Caught in the act: The structural pathway of liquid metals to vitrification monitored in situ by synchrotron X-ray diffraction.

When a metallic melt is undercooled fast enough below its liquidus temperature, crystallisation can be avoided and a metallic glass, i.e. a metallic solid with amorphous structure, be formed. This kind of solidification is called vitrification. The prerequisites for this phenomenon are still not clear. An extensive review of the available relevant literature was carried out. To reveal the structural changes taking place at the atomic scale during undercooling and vitrification, data obtained by ultrafast synchrotron X-ray diffraction during aerodynamic-levitation experiments of different metallic-glass forming liquids was analysed. The complete pathway from temperatures well above the liquidus temperature during undercooling and vitrification down to temperatures well below the glass-transition temperature Tg was studied. During undercooling, a non-linear evolution of structural metrics in real as well as in reciprocal space takes place. Especially the height of the first maximum in the structure factor can be described by a structural analogue to the Curie-Weiss law. This behaviour was also found in published data re-analysed here. Indications of universal behaviour among the investigated alloys below a certain temperature as well as for a liquid-liquid crossover in Ti₄₀Cu₃₄Pd₁₄Zr₁₀Sn₂ were found. Small differences in the temperature dependence of the structural behaviour among the different alloys are possibly related to their different glass-forming abilities. To facilitate the analysis of the real-space structure the novel concept of the anti-shell was introduced. Temperature affects different length scales differently. Below Tg the structural behaviour is dominated by the Debye-Waller factor as well as by normal thermal-expansion behaviour. Above Tg an apparent negative thermal expansion of the first nearest-neighbour distance can be attributed to the influence of the structure-forming processes. In addition to short- and medium-range order, a third structural range for distances beyond the third nearest-neighbour is proposed. A disordering of the atomic structure of metallic glasses by the introduction of further alloying elements, facilitated by emergent effects among the components, could be demonstrated. The importance of the influence of global electronic interactions on structure formation was shown as well as their limitation to distances beyond the third nearest-neighbour.