Magnetic field-induced surface roughness of an undercooled Co–B alloy: Experiment and modeling

Surface roughness (R_a) is a crucial parameter for determining the macroscopic properties of metallic materials. Developing advanced techniques for manipulating R_a has become a frontier hotspot in the interdisciplinary field of mechanical and materials engineering. R_a is directly manifested as the fluctuation of the interface of the melt. A high magnetic field, due to its noncontact and high-energy density characteristics, exerts a significant effect on the interface modification during solidification, showing promise for tailoring R_a. This study systematically investigates the effect and underlying mechanisms of magnetic field intensity on the R_a of an undercooled Co₈₀B₂₀ alloy. The results show that under an identical undercooling degree (ΔT), R_a increases monotonically as the magnetic field intensity increases, rising from 2.633 μm at 0 T to 5.571 μm at a 10 T magnetic field. The macroscopic morphology of the melt transitions from equiaxed spheres to elongated cylinders with a high aspect ratio along the magnetic field direction. A surface tension model imposed on a high magnetic field reveals that the thermoelectric magnetic convection (TEMC) and magnetic gradient force introduced by the magnetic field drive the migration of Co solute with higher surface tension toward the tube wall, increasing the surface tension of the melt and reducing the wettability at the melt–tube wall interface, resulting in enhanced interface fluctuations and increased R_a. This research provides a novel theoretical basis and technical guidance for tuning the macroscopic surface quality of alloys under external magnetic fields.