Formation of quasicrystals in bulk glass forming Zr-Cu-Ni-Al alloys

Zr65Cu17.5Ni10Al7.5 as well as Zr69.5Cu12Ni11Al7.5 belong to the best glass forming alloys known. Transmission electron microscopy (TEM) and X-ray analysis of samples annealed above the glass transition temperature exhibit the formation of quasicrystalline microstructures. The metastable icosahedral phase is primitive with a quasilattice constant a = 0.253 nm; its composition is close to Zr69.5Cu12Ni11Al7.5. The growth of spherical shaped quasicrystals has been observed to be time-dependent (r ∞ t0.5), thus indicating a diffusion controlled transformation. The nucleation of the quasicrystals in Zr69.5Cu12Ni11Al7.5 was investigated by means of crystallization statistics and found to be heterogeneous and time-dependent. The nucleation rate reached a maximum at 410 °C. The temperature dependence of nucleation and growth rates can be described best assuming a Vogel-Fulcher-Tammann equation with B = 5710 K and T0 = 421 K. From differential scanning calorimetry (DSC) the enthalpy for the quasicrystal formation in Zr69.5Cu12Ni11Al7.5 has been estimated to be 2.4 kJ mol−1. Simulation of the observed nucleation and growth rates indicates an interfacial energy of about 13 mJ m−2. The heterogeneous nucleation of quasicrystals might result from quenching-in clusters with icosahedral structure.