Poor glass-forming ability of Fe-based alloys: Its origin in high-temperature melt dynamics

The relatively poor glass-forming ability (GFA) of Fe-based alloys has limited the mass production of Fe-based bulk metallic glasses (MGs), and hence their applications. To clarify the origin of the low GFA of the Fe-based alloys, we investigate the dynamic behaviors of these alloy systems well above their respective liquidus temperatures. By using a torsional oscillating viscometer we have observed a dynamic transition, where the slope of the viscosity-temperature relation of the Fe-based metallic liquid increases upon cooling, and a viscosity hysteresis occurs between the heating and cooling processes. By using the concept of fluid cluster and supercooled liquid fragility in metallic liquids, it has been found that this dynamic transition makes the Fe-based supercooled liquids become more unstable, which leads to the poor GFA of Fe-based alloys. Further, it has been found that the degree of the dynamic transition relates to the different GFA among Fe-based alloys. By taking into account the thermodynamic effect accompanied with the dynamic transition in Fe-based melts, a possible scenario has been proposed from the aspect of the underlying structural evolution during cooling above the liquidus temperature.