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The ability to undercool and solidification mechanism of liquid quinary Ni40Zr28.5Ti16.5Al10Cu5 alloy are investigated by electromagnetic levitation (EML) and drop tube (DT) technique. Under the EML condition, the maximum undercooling of levitated alloy can reach up to 290 K (0.21TL). Under the DT condition, the alloy achieves higher undercooling than EML, and solidifies finally into metallic glass. At lower undercooling, the solidification structure of the alloy is composed of primary Ni3Ti phase, secondary Ni10Zr7 phase and eutectic (Ni10Zr7+Ni21Zr8) phase. With the rise of undercooling, the solidification structure displays the following evolution events: phase morphology refinement, primary phase inhibition, phase number reduction, and amorphous phase formation. By using the high-speed cinematography technique, three nucleation modes are distinctly observed on the levitated alloy melt surface at the beginning of solidification, that is, single-point nucleation, multi-point nucleation and annular nucleation. The levitation state corresponding to single-point mode nucleation is relatively stable, and the alloy undercooling is also relatively low. The annular nucleation only occursin the case with high rotation speed, and the undercooling is greater than 208 K. The discrepancy between nucleation modes is due to the He gas flow for forced cooling. The theoretical calculations indicate that the alloy droplets achieve high undercoolingand large cooling rate under the DT condition. The experimental results show that when the droplet diameter decreases to 498 μm, the amorphous phase begins to appear in the alloy particles. It is noteworthy that the amorphous phase is preferentially formed inside the droplet, but not on the outer surface. The morphology of solidification structure reveals that different regions of the droplet have various local undercoolings, which result in the distribution characteristics of amorphous phase. The volume fraction of amorphous phase increases linearly with the decrease of particle diameter. When the droplet diameter decreases to 275 μm, the alloy droplets are completely frozen into glassy particles. The average eutecticspacing values are also measured at different alloy undercoolings. Compared with the classical binary eutectic growth model, the experimental eutectic growth law exhibits a large deviation in index. This indicates that the eutectic growth in multicomponent alloys displays more complex kinetic characteristics.

In the electromagnetic levitation experiment, the liquid flow in the undercooled liquid alloy remarkably affects the relevant thermodynamic property measurement and solidification microstructure. Therefore, it is of great importance to understand the fluid convection inside the undercooled melt. Theoretical calculation and electromagnetic levitation experiment have been used to investigate the internal velocity distribution and rapid solidification mechanism of Fe50Cu50 alloy. Based on axisymmetric electromagnetic levitation model, the distribution patterns of magnetic flux density and inducted current for levitated Fe50Cu50 alloy are calculated together with the mean Lorenz force. The Navier-Stokes equations are further taken into account in order to clarify the internal fluid flow. The results of the theoretical calculation reveal that the fluid velocity within levitated melt is strongly dependent on three factors, i.e., current density, current frequency and melt undercooling. As one of these factors increases, the maximum fluid velocity decreases while the average fluid velocity increases. Meanwhile, the area with fluid velocity larger than 100 mm·-1 is significantly extended. Furthermore, the fluid flow within levitated melt displays an annular tubular distribution characteristic. The Fe50Cu50 alloy melt is undercooled and solidified under electromagnetic levitation condition. In this undercooling regime △ T50Cu50 alloy melt has suppressed phase separation substantially. Once the undercooling attains a value of 150 K, metastable phase separation leads to the formation of layered pattern structure consisting of floating Fe-rich zone and sinking Cu-rich zone. A core-shell macrosegregation morphology with the Cu-rich zone distributed in the center and outside of the sample and Fe-rich zone in the middle occurs if the undercooling increases to 204 K. With the enhancement of undercooling after phase separation, the grain size of α -Fe dendrites in Cu-rich zone presents a decreasing trend. In contrast to the phase separated morphology of Fe50Cu50 alloy under the glass fluxing condition, the phase separated morphologies show obviously different characteristics. In such a case, the forced convection induced by electromagnetic stirring results in the formation of wavy interface between Fe-rich and Cu-rich zones, the distorted morphology of the Cu-rich spheres distributed in the Fe-rich zone, and the increased appearance probabilities of Cu-rich spheres at the upper part of electromagnetically levitated sample. Experimental observations demonstrate that the distribution pattern of Cu-rich spheres in Fe-rich zone is influenced by the tubular fluid flow inside the melt.