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

Abstract

The substantial undercooling and rapid solidification of liquid quinary Zr<sub>57</sub>Cu<sub>20</sub>Al<sub>10</sub>Ni<sub>8</sub>Ti<sub>5</sub> alloy were achieved by electromagnetic levitation (EML) technique. The amorphous solidification mechanism was revealed with molecular dynamics (MD) simulation. It was observed in EML experiment that the containerlessly solidified alloy was characterized by a core-shell structure, with mainly amorphous phase becoming the core and crystalline ZrCu, Zr<sub>2</sub>Cu and Zr<sub>8</sub>Cu<sub>5</sub> phases forming the shell. The volume fraction of amorphous core structure increased with undercooling and attained a value up to 81.3% at the maximum experimental undercooling of 300 K, which provided an estimated critical undercooling for complete amorphous solidification as 334 K. Based on TEM analyses, the alloy microstructure was mainly composed of Zr<sub>8</sub>Cu<sub>5</sub> phase, whereas the ZrCu and Zr<sub>2</sub>Cu phases were suppressed when liquid undercooling approached this threshold. Once the critical undercooling was reached, amorphous solidification prevailed over the crystallization of Zr<sub>8</sub>Cu<sub>5</sub> phase. In addition, a small amount of amorphous phase was found in the crystalline shell and a little trace of Zr<sub>8</sub>Cu<sub>5</sub> nano-clusters was detected among the amorphous core. It was further verified by MD simulation that the formation of amorphous phase in the shell was caused by the microsegregation-induced solutal undercooling when liquid alloy was attaining the critical undercooling, while the nano-clusters within the core was mainly ascribed to the micro-thermal fluctuation effect inside highly undercooled liquid phase.