Microscopic structure evolution and amorphous solidification mechanism of liquid quinary Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅ Alloy

The substantial undercooling and rapid solidification of liquid quinary Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅ alloy are achieved by electromagnetic levitation (EML) technique. The amorphous solidification mechanism is revealed with molecular dynamics (MD) simulation. It is observed in EML experiment that the containerlessly solidified alloy is characterized by a core-shell structure, with mainly amorphous phase becoming the core and crystalline ZrCu, Zr₂Cu and Zr₈Cu₅ phases forming the shell. The volume fraction of amorphous core structure increases with undercooling and attains a value up to 81.3% at the maximum experimental undercooling of 300 K, which indicates that the critical undercooling required for complete amorphous solidification is 334 K. TEM analyses show that the alloy microstructure is mainly composed of Zr₈Cu₅ phase, whereas the ZrCu phase and Zr₂Cu phase are suppressed when liquid undercooling approaches this threshold. Once the critical undercooling is reached, amorphous solidification prevails over the crystallization of Zr₈Cu₅ phase. In addition, a small quantity of amorphous phases are found in the crystalline shell and a little trace of Zr₈Cu₅ nano-cluster is detected among the amorphous core. It is further verified by MD simulation that the formation of amorphous phase in the shell is caused by the microsegregation-induced solutal undercooling when liquid alloy attains the critical undercooling, while the nano-clusters within the core is mainly ascribed to the micro-thermal fluctuation effect inside highly undercooled liquid phase.