OPTIMIZATION OF SOLIDIFICATION AND PHASE SELECTION SEQUENCES IN WATERQUENCHED ZR-AL-NI-CU ALLOYS

Solidification and phase selection sequences have been investigated in Zr65.0Al7.5Ni10.0Cu17.5 samples prepared by arc melting or rapidly qu enching the melt sealed in quartz ampoules in water. The solidificatio n microstructures produced during conventional are melt processing dis play micrometer scale eutectic colonies. Water quenched samples in 4 m m or smaller diameters retain the primarily amorphous phase. Controlle d crystallization of these amorphous samples yield nanocrystalline com posites with an average crystallite size of the order of 20 nm. During isothermal annealing of quenched samples, at temperatures below the g lass transition temperature, T-g, excess structural energy is released with the structure being relaxed towards a full metastable equilibriu m state. The fully annealed samples yield a pronounced endothermic ste p at the glass transition (T-g = 655 K with Delta C-p = 22.4 J/g atom K) followed by exothermic crystallization at T-x = 746 K during heatin g at 0.67 K/s. These results are useful in designing a refined crystal lization microstructure to optimize the mechanical properties.