Atomic structure and structural stability of Fe90Sc10 nanoglasses

• Main Author: Wang, Chaomin
• Format: Others
• Language: en
• Published: 2017
• Online Access: https://tuprints.ulb.tu-darmstadt.de/6981/1/CMW-Atomic%20Structure%20and%20Structural%20Stability%20of%20Fe90Sc10%20Nanoglasses.pdf; Wang, Chaomin <http://tuprints.ulb.tu-darmstadt.de/view/person/Wang=3AChaomin=3A=3A.html> (2017): Atomic structure and structural stability of Fe90Sc10 nanoglasses.Darmstadt, Technische Universität, [Ph.D. Thesis]

Nanoglasses are non-crystalline solids whose internal structure is characterized by fluctuations of the free volume. Due to the typical dimensions of the structural features in the nanometer-range and the disordered atomic structure of the interfacial regions, the atomic structure and the structural stability of nanoglasses is not yet completely understood. Nanoglasses are typically produced by consolidation of glassy nanoparticles. Consequently, the basis for the understanding of the atomic structure of nanoglasses lies in the atomic structure of the primary glassy nanoparticles. Using electron energy loss spectroscopy, the elemental distribution in the Fe90Sc10 primary glassy nanoparticles and in the corresponding nanoglasses produced by consolidation of these glassy nanoparticles have been studied. Due to surface segregation, Fe has been found to be enriched at the surface of the primary Fe90Sc10 glassy nanoparticles. This behavior was found to be consistent with theoretical results based on a monolayer model for surface segregation behavior of the binary liquid alloys. In addition, the heterogeneous structure of Fe90Sc10 nanoglasses with Fe enriched interfaces was also directly observed, and may be attributed to the segregation of the primary glassy nanoparticles on the surface. Furthermore, the electron density of the isolated and loosely compacted primary glassy nanoparticles was investigated using small- and wide- angle X-ray scattering. The results indicate that the surface shells of glassy nanoparticles have an electron density that is lower than the electron density in the cores of the glassy nanoparticles. The lower electron density seems to result mainly from a lower atomic packing density of the surface shells rather than from compositional variations due to the surface segregation. During the consolidation of the glassy nanoparticles, the inhomogeneous elemental distribution and the short-range order in the shells of Fe90Sc10 glassy nanoparticles can be transferred into the interfaces of the resulting bulk Fe90Sc10 nanoglasses. The free volume within the shells of the Fe90Sc10 glassy nanoparticles may delocalize into the interfaces between the Fe90Sc10 glassy nanoparticles resulting in interfacial regions of lower atomic packing density in the Fe90Sc10 nanoglasses. The structural stability of Fe90Sc10 nanoglasses has been studied by means of low temperature annealing in situ in a transmission electron microscope, and ex situ in an ultra-high-vacuum tube-furnace. The analysis of both experiments showed similar results. The structure of the Fe90Sc10 nanoglasses was stable for up to 2 hours when annealed at 150 °C. Annealing of nanoglasses at higher temperatures resulted in the formation of a metastable nanocrystalline bcc-Fe(Sc) with Sc-enriched interfaces. The crystallization process of Fe90Sc10 nanoglasses was clarified and a plausible mechanism for the structural stability was proposed.