| Metallic glass (MG), as a relative new member of glass family, offers novel mechanical, magnetic and electrochemical properties and has perspective potential applications in industry. Its unique nondirectional and densely packed structure provides an idea model system for studies of fundamental problems in condense matter physics, e.g. glass structure, glass forming, supercooled liquid behavior etc. It has been at the cutting edge of material researches for decades. The phase transitions in materials closely correlate with their structures and applications of switchable properties. The polyamorphic transitions in traditional amorphous materials have improved and extended our knowledge of amorphous matter. Generally, polyamorphic transitions occur in open-network glasses (coordination number<6) linking with the structure fluctuation in their relevant liquids. Thus, in principle it was thought that no polyamorphic transition would occur in MG because of its non-directional densely packed structure. In this thesis, by applying the state-of-the-art in-situ high-pressure synchrotron x-ray technologies to Ce-based MG as a model system, we discovered the polyamorphism in the Ce-based MG, as well as its underlying mechanism, and the emergent properties accompanying with the polyamorphic transitions. The results from this work also promise a new material synthesis method under high pressure in polyamorphous MGs. The main results are summarized as follows.(1) With in-situ high-pressure synchrotron x-ray diffraction (XRD) techniques, we investigated the compression behavior of Ce32La32Al16Ni5Cu15 bulk metallic glass (BMG), an amorphous-to-amorphous polyamorphic transition was discovered in BMG for the first time. Using in-situ high-pressure synchrotron XRD and x-ray absorption spectroscopy (XAS) techniques; we experimentally confirmed a low density amorphous (LDA) to high density amorphous (HDA) polyamorphic transition in a binary Ce75Al25 MG with high Ce concentration under hydrostatic pressure condition. Densification in this novel type of polyamorphic transition is dictated by the Ce 4f electronic transition from localized to itinerant state, which causes volume collapse. This transition is fundamentally different from the normal structural polyamorphism, in which coordination number changes and topological rearrangement of atoms occurs. These results in MGs extend our understanding about polyamorphism and may promote the searching for polyamorphism in other densely packed MGs, which could have pressure-tuned electron transitions. In addition, it was found that the pre-peak in the structure factor of Ce75Al25 MG showed less compressible behavior under pressure, which may improve our understanding of pre-peak in MGs.(2) Combining in-situ high-pressure and high-temperature energy dispersive XRD with in-situ high-pressure, low-temperature, four-probe resistance and magnetic measurements, we investigated the thermodynamic stability, electronic transport and magnetic properties accompanying the polyamorphic transition between LDA and HDA in Ce75Al25 MG. Compared with the La75Al25 MG sample, the change of properties was determined to associate with the 4f electron delocalization in Ce75Al25 MG. To the best of our knowledge, this is the first time in MGs that a pressure-tuned temperature coefficient of resistance (TCR), composition and magnetic field-tuned magnetoresistance were observed to change from negative to positive values. These obtained results will trigger more studies to discover polyamorphous MGs with interesting properties and potential applications. Moreover, they could be an interesting model system for the investigation of 4f electron behaviors. Additionally, we conducted the first successful in-situ high-pressure XRD experiment at the 15U, SSRF, which revealed another MG system having polyamorphic transition under pressure and minor alloying, e.g., Si, could modify the transition pressure and properties of LDA and HDA MGs.(3) Using in-situ high-pressure synchrotron XRD, we further investigated the high-pressure behavior of HDA Ce75Al25 MG. We discovered a crystallization of a new face-centered cubic (fcc) solid solution alloy from HDA Ce75Al25 MG at about 25 GPa. Normally, the Ce and Al with big radii difference (28%) and electronegativity difference (0.45) could not form substitutional solid solution alloys according to the Hume-Rothery rules. Synchrotron XRD, Ce L3-edge XAS, and ab-initio calculations revealed that the pressure-induced volume collapse and 4f electron delocalization of Ce reduced the differences in atomic size and electronegativity between Ce and Al and brought them within the Hume-Rothery limit for substitutional alloying. The novel alloy remained after complete release of pressure which was also accompanied by the transformation of Ce back to its ambient 4f electron localized state and reversal of the volume collapse, resulting in a novel non-Hume-Rothery alloy at ambient conditions.(4) Utilizing in-situ high-pressure synchrotron XRD, we discovered a novel pressure-induced "single crystal-like" crystallization in Ce75Al25 MG at room temperature. The transition was very fast and the crystalline fcc phase showed a fixed orientation with the starting Ce75Al25 MG ribbon sample, which was confirmed to be attributed to the intrinsic structure of the MG ribbon sample by many diagnostic XRD experiments. High resolution electronic microscopy and high resolution XRD results showed no inhomogeneity or stress existing in the starting Ce75Al25 MG ribbon sample. With the help of ab-initio calculations, we proposed that some kind of long range orientation structure was inherited during the melt-spinning synthesis process of the Ce75Al25 MG ribbon sample (possible some kind of cluster packing orientation). The pressure-induced "single crystal-like" crystallization in Ce75Al25 MG may not need diffusion process, rather collective adjustments, thus the hidden long range orientation in MG ribbon sample was awakened by such a transition to a single crystal. The origin of this novel phenomenon is not clear yet and needs further studies. However, these results obtained showing a novel type of pressure-induced single crystal-like polymorphic crystallization in MG, may promise a new method for synthesizing single crystal materials with controlled orientation from non-equilibrium systems.
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