Oxidation behavior of silver-bearing zr-based metallic glasses
Student thesis: Doctoral Thesis
Oxidation behavior of silver-bearing zirconium-based (Zr-based) bulk metallic glasses and the corresponding silver-free ones has been studied through continuous heating and isothermal annealing repeatedly. Four types of Zr-based bulk metallic glasses, namely Zr42Cu42Al8Ag8, Zr46Cu46Al8, Zr53.2Co26.6Al15.2Ag5 and Zr56Co28Al16 alloys, were used in this investigation. Particular attention is directed to compare oxidation resistance between silver-free and silver-bearing alloys. The oxidation mechanisms are analyzed and discussed. The oxidation behavior of Zr42Cu42Al8Ag8 bulk metallic glass was studied over the temperature range of 603 to 733 K. The oxidation kinetics, measured with thermogravimetric analysis, of the metallic glass follows a single parabolic rate law from 603 K to 663 K and a two-stage parabolic rate law from 693 K to 733 K. Silver precipitations on the topmost oxide layer of the metallic glass were revealed by energy-dispersive X-ray spectroscopy together with X-ray diffraction pattern. Observation using scanning electron microscopy shows that metallic silver precipitated at all temperatures and some islands were formed on the outermost copper oxide layer at high temperatures. Multilayered oxide scales were also observed with silver precipitates sandwiched between copper-rich oxide layers and zirconium-rich oxides layers. Particular attention is drawn to the segregation of silver in the metallic glass induced by oxidation. Results showed that segregation is strongly dependent on annealing environment. The formation mechanism and the effect of segregation phenomenon during oxidation are discussed in view of composition, atomic mobility and chemical affinity of elements in the alloy system. The oxidation behavior of Zr46Cu46Al8 bulk metallic glass was studied over the temperature range of 663 to 723 K for comparison. The oxidation kinetics of the metallic glass follows a single parabolic rate law at 663 K and a two-stage parabolic rate law from 693 K to 723 K. Nodule shape copper oxides on the topmost oxide layer of the metallic glass were revealed at low temperature (below Tg) and needlelike whisker copper oxides were found on the nodule copper oxide. Comparison of the oxidation resistance between Zr46Cu46Al8 and Zr42Cu42Al8Ag8 shows that silver-bearing alloys have a better oxidation resistance below Tx. The possible reasons are discussed in terms of the oxide structures as well as diffusion coefficient of anions and ions in the glassy state and supercooled state. The oxidation behavior of Zr56Co28Al16 bulk metallic glass was studied in synthetic air over the temperature range of 673-923 K. The oxidation kinetics of the metallic glass follows a two-stage or single parabolic rate law from 773K to 923K which indicates that diffusion is the rate controlling step of the oxidation process. Cobalt oxides precipitated on the topmost oxide layer of the metallic glass during oxidation. Below the crystallization temperature, the scale formed after oxidation for ten hours was less than 2μm-thickness. The oxidation was accelerated at 823K. However, the oxidation resistance was enhanced at a further increased temperature 923K. The oxidation process is governed by the inward diffusion of oxygen and the outward diffusion of Co. The oxidation behavior of Zr53.2Co26.6Al15.2Ag5 metallic glasses was studied over the temperature range of 723 K to 923 K. The oxidation kinetics of the metallic glass follows a single parabolic rate law at 773 K and a two-stage parabolic rate law at 723 K to 923 K. Nodule shape cobalt oxides were also observed on the top surface and the amount and size increased with increasing temperature except that at 823 K. Silver phases were also observed in the XRD pattern but were not observed on the surface. As compared to the silver-free Zr-Co-Al metallic glasses, silver addition accelerates the oxidation of Zr56Al16Co28 bulk metallic glasses except that at temperature 823K. The possible reasons are discussed in terms of the oxide structures as well as diffusion coefficient of different elements in the alloys at different temperatures.
- Oxidation, Corrosion, Metallic glasses