady-state growth is richer than expected. It consists of three primary instabilities, two of which are oscillatory, which bound the domain of the commonly observed axisymmetric lamellar morphology, and two secondary oscillatory instabilities, which bound the domain of the nonaxisymmetric (tilted) lamellar morphology. Four stable oscillatory microstructures, at least three of which have been seen experimentally, are predicted to occur in unstable regimes. In the model alloy, the structure is qualitatively similar, except that a stable domain of tilted steady-state growth is not found, in agreement with previous random-walk simulations. Furthermore, the composition range of stability of the axisymmetric morphology decreases sharply with increasing spacing away from minimum undercooling but extends further off-eutectic than predicted by the competitive growth criterion. In addition, oscillations with a wavelength equal to two {lambda} lead to lamella termination at a small distance above the onset of instability. The implications of these two features for the eutectic to dendrite transition are examined with the conclusion that in the absence of heterogeneous nucleation, this transition should be histeritic at small velocity and temperature gradient.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 237774
- Journal Information:
- Metallurgical Transactions, A, Journal Name: Metallurgical Transactions, A Journal Issue: 3 Vol. 27; ISSN 0360-2133; ISSN MTTABN
- Country of Publication:
- United States
- Language:
- English
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