Morphological stability analysis for planar interface during rapidly directional solidification of concentrated multi-component alloys

Coupling with the thermodynamic database and incorporating the effects of local non-equilibrium and off-diagonal diffusional interactions, an extended morphological stability analysis was performed for a planar interface upon rapid solidification of concentrated multi-component alloys. Compared with the previous work, the model is able to clarify the effects of concentration dependence and diffusional interactions on the interface stability. Taking the Al-Mg-Zn alloy as an example, the neutral stability, i.e. the critical Zn concentration (with Mg concentration fixed) for the breakdown of the planar interface, was studied. The stability mechanisms for the neutral Zn concentration subjected to different Mg concentrations and the off-diagonal diffusion effect were clarified. In particular, for high Mg concentrations, a stage of absolute instability dependent on the Mg diffusion effect but independent of the Zn concentration happens. Although the off-diagonal diffusion is several orders of magnitude smaller than the solute diffusion coefficients, it can significantly change the Zn concentration for the neutral stability conditions.