Slower nucleation kinetics with stirring in a supercooled Zr80Pt20 liquid-Terrestrial and microgravity experiments on...

Under terrestrial conditions, liquids are stirred by Marangoni and gravity-induced flows. A nucleation model that couples the stochastic fluxes of long-range diffusion and interfacial attachment [the Coupled-Flux Model (CFM)] predicts that the nucleation kinetics in metallic liquids should be faster with increased stirring for cases where long-range diffusion is required for nucleation due to an additional mechanism for the transportation of atoms to the nucleating cluster. Unfortunately, few experimental studies of stirring effects exist for metallic liquids. Here, the effect of stirring on hundreds of nucleation cycles is presented for a Zr80Pt20 eutectic liquid using the quiescent environment of ground-based electrostatic levitation and the controlled stirred environment of the electromagnetic levitation facility on the International Space Station. While the Zr80Pt20 liquid should solidify to a eutectic phase mixture, ground-based synchrotron x-ray studies of the crystallizing liquid presented here show that primary nucleation is to an icosahedral phase (i-phase). Approximately 5 s later, the i-phase/liquid transforms into the equilibrium eutectic phase mixture. Since Zr80Pt20 is the eutectic composition, little effect of stirring is expected since diffusion only occurs over short distances during crystallization. Stirring should also have little effect on the nucleation of the i-phase, which has a similar composition to that of the liquid. In contrast, the experimental results show that stirring slows down the nucleation kinetics of the i-phase. However, why the nucleation is heterogeneous, not homogeneous, is yet unknown; a few plausible explanations are suggested for this and the mechanism for the decrease in the nucleation kinetics with stirring.