Nucleation Kinetics and Microstructure Evolution of Traveling ASTM F75 Droplets

Abstract

AbstractA method is devised for the prediction and control of the nucleation kinetics and solidification microstructure of traveling droplets of a cobalt‐base biocompatible alloy ASTM F75. Mono‐size droplets produced by the uniform‐droplet spray (UDS) process (a capillary jet breakup process) are quenched on substrates at different flight distances to determine nucleation distance from the splat morphology. Using the nucleation temperatures estimated from the splat morphology as references, the nucleation kinetics of F75 droplets is predicted and presented in the form of a continuous‐cooling transformation diagram. Cooling 300 µm F75 UDS droplets in argon gas resulted in an estimated supercooling of 132 K and formation of a well‐defined dendritic microstructure, whereas cooling the same droplets in helium gas produced a larger supercooling of 377 K and a microcrystalline structure indicative of dendrite fragmentation. This process, with stringent control of nucleation kinetics, can produce F75 balls with a rapid solidification microstructure suitable for bio‐implant applications.