Icosahedral quasicrystal structure of the Mg40Zn55Nd5 phase and its thermodynamic stability

The quasicrystal phase is beneficial to increasing the strength of magnesium alloys. However, its complicated structure and unclear phase relations impede the design of alloys with good mechanical properties. In this paper, the Mg40Zn55Nd5 icosahedral quasicrystal (I-phase) structure is discovered in an as-cast Mg-58Zn-4Nd alloy by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). A cloud-like morphology is observed with Mg41.6Zn55.0Nd3.4 composition. The selected area electronic diffraction (SAED) analysis shows that the icosahedral quasicrystal structure has 5-fold, 4-fold, 3-fold, and 2-fold symmetry zone axes. The thermodynamic stability of the icosahedral quasicrystal is investigated by differential scanning calorimetry (DSC) in the annealed alloys. When annealed above 300 degrees C, the Mg40Zn55Nd5 quasicrystal is found to decompose into a stable ternary phase Mg35Zn60Nd5, a binary phase MgZn, and alpha-Mg, suggesting that the quasicrystal is a metastable phase in the Mg-Zn-Nd system.

Automated summary

In this study, the Mg40Zn55Nd5 icosahedral quasicrystal (I-phase) structure was observed in an as-cast Mg-58Zn-4Nd alloy using high-resolution electron microscopy. The thermodynamic stability of the quasicrystal was investigated and it was found to decompose into other phases after annealing.