Ryan Reeves1 Etop Esen1 Michael Roberts1

1, International Space Station U.S. National Laboratory, Melbourne, Florida, United States

High-entropy alloys exhibit many desirable mechanical properties, including high fracture toughness, wear resistance, and crack resistance. However, the large atomic mass differential between constituent elements can lead to undesired compositional segregation and inhomogeneity. Gravitational effects (e.g., convection and sedimentation) during the melt cause density-driven phase segregation leading to inhomogeneity in the composition of the alloy. This causes reduced mechanical strength and toughness.
The International Space Station (ISS) U.S. National Laboratory offers a unique environment of persistent microgravity and, if desired, exposure to the harsh space environment in low Earth orbit. Microgravity mitigates gravitational phenomena such as buoyancy-driven convection, density-driven sedimentation and segregation, and heterogeneous nucleation at surface interfaces. Early studies of the solidification of high-entropy alloys in microgravity have demonstrated a higher degree of compositional homogeneity compared with terrestrial solidification. In addition, microgravity provides the opportunity to decouple the effects of Stokes sedimentation from Marangoni flow in order to better understand the heat and mass transfer dynamics in the melt and during solidification.
We will introduce the underlying phenomena directing the physics of the high-entropy alloy melt in persistent microgravity and will present the roles of Stokes sedimentation, Marangoni flow, and buoyancy-driven convection during the melt and solidification of high-entropy alloys. Additionally, we will present case studies of high-entropy alloy solidification in microgravity and compare the results with terrestrial experiments. We will also discuss translational lessons learned from microgravity experiments that inform and direct terrestrial research and manufacturing. Finally, we will present opportunities for future microgravity experiments and access to ISS facilities through the ISS National Laboratory.