Role of polyhedral order in glass to crystal transition dynamics in Zr60Cu10Al15Ni15 glass forming alloy

Understanding the structural characteristics in bulk metallic glass forming alloys is important in order to understand the atomic origin of its high glass forming ability and associated physio-chemical properties. Zr60Cu10Al15Ni15 alloy was cast in a water cooled copper mold. Due to difference in cooling rate the bottom regions of the cast rod shows amorphous phase with randomly oriented short range ordered domains while in the top part of the cast rod having faceted or dentritic crystals of cF96 Zr2Ni and tP20 Zr3Al2 phases were observed. The size of the short/medium range ordered domains is ~ 0.3 nm. Complete structural analysis of the crystalline phases indicates that complex interconnected polyhedral order exists in the crystalline phases which are related to Bernal deltahedra and Frank-Kasper polyhedra. Correlation with the size of short range ordered regions indicates that similar polyhedra may also exist in the liquid, the specific interaction of which leads to the formation of short range ordered domains and crystal nuclei. Morphology of the crystalline phases indicates strong interplay of solid-liquid interfacial energy; break down of solid-liquid planar interface and solute segregation during solidification and crystal nucleation.