Undercooling, local structure and phase transition in icosahedral quasicrystals forming titanium-zirconium-nickel alloy liquids

While lacking the long-range translationally periodic order of crystalline solids, liquids contain a significant amount of short-range order (SRO), which distinguishes them from gases. The local structure of a liquid is important to understand its chemical and physical properties. One of the remarkable phenomena related to the short-range order of liquids is undercooling. That is, liquids can be retained below their melting temperature for a long time without crystallization. Turnbull first demonstrated this for metallic liquids in 1952. To explain the surprising results, Frank hypothesized in 1952 that the local structure of liquid metals is icosahedral. This structure is quite different from those of crystal phases giving a large nucleation barrier and making the undercooling of liquids possible. However, a complete verification of Frank's hypothesis has not been possible thus far. In this dissertation, this goal has been achieved by demonstrating a direct connection between the nucleation barrier and the icosahedral SRO (ISRO) in Ti-Zr-Ni alloy liquids. Containerless environments and in-situ x-ray scattering experiments, essential for such studies, were possible because of the development of electrostatic levitation (ESL) and Beamline-ESL techniques. In addition, distorted icosahedral SRO in liquids will be shown, which has been expected but never observed.;The other important topic related to the undercooled liquids is a liquid-liquid (L-L) transition. Since the undercooled liquids are essentially metastable, the L-L transition could be expected. However, L-L transitions in undercooled liquids have not been observed experimentally, although elemental liquids of P, C, and Si have shown first order transitions above the liquidus temperatures under high pressure. From specific heat measurements of a series of Ti-Zr-Ni alloys by the ESL technique, a maximum in the specific heat at constant pressure was observed in a few quasicrystal forming alloy compositions in the undercooled state, indicating a L-L transition. Surprisingly, the configurational specific heat for the quasicrystal-forming liquid rapidly approaches zero below the maximum, indicating the attainment of a state of constant entropy. Assuming that this is a configurational ground state (at least a constrained equilibrium one), a modified two-level model fits the maximum specific heat data well.