Transition from hypereutectic to hypoeutectic for rapid solidification in an undercooled Co-B alloy

Phase selection, a common and important phenomenon in rapid solidification, determines the solidification sequence and the mechanical/physical properties. In this work, a Co-20 at%B hypereutectic alloy was undercooled by the melt fluxing technique and the microstructure was characterized by the back-scattering diffraction technique. A transition from hypereutectic to hypoeutectic was found at a critical undercooling of ΔT = 119 K. When ΔT < 119 K, a primary directional dendritic β-Co₃B phase surrounded by the regular α-Co + β-Co₃B lamellar eutectics was found. When ΔT > 119 K, the above hypereutectic microstructure changes into hypoeutectic structure with the α-Co phase as the primary phase. According to dendrite growth model, the transition from hypereutectic to hypoeutectic can be ascribed to the higher growth velocity of the α-Co phase than the β-Co₃B phase, i.e., the growth-controlled mechanism. The current work shows also that there is a coupled zone skewed to the β-Co₃B phase in the Co-B alloys system.