Grain refinement has been a topic of extensive research due to its scientific and technological importance as a common industrial practice for over seven decades. The traditional approach to grain refinement has been to reduce nucleation undercooling by the addition of potent nucleant particles. Here we show both theoretically and experimentally that more significant grain refinement can be achieved through increasing nucleation undercooling by using impotent nucleant particles. Based on the concept of explosive grain initiation, this new approach is illustrated by grain initiation maps and grain refinement maps and validated by experiments. It is anticipated that this new approach may lead to a profound change in both nucleation research and industrial practice well beyond metal casting. Crystallization from liquids is a widespread phenomenon in both nature and technology, and has countless consequences in our everyday life 1,2. For example, formation of ice in the atmosphere affects climate change 3 ; controlling nucleation of molecular crystals from solutions is highly relevant to drug design and production 4 ; protein crystal formation in living beings is responsible for many neurodegenerative disorders such as Alzheimer's disease 5 ; and grain refinement during solidification is critical for high performance engineering alloys 6. In this paper we will focus our attention on grain formation during solidification of metallic materials. Although nucleation plays a critical role in determining the solidified microstructure, it has been very much under-investigated due its associated experimental difficulties, with the majority of solidification research so far being concentrated on grain growth 7. Classical homogeneous nucleation theory 8 uses a thermodynamic approach to identify the critical cluster size and energy barrier, and deploys statistical mechanics to determine the nucleation rate 9-11 , rendering local fluctuation in atomic configuration, chemical composition and temperature extremely important. Based on this, heterogeneous nucleation on a substrate is facilitated by the liquid/substrate interface through reduction of the energy barrier for nucleation 12. It is now generally accepted that nucleation in metallic systems is heterogeneous due to the inevitable existence of solid inclusions in metallic melts 13. More recent advances in nucleation research include realization of the prenucleation phenomenon 14,15 , development of the epitaxial nucleation model based on structural templating 16 and understanding the effect of substrate structure 15 , substrate chemistry 17 and substrate surface roughness 18 on heterogeneous nucleation. Grain refinement during solidification processing is usually achieved by the addition of grain refiners (i.e., chemical inoculation) 19-21. The traditional wisdom for grain refiner development is to search for the most potent solid particles practically available to reduce nucleation undercooling (∆T n). The best example is the Al-5Ti-1B (all compositions are in wt.%) grain r...
Growth restriction refers to the phenomenon of reduced growth velocity due to the solute enrichment/depletion at the solid/liquid interface during alloy solidification. Although significant progress has been made to understand this phenomenon, so far there has been no effective parameter to quantify growth restriction. In this paper, we have derived a new parameter, β, to quantify the growth restriction in multicomponent systems effectively, and which incorporates the nature of solutes, solute concentrations and solidification conditions holistically. Theoretical analysis and phase field simulations have confirmed that growth velocity is a unique function of β regardless of the nature of solutes, solute concentrations and solidification conditions, but it is not a unique function of the widely used growth restriction factor, Q. Our analysis suggests that the overall β for a multicomponent alloy system can be either calculated accurately by the ratio of the liquid fraction to the solid fraction (β = f L / f S ) or approximated with great confidence by a linear addition of the β values of the constituent binary systems. In addition, we have shown theoretically that for a given alloy system solidifying under a given undercooling, there is a critical solute concentration, below which solidification becomes partitionless and therefore there is no growth restriction during solidification. Furthermore, our analysis has shown that the physical origin of growth restriction is the blockage of the supply of the critical elements for crystal growth, i.e., solvent atoms in the case of eutectic-forming.
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