Structural Origins of Crystallization of Metallic Glasses in the Supercooled Liquid Region


Student thesis: Doctoral Thesis

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Award date27 Jul 2018


Bulk metallic glasses (BMGs) have attracted widespread attention. On one hand, their superior mechanical, chemical, and physical properties promote practical applications in technological fields. On the other hand, their disordered atomic structure and unique glass transition would attribute to the scientific understanding of glass formation. Upon heating to the supercooled liquid region, metastable metallic glasses (MGs) turn into unstable supercooled liquids and eventually suffer from devitrification. The devitrification behavior is closely related to the supercooled liquid phase stability, described by glass-forming ability (GFA), the nature of which is still puzzling. In this thesis, the crystallization pathway and atomic structural evolutions during the amorphous-to-crystalline transformations between a good glass former, Zr46Cu46Al8, and an average glass former, Zr56Cu36Al8, upon isothermal annealing in the supercooled liquid region have been experimentally compared with several kinds of diffraction techniques. By combining the pair distribution function (PDF) and molecular dynamic (MD) simulation, the long-range order development in a Zr56Cu36Al8 supercooled liquid from the perspective of cluster connectivity is presented.

In-situ transmission electron microscopy (TEM) and time-resolved neutron diffraction were used to study crystallization kinetics of two ternary BMGs during isothermal annealing in the supercooled liquid region. It is found that the crystallization of Zr56Cu36Al8, an average glass former, follows continuous nucleation and growth, while that of Zr46Cu46Al8, a better glass former, is characterized by site-saturated nucleation, followed by slow growth. Possible mechanisms for the observed differences and the relationship to the glass forming ability are discussed.

The atomic structural evolutions during the amorphous-to-crystalline transformation between Zr46Cu46Al8 and Zr56Cu36Al8 upon isothermal annealing in the supercooled liquid region were experimentally compared. In-situ synchrotron pair distribution function (PDF) measurements were conducted at the Advanced Photo Source (APS) of the Argonne National Laboratory (ANL) with a data rate of 1 second per patten. The PDF analyses showed that the initial states of both two glasses were almost identical. During devitrification, the short-range order (SRO) experienced a positive expansion in Zr56Cu46Al8, while in Zr46Cu46Al8, the SRO arrangement became denser during devitrification. Furthermore, the incubation time of the newly forming bonds in Zr46Cu46Al8 widely varied, while all the correlation shells of Zr56Cu36Al8 started to develop at almost the same time. Moreover, the high-resolution high voltage electron microscopy (HVEM) results illustrated that the isolated distributed nanocrystals with mutual orientation easily grew from the Zr56Cu36Al8; while the density population of poorly ordered atomic clusters with large misorientation suspended the growth in Zr46Cu46Al8. The medium-range ordering clusters could be responsible for the saturated nucleation and slow growth in Zr46Cu46Al8. In addition, Zr46Cu46Al8 finally crystallized into a crystalline phase with confined order; in contrast, Zr56Cu36Al8 crystallized into a crystalline phase with much more extended ordering. The results reveal that during devitrification, medium-range order (MRO) could be a key structural factor that affects the crystallization pathways.

The devitrification in the Zr56Cu36Al8 supercooled liquid from the perspective of cluster connectivity has been investigated by synchrotron diffraction and Molecular dynamic simulation modelling. The long-range order developed via cluster connectivity in the supercooled liquid. During devitrification, 2- and 4-atom connection motifs turned to be the most energetically-favored schemes and dominant in the final structure, while 3-atom shared connections resisted reducing the energy.

Overall, the crystallizations in two Zr-based supercooled glass-forming liquids were systematically investigated by both experimental characterization and simulation modelling. Our results reveal the strong correlation between crystallization behavior and the GFA. A detailed microscopic understanding of the crystallization kinetics and phase stability in supercooled liquids has indicated that the crystallization pathway is affected by MRO and final crystalline structure. Furthermore, we uncovered the original structural mechanisms behind the crystallization with respect to cluster connectivity at MRO range in MGs and suggest that the face-sharing connection scheme plays a key role in stabilizing the supercooled MG forming liquid and frustrate the crystallization. The outcome of this study has broad implications for the fundamental understanding of crystallization in disorder systems and glass formation.

    Research areas

  • Crystallization, Metallic glasses, Neutron Scattering, Synchrotron diffraction, Pair distribution function analysis