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Investigating the Effects of Heat Treatment and Y Addition on the Microstructure, Mechanical Properties and Degradation Behavior of Bio- Mg–Zn-Y Alloys

Sharifi, Ali | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58310 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Alizadeh, Reza; Ashori, Hossein
  7. Abstract:
  8. The three main secondary phases in Mg-Zn-Y alloys are W (Mg3Zn3Y2 or Mg2Zn3Y3), (X-Phase) LPSO and I (Mg3Zn6Y), the amount and type of phases formed in the field depends on the Zn/Y ratio. When this ratio is less than 1, LPSO phase is formed more than other phases, when this ratio increases to 1.5-2, W phase begins to form and the phase becomes dominant, and with a further increase of this ratio, phase I begins to form and Finally, in a ratio greater than 5, only phase I is formed. In this research, four alloys Mg-1Zn-1Y, Mg2-Zn-1Y, Mg-2Zn-2Y, Mg-3Zn-1Y were studied. Microstructural studies were performed using optical and scanning electron microscopes, and the phases were identified using X-ray diffraction. It was observed that the LPSO phase is a single phase and the I and W phases are formed as a layer together with the background phase which is Mg-Zn (α-Mg) solid solution. The measurement of the distance between dendrites showed that the distance between dendrites in Mg-2Zn-2Y alloy is reduced by about %40 compared to Mg-1Zn-1Y alloy. Mechanical properties were evaluated using a shear punch test. In general, LPSO phase has more strengthening than phase I and phase I has more strength than phase W. Also, I and LPSO phases cause high deformability of Mg-Zn-Y alloys due to their compatibility with the background, but the presence of W phase in the background reduces the deformability of the alloy due to their compatibility with the background. The secondary phases have a higher electrochemical voltage than the α-Mg background and cause microgalvanic corrosion of the background, but in the continuation of the corrosion, they act as a barrier against the growth of corrosion and increase the corrosion resistance. In this context, LPSO phase with medium volume and by creating a continuous network can have the greatest effect in increasing corrosion resistance. Phase I can also prevent the growth of corrosion, but phase W has less effect in increasing corrosion resistance due to intergranular corrosion. Potentiodynamic polarization, EIS and hydrogen release corrosion tests of cast alloys were performed. The results showed that the Mg-2Zn-2Y alloy has the highest corrosion resistance, which is due to the presence of continuous precipitations of LPSO with medium volume in this alloy. The lowest corrosion resistance for Mg-2Zn-1Y alloy was due to the presence of W phase and the creation of intergranular corrosion in this alloy, and it is the most unstable oxide layer for this alloy. In addition, heat treatment at a temperature of 400 °C for 24 h was performed on the cast samples and the properties of the alloys were evaluated. After annealing: some of the precipitates are dissolved and the precipitates become more spherical, as a result the consistency of the precipitates decreases, the amount of LPSO phase decreases and the amount of W phase increases slightly. The strength of the samples decreased by about %13 after heat treatment. Also, in terms of corrosion, it was found that heat treatment increases corrosion resistance, which is due to the reduction of residual stresses and the formation of precipitations with uniform dispersion. After 240 h of immersing the samples in Hank's solution, precipitations were observed without corrosion
  9. Keywords:
  10. Heat Treatment ; Mechanical Properties ; Corrosion Behavior ; Yttrium ; Magnesium-Zinc-Yttrium Alloys ; Long Period Variable Stars ; Long-Period Stacking Ordered (LPSO)Phase

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