The Solidification of Alloys is Typically Controlled by Solute Diffusion Due to the Solute Partitioning Happening at the Solid-Liquid Interface. in This Study, We Show that the Switching from Solute Diffusion-Controlled Growth to Thermal Diffusion-Controlled Growth May Happen at the Solidification Front during Rapid Solidification Processes of Alloys Such as Additive Manufacturing using a Phase-Field Model. the Switching is Found to Be Triggered by the Cooling of the Solid-Liquid Interface When It Becomes Colder Than the Solidus Temperature. the Switching Introduces a Sudden Jump of Growth Velocity, an Increase in Solute Concentration, and the Refining of the Resulting Microstructures. All Those Changes Predicted by the Phase-Field Simulations Agree with Experimental Observations Quantitatively. the Switching of Control Mechanisms Can Be Exploited by Manipulating the Processing Conditions to Form Refined Microstructures or Layered Structures for Improved Mechanical Properties.