Formation of a Three-Phase Spiral Structure Due to Competitive Growth of a Peritectic Phase with a Metastable Eutectic

Metastable spiral MgZn₂/Zn eutectics grow in binary Zn-3 wt.% Mg alloys at small liquid undercooling. In this article, we report growth of a new class of three-phase spiral structures in metastable solidification of Ni-doped Zn-3Mg alloys. Thermodynamic calculations demonstrated that a minor Ni substitution for Zn allows for peritectic growth of the Mg₂Zn₁₁ phase following secondary MgZn₂ growth. In situ synchrotron x-ray diffraction confirmed such metastable solidification reactions and suggested concomitant growth of MgZn₂, Mg₂Zn₁₁ and Zn close to the end of solidification. Synchrotron x-ray nano-tomography revealed a localized three-phase spiral structure, wherein Mg₂Zn₁₁ grows at the phase-interface of the metastable eutectic. Such a novel spiral structure can be attributed to competitive growth of a metastable binary eutectic and a peritectic phase in metastable solidification. Ni doping increases the nucleation rate of MgZn₂ and the driving force for peritectic growth of Mg₂Zn₁₁, thus leading to the formation of the three-phase spiral structure.