Constitutional supercooling and corresponding microstructure transition triggered by high magnetic field gradient during directional solidification of Al-Fe eutectic alloy

Directional solidification experiments of eutectic Al Fe alloys were carried out under different high magnetic field gradients. High magnetic field gradient changed the microstructure selection during solidification process, induced the solidification microstructure to undergo eutectic instability, cellular transition, single-phase instability, and dendrite transition, which is similar to the planar-cellular-dendritic crystal growth pattern transition usually induced by an increasing growth velocity without magnetic field. The single phase was proved to be formed through an independent nucleation mechanism during single-phase instability. The effect of high magnetic field gradient on solidification was analyzed. Through the coupling effect of magnetic force and Lorentz force on solute migration and diffusion during solidification, high magnetic field gradient triggered a constitutional supercooling at the front of solid-liquid interface, which led to the growth pattern transition. A solidification model of eutectic alloy under high magnetic field gradient was proposed, and the microstructure selection map of Al Fe alloy under gradient magnetic field was qualitatively drawn for the first time. This work proved that an alloy material with a microstructure usually obtained at high growth velocity can be obtained at a low growth velocity by applying a high magnetic field gradient, and suggested a new potential method for the future controlling of alloy structures and properties. • High magnetic field gradient changes the microstructure selection during alloy solidification. • Critical solidification velocity for crystal growth pattern transition decreased by high magnetic field gradient. • Under high magnetic field gradient, the left boundary of coupled zone of Al Fe eutectic alloy moves right. • Coupling effect of Lorentz force and magnetic force induces constitutional supercooling during solidification. • High magnetic field gradient has a same effect as increasing solidification velocity on alloy solidification.