Prof Lindsay Greer - University of Cambridge, UK
Principles of Alloy Grain Refinement in Solidification Processing
A. L. Greer
University of Cambridge, Department of Materials Science & Metallurgy, UK
The development of grain structure in cast alloys is reviewed. The development involves the initiation of new grains, their growth, and possibly subsequent solid-state changes such as recrystallization and grain growth. The analysis of the grain size resulting from solidification itself involves consideration of the kinetics of both grain initiation and grain growth, and in essence the grain size is determined by a competition between the rates of initiation and growth. There are many possible origins of grain initiation: homogeneous crystal nucleation in the melt; heterogeneous nucleation on extrinsic particles (intentionally added to the melt, or otherwise); and seeding, for example through dendrite fragmentation. The various forms of grain initiation are compared, focusing on the required degree of supercooling. The mechanisms of growth restriction are also reviewed, considering the principles for the selection of solutes suitable for promoting grain refinement. The supercooling at which solidification is initiated is very significant. In conventional solidification processing, that is shaped casting and casting of wrought alloys, the solidification is initiated at small supercooling. In such cases, grain initiation is by seeding or by potent heterogeneous nucleation. The conditions for successful grain refinement by seeding are narrowly defined, and heterogeneous nucleation is the more likely practical method for achieving a fine grain size. The requirements for successful heterogeneous nucleation will be considered. Grain refinement through inoculation by extrinsic particles is widely practised for aluminium alloys, and key findings will be reviewed, focusing on the importance of nucleant particle size. The same principles of grain refinement apply in other alloy systems, and some practical examples will be compared. At the other extreme, much interest has arisen in melts taken to extremes of large supercooling. In such melts, on the border of glass formation, grain refinement can also be achieved. The origins and potential applications of such refinement will be surveyed. The talk will conclude with consideration of how such studies at large supercooling may be of practical interest in a wide range of alloy systems and applications.