Phase equilibria and phase separation processes in immiscible alloys

Abstract

Alloys being immiscible in the liquid state exhibit a wide variety of microstructures depending on details of the liquid-liquid phase decomposition process and their phase diagram, especially the shape of the miscibility gap, the solidus surface and thermophysical properties related to the phase diagram. On cooling through the miscibility gap phase decomposition exhibits all classical features known from precipitation processes: nucleation, growth and coarsening of the minority phase droplets. Their simultaneous action is, however, complicated by collisions and coagulations of droplets induced by fluid flow, like sedimentation and Marangoni motion and buoyant convections of any kind. The final microstructure being observed in the solidified state is in addition a consequence of the interaction between the droplets stemming from the decomposition process in the liquid state and their interaction with a dendritic transition zone (mush) determined by the phase diagram and non-equilibrium solidification conditions. The lecture presents experimental and theoretical results on the phase separation and microstructure evolution in binary alloys (Al-Bi, Al-Pb), ternary alloys of Al-Pb with Si, Cu, Cr and Ni and Al-Bi alloys with additions of Sb, Ni, Cu and ternary AlZnBi alloys of various compositions. The alloys were cooled through the miscibility gap and liquidus-solidus interval with different cooling rates leading to quite different microstructures. Their microstructural features like Pb and Bi particle sizes, distribution in size and space and their morphology are discussed with respect to the phase diagram and the kinetics of phase separation.