5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Recent Advances in Understanding the Early Stages of Solidification Z Fan BCAST, Brunel University London, Uxbridge, Middlesex, UB8 3PH, UK (zhongyun.fan@brunel.ac.uk) Solidification of single phase alloys occurs in a number of distinctive stages during the cooling process. For isothermal solidification these may include prenucleation (atomic ordering in the liquid adjacent to the liquid/substrate interface), heterogeneous nucleation, grain initiation, spherical growth, morphological instability, dendritic growth and grain impingement. Here, we define the early stages of solidification as the process between prenucleation and the point of morphological instability. Since the majority of historic research has concentrated on dendritic growth, our current understanding on the early stages of solidification has been very limited despite it contributing predominantly to the formation of the solidified microstructure. In recent years, we have been devoting our research effort to understanding the early stages of solidification and have made good progress. In this contribution we present an overview of the recent advances in understanding the early stages solidification, focusing on heterogeneous nucleation, grain initiation and their effect on grain refinement. We will demonstrate both theoretically and experimentally the following key conclusions: • The current grain refining approach (take Al-5Ti-1B grain refiner as an example) has reached a saturation point; it is difficult to achieve any further grain refinement; • More significant grain refinement can be achieved by impeding heterogeneous nucleation using less potent nucleant particles (larger nucleation undercooling), which is in contrast to the conventional approach to grain refinement by enhancing heterogeneous nucleation on the most potent nucleant particles. • Al- and Mg-alloys do not need grain refiners since they contain sufficient native solid particles to achieve grain refinement once they are made available by appropriate melt treatment, for example, by intensive melt shearing.