| Physical properties and structural evolutions during solidification of metallic glass-forming liquids are always the frontier and hot-spot issues in the domain of condensed physics and materials. Study on such issues will not only theoretically help us deeply understand the nature of glass transition and their competitive relationships with crystallization behavior, but also have significance of controling or improving properties of amorphous solids by utilizing liquid properties. According to the traditional Adam-Gibbs’s theory, dynamic properties and the structural evolution with temperature of metallic glass-forming liquids should be monotomic trends. However, recent work demonstrates that dynamic evolutions of metallic glass-forming liquids are complicated. Moreover, enthalpy relaxation patterns in some metallic glass solids show non-contenious three-stage trends. These discoveries call out new challenges to the evolution features of metallic glass-forming liquids during solidification. Based on this background, the current project took Cu-based alloys as a starting point, and systematically investigated the dynamic behavior of both superheated melts and supercooled liquids of various metallic glass-forming liquids, and also discussed the correlated struccural evolution features. The following work has been carried out:1. We uncovered the liquid-liquid phase transition in CuZr-based melts through the temperature dependence of viscosity above the liquidus temperature, which is also accompanied with exothermic peaks by calorimetric method. Combined with DSC cycle curves and quantitative calculated results by fluid clusters model, we believe this reversible liquid-liquid phase transition is attributed to the structural transition from the strongly ordered high-density liquids to the weak-local low-density liquids upon cooling.2. We observed the dynamic fragile-to-strong transition in CuZr(Al) glass-forming liquids (GFLs). A detailed analysis of the dynamics of 98 glass-forming liquids demonstrated that the F-S transition occurs around Tf-s≈1.36Tg. Both the three-stage evolution pattern of medium-range ordering (MRO) structures during cooling and the significant differences of cluster stabilities of different temperature region indicated that the F-S transition in CuZr(Al) GFLs is attributed to the competition among the MRO clusters composed of different locally ordering configurations. A phenomenological scenario was proposed to explain the structural evolution from the fragile to the strong phase in the CuZr(Al) GFLs based on the structural evolution of cluster sizes and basic units in metallic glasses.3. In order to investigate the crystallization behavior of metallic glass-forming liquids with dynamic anomalies, we explored the effects of thermal histories in detail such as cooling rate, physical aging temperature and re-upscan rate in Ce-based, Pr-based, La-based, Cu-based and Zr-based metallic glasses. The results showed that similar to other dynamic properties, crystalline behavior of metallic glasses in nature has abnormal trends as well. These crystalline anomalies accord with the non-monotonic dynamic behavior in metallic glass-forming liquids.4. To uncover the connection between the slow β relaxation and liquid fragility, we investigated the activation energy of the slow β relaxation in ten metallic glass systems with different cooling rates by calorimetric method. Experimental results showed that for a certain metallic glass, a faster cooling rate of metallic glass corresponds to a higher fictive temperature, leading to smaller activation energy of slow β relaxation. A quantitative link between the critical fictive temperature Tf’ where slow β relaxation deviates from a relaxation and liquid fragility index m is derived:Tf,/Tg=1.054+0.00354m. |
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