The criterion of planar instability in alloy solidification under varying conditions: A viewpoint from free energy

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Abstract
In alloy solidification, the transport processes of heat and solute result in morphological instability of the interface, forming different patterns of the solidification structure and determining the mechanical properties of components. As the first observable phenomenon of morphological instabilities, planar instability influences the subsequent stages significantly, deserving in-depth investigations. In this paper, the planar instability in alloy solidification under varying conditions is studied. First, the dynamical evolution of the planar instability is performed by the theoretical model and the phase-field model, respectively. Second, to represent the history-dependence of solidification, varying parameters are adopted in the simulations. Then, the criterion of planar instability under varying conditions is discussed. This paper considers that the critical parameters of planar instability are the excess free energy at the interface and the corresponding interfacial energy. Finally, to validate the criterion, comparisons between the phase-field and theoretical models are carried out, showing good consistency. Moreover, solidification processes with different preferred crystallographic orientations are performed, demonstrating the effect mechanism of the excess free energy and interfacial energy on planar instability. The idea of the interfacial energy influencing the planar instability could be applied to investigating other patterns induced by interfacial instability.

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