Structure and mechanical properties of metallic nanoglasses

• Main Author: Nandam, Sree Harsha
• Format: Others
• Language: en
• Published: 2019
• Online Access: https://tuprints.ulb.tu-darmstadt.de/8702/1/Doctoral%20Thesis_Sree%20Harsha%20Nandam.pdf; Nandam, Sree Harsha <http://tuprints.ulb.tu-darmstadt.de/view/person/Nandam=3ASree_Harsha=3A=3A.html> (2019): Structure and mechanical properties of metallic nanoglasses.Darmstadt, Technische Universität, [Ph.D. Thesis]

Metallic nanoglasses are a new class of amorphous materials with interesting magnetic and mechanical properties. They are characterized by interfacial regions with enhanced free volume compared to the core of the nanoparticles. Till now, nanoglasses are primarily synthesized by using thermal evaporation in inert gas condensation (IGC). However, due to the different vapour pressure of constituent elements and reproducibility issues in thermal evaporation, it is difficult/impossible to synthesize different glassy compositions. In this work, by using magnetron sputtering in IGC, Cu50Zr50, Cu60Zr40 and Pd84Si16 nanoglasses are produced with completely amorphous nature and good reproducibility. By varying several parameters, the yield of the sputtering process in IGC is optimized to make sufficient amount of material to obtain a nanoglass pellet. The influence of several processing parameters like inert gas pressure, sputtering power, the type of material etc., on the yield of the process are studied in the current work. The primary aim of the current work is to study the properties of the nanoglasses and compare them with conventional metallic glasses produced by melt-spinning and thus comment on the relation between the structure and properties of nanoglasses. Structural characterization of the metallic nanoglasses showed that the samples are amorphous in nature. Elemental segregation in the samples was studied by atom probe tomography and significant segregation was found in Cu-Zr alloys while very little chemical inhomogeneity was observed in Pd-Si nanoglasses. Crystallization temperature was higher in Cu-Zr nanoglasses than that in melt-spun ribbons while Pd-Si nanoglasses showed lower glass transition and crystallization temperature compared to melt-spun ribbons. Mechanical properties of the nanoglasses and melt-spun ribbons were tested by indentation and micropillar compression tests. Hardness and elastic modulus were found to be higher in Cu-Zr and lower in Pd-Si nanoglasses compared to their corresponding melt-spun ribbons. Deformation mode was also found to be different in Cu-Zr and Pd-Si nanoglasses. While Cu-Zr nanoglasses deformed homogenously without the formation of shear bands during indentation, Pd-Si alloys showed shear bands around the indents. Similar results were also observed in micropillar tests of Pd-Si and Cu-Zr nanoglasses. Cu-Zr nanoglasses showed less catastrophic deformation compared to the melt-spun ribbons while shear banding was observed in both Pd-Si nanoglasses and melt-spun ribbons. With the help of molecular dynamic simulations, the effect of topological structure at the interfacial regions was studied in Pd-Si metallic nanoglasses. Simulation results conveyed that the fraction of major Si polyhedra i.e. Si[0,3,6,0] played an important role in determining the shear band formation and consequently the ductility of glassy Pd-Si alloys. With the increase in the fraction of Si[0,3,6,0] in the interfacial regions of Pd-Si nanoglasses, the mode of deformation changed from homogenous to heterogeneous one. The importance of chemical inhomogeneity on the thermal and mechanical properties of nanoglasses was described in detail based on a segregation model. Finally, Pd80Si20 thin film nanoglasses synthesized by conventional magnetron sputtering were also studied in the current work. No elemental segregation was observed in thin films. Annealing the nanoglassy thin films did not lead to any change in the globular nanostructure even after crystallization. The mode of deformation was practically the same as that in the rapidly quenched ribbon. The reasons for similar behaviour of the thin films and melt-spun ribbons are discussed.