Optimization of solidification and phase selection sequences in water quenched Zr-Al-Ni-Cu alloys

Abstract

Solidification and phase selection sequences have been investigated in Zr 65.0Al 7.5Ni 10.0Cu 17.5 samples prepared by arc melting or rapidly quenching the melt sealed in quartz ampoules in water. The solidification microstructures produced during conventional arc melt processing display micrometer scale eutectic colonies. Water quenched samples in 4 mm or smaller diameters retain the primarily amorphous phase. Controlled crystallization of these amorphous samples yield nanocrystalline composites with an average crystallite size of the order of 20 nm. During isothermal annealing of quenched samples, at temperatures below the glass transition temperature, T g, excess structural energy is released with the structure being relaxed towards a full metastable equilibrium state. The fully annealed samples yield a pronounced endothermic step at the glass transition ( T g = 655 K with Δ C p = 22.4 J/g atom K) followed by exothermic crystallization at T x = 746 K during heating at 0.67 K/s. These results are useful in designing a refined crystallization microstructure to optimize the mechanical properties.