Rapid solidification

Abstract

Points of progress in current understanding of rapid liquid to solid transformation are reviewed. Emphasis is placed on those aspects of the emerging science that would permit development of guidelines and predictive models for alloy design and process control to achieve desired microstructures and properties. Recently available computerbased phase-stability data compilations could be coupled to predictive models to provide a complete description of phase relations (both equilibrium and metastable) for design of new alloys. Heat-flow models are now available for the rapid solidification of ribbons, surface layers, and atomized droplets. These models predict strict limitations on achievable cooling rates in the various processes. The need to balance emphasis on heat-extraction rate and other factors, such as nucleation rate and undercooling governing the attainment of unique microstructures, is now clearly recognized. The extension of morphological stability theory to high interface velocities has shown that plane-front solidification becomes increasingly stable with increasing interface velocity. While some metastable structures obtained during rapid solidification indicate departures from local equilibrium at the solid/liquid interface, the exact form or magnitude of these departures are not known. Amorphous solidification in the presence of a crystalline phase can now be explained in term s of thermodynamically forbidden regions of a phase diagram for partitionless solidification of a single phase. Microstructural observations made on submicrometre powders are discussed with particular reference to recent theoretical developments in interface stability and heat-flow concepts. Finally, the potential for coupling non-destructive testing for online control of parameters during rapid solidification processing is noted.