Oxidation Behaviors Of A Zr-Based Bulk Metallic Glass

Bulk Metallic Glasses?BMGs?are a kind of relatively recent materials characterized by an amorphous structure and interesting properties,which are different from their crystalline counterparts.Unique properties make them promising for industrial uses as structural and engineering materials.Zr-based bulk glassy alloys particularly show outstanding glass-forming abilities.In this thesis,the oxidation of a Zr50.7Cu28Ni9Al12.3 BMG is studied at four temperatures,i.e.400°C,430°C?Tg?,460°C,and 520°C?Tx?for different durations.The microstructures of the oxide scale and the matrix,as well as the diffusion mechanism during the oxidation process,were studied.After an introduction on the available literature concerning the characteristics of BMGs,the procedure of sample preparation is presented.Samples with different crystallization volume fractions were obtained by annealing the studied Zr-based BMG in vacuum for different durations.The method determining the glass-transition and crystallization temperatures is also described.The observation of the oxidation kinetics of the samples oxidized at the different temperatures for a maximum duration of 10 hours show a decrease of the oxidation rate after some time,which is assumed to be caused by the crystallization of the amorphous matrix.X-Ray Diffraction?XRD?of the oxidized samples showed that the oxide scale is mostly composed of tetragonal zirconia?tZr O₂?at low temperatures?400°C?and/or short oxidation durations.With the time and temperature increasing,crystalline Cu and Cu O appear,together with some monoclinic zirconia?m-Zr O₂?.It is believed that the occurrence of m-Zr O₂ corresponds to the beginning of crystallization.During the oxidation process,the metastable t-Zr O₂ is converted into m-Zr O₂.For longer oxidation durations,several crystalline metallic phases appear.The first one identified is Al5Ni3 Zr phase,while Ni10Zr7 and Cu10Zr7 phases are observed for higher oxidation temperatures.At 520°C,the Ni10Zr7 phase is not observed in a sample annealed in vacuum for 10 hours.The oxidation of partially crystallized samples revealed that the crystalline phase is more resistant to oxidation than the amorphous one.The analysis of the microstructure through X-ray Photo-Spectroscopy?XPS?of a sample at early stages of oxidation reveals the presence of Al2O3 in the oxide scale.Al and Zr seem to be the first elements to be oxidized.Secondary Emission Microscopy?SEM?surface imaging coupled with Energy Dispersive Spectroscopy?EDS?showed that copper and copper oxide particles aggregate at the surface of the samples during the oxidation process.This suggests an outward diffusion of the copper atoms.The SEM observations on partially crystallized samples show that with the crystallization volume fraction of the samples increasing,the density of copper particles at the surface decrease.This confirms the role of the crystalline phase in slowing down the oxidation process.Transmission Electron Microscopy?TEM?allowed to observe and identify Ni10Zr7 and Al5Ni3 Zr crystals in the amorphous matrix of an oxidized BMG in the early stages of crystallization.The observation of the oxide-substrate interface of a sample oxidized for 10 hours at 460°C suggests that the nano-scale m-Zr O₂ forms at the frontier between the oxide and the matrix.According to the EDS mapping realized,the Ni atoms appear to be absent of the oxide layer near the interface.The spatial ordering of the atoms near the interface is described using HighResolution TEM images.The estimation of the thicknesses of the oxide layers of the different samples analyzed by cross-section EDS corroborate with the fact that the occurrence of crystallization during oxidation decreases the rate of the phenomenon.The evolution of the elements atomic percentage with the distance from the surface showed that the O anions diffuse inward and seem to confirm the outward diffusion of the Cu cations.The Zr cations might be diffusing inward during the oxidation process.The EDS also showed that the Al content remains relatively unaffected by the formation of the oxide scale.At high oxidation temperatures or long durations,the Ni atoms seem to be back-diffusing in the oxide scale to form a Ni-rich zone close to the interface that might favor the formation of the Ni10Zr7 crystalline compound.The oxide layers of the samples with different crystallization volume fractions showed almost the same thickness.