Monotectic Alloys and their Growth Morphologies

Alloys of exact monotectic composition can decompose at the monotectic temperature into different two phase microstructures. The minority phase can be arranged as fibers similar to eutectics, as string of pearls or as irregularly distributed droplets. The directional solidification of three Al-base monotectic alloys was investigated: Al-Pb, Al-In and Al-Bi utilizing aerogel base furnace technology to ensure constant gradient and solidification velocity over the processing length. The solidified microstructures are characterized as a function of temperature gradient ahead of the solidification front and the solidification velocity. The experimental results are presented in form of stability diagrams for the three microstructures showing the regions in which fibers, string of pearls or irregular morphologies exist. The inter fiber spacing is analyzed and presented in from of Jackson and Hunt type relations and the transitions between the microstructures are discussed.

Kinetics of Isothermal Phase Transformations by Phase-Field Simulations: An Analogy between the Peritectic and Monotectic Systems

We present phase-field simulations of isothermal phase transformations in the peritectic system below and above the peritectic temperature TP , and in the monotectic system below the monotectic temperature TM. We focus particularly on the Liquid-Film-Migration (LFM) mechanism, which appears to be the generic process for phase transformations above TP . Below TP , we obtain an assymetric LFM, suggesting the existence of a doublon structure in free space. In the monotectic system, the transformation from a liquid L1 to a solid+liquid L2 mixture proceeds via the migration of a L2 film, which is the analogous of the LFM process. When the metastable state consists of a liquid-liquid mixture, a dendritic-like solidification is obtained.

A Study of the Coalescence Process Inside the Miscibility Gap in Zn-Bi Alloys

ABSTRACTThe coalescence process has been studied in four different hyper-monotectic Zn-Bi alloys with 4–10wt % Bi . The alloys have been studied by isothermal treatments at a temperature just above the monotectic temperature and at different holding times. The alloys have been studied both at microgravity and at normal gravity. The change of the particle distribution as a function of composition and holding time has been investigated. The changing of the particle distribution has been compared with the theory of particle collision. The validity of the collision theory is investigated with respect to the theory of liquid phase sintering.