Structure evolution upon non-equilibrium solidification of bulk undercooled Fe-B system

Bulk Fe-rich Fe-B alloy melts were undercooled and even hypercooled by adopting molten glass purification combined with high-frequency vacuum melting and cyclical superheating. The cooling curves of eutectic and off-eutectic alloys upon solidification which were subjected to different undercoolings are described and compared. Then systematic investigations for structural evolution, including a morphological change of the primary phase and transformation mechanisms for hypoeutectic, hypereutectic and eutectic alloys were carried out. In particular, for the eutectic Fe₈₃B₁₇ alloy melt subjected to hypercoolings of 300-460 K, phase selection between the stable Fe₂B and the metastable Fe₃B was found and described. A bulk Fe-B eutectic structure with metastable Fe₃B can be directly solidified from the hypercooled melt and then be preserved at room temperature, provided the undercooling exceeds 386 K. These phenomena were also observed for some hypereutectic alloys with some undercooling. On these bases, an evolution map of the eutectic zone as a function of composition and temperature was deduced.