The understanding of rapid solidification behaviour, e.g. the undercooling vs. growth velocity relationship, is crucial for tailoring microstructures and properties in metal alloys. In most rapid solidification processes, such as additive manufacturing (AM), in-situ investigation of rapid solidification behaviour is missing because of the lack of accurate measurement of the cooling rate and nucleation undercooling. In the present study, solidification of single micro-sized Al-Si alloy (AlSi1, AlSi10, AlSi12 and AlSi20 (mass%)) particles of various diameters has been investigated via differential fast scanning calorimetry (DFSC), employing controllable cooling rates and solidification temperatures. Based on nucleation undercooling and on microstructure analysis of rapidly solidified single powder particles, different heterogeneous nucleation mechanisms of Si and α-Al phases are proposed. The present work not only proposes new insight into rapid solidification processes, but also provides a theoretical foundation for further understanding of microstructures and properties in additively manufactured materials.