抄録
As part of the MONOPHAS (Advanced bearing alloys from immiscibles with aluminium) program of the European Space Agency, in-situ investigations of hypermonotectic Al-Bi(-Zn) alloys were undertaken to investigate liquid-liquid phase separation under terrestrial conditions. Hypermonotectic alloys are distinguished by a temperature region for which the homogeneous melt decomposes into two liquid phases. In Al-based hypermonotectics the minority phase is much higher in density than the matrix melt phase, and consequentially macro-segregation due to sedimentation is an inherent problem when casting these alloys1). Under the correct solidification conditions, however, it may be feasible to counteract sedimentation by thermocapillary forces arising due to the thermosolutal dependence of the surface tension between the two liquid phases. The current investigation involved in-situ X-ray video microscopy studies during directional solidification of Al-Bi samples of various compositions employing a Bridgman furnace. It was found that during liquid-liquid decomposition, the hydrodynamic forces coupled to external fields (i.e. temperature gradient and gravity) dominate L2 droplet motion and interaction. As a consequence these flow fields are superimposed on the short range coagulation mechanisms making quantitative data difficult to obtain. In-situ investigations in micro-gravity would eliminate the effect of gravitational field allowing short range coagulation mechanisms to be investigated providing important data for modelling of hypermonotectic systems. Keywords. MONOPHAS, hyper-monotectic alloys, directional solidification, X-Radiography.
