Rapid Solidification Processing of Alloys Enables the Formation of Exotic Nonequilibrium Microstructures. However, the Interrelationship between the Processing Parameters and the Resulting Microstructure is Yet to Be Fully Understood. in Melt Spinning (MS) and Additive Manufacturing (AM) of Rapidly Solidified Alloys, Opposite Microstructure Development Sequences Were Observed. a Fine-To-Coarse Microstructural Transition is Typically Observed in Melt-Spun Ribbons, Whereas Melt Pools in AM Exhibit a Coarse-To-Fine Transition. in This Paper, the Microstructural Evolutions during These Two Processes Are Investigated using Phase-Field Modeling. the Variation of All Key Variables of the Solid-Liquid Interface (Temperature, Composition, and Velocity) throughout the Entire Rapid Solidification of AM and MS Processes Was Acquired with High Accuracy. It is Found that the Onset of Nucleation Determines the Selection of the Solidification Pathway And, Consequently, the Evolution of Temperature and Velocity of the Interface during the Rapid Solidification. the Switching of Control Mechanisms of the Solid-Liquid Interface, Which Happens in Both Processes But in Opposite Directions, is Found to Cause the Velocity Jump and Disrupt the Microstructure Development.