Investigating the Microstructure, Mechanical Properties and Corrosion Resistance of Biodegradable Mg-Li-Zn Alloys

Zohrevand, Mohammad | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55370 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Alizadeh, Reza; Tavakoli, Rouhollah
  7. Abstract:
  8. Due to its biodegradability, magnesium can be a good option for making degradable medical implants that disappear in the body over time and thus there would be no need for secondary surgery. However, high rate of corrosion of magnesium in the body environment and low formability at room temperature are the two main problems of magnesium, which lead to rapid destruction of the sample in a short time and serious problems. For these reasons, much research is currently being done to improve the corrosion behavior and mechanical properties of magnesium alloys through microstructure modification. In this regard, special attention has been paid to Mg-Li alloys as the lightest magnesium alloys. The element Li, with a density lower than Mg, is known as one of the essential elements for the human body. The crystal structure of Mg-Li alloys depends on the amount of lithium. In the present study, in addition to investigating the mechanical properties and biocorrosion behavior of Mg-4Li-4Zn and Mg-8Li-4Zn alloys, the effect of Ca addition and multidirectional forging (MDF) process on the first alloy and the effect of hydrothermal coating on the second alloy were evaluated. According to the obtained results, by the addition of Ca to the Mg-4Li-4Zn alloy, in addition to the (Mg,Li)Zn2 particles, Ca- and Zn-rich secondary phases with the possible composition of Ca2(Mg, Li)6Zn3 are formed as a continuous network at the grain boundaries, which prevents the advancement of pitting corrosion and also makes it possible to achieve further strengthening from precipitate hardening. Multidirectional forging process also improved the mechanical properties and corrosion behavior of Mg-4Li-4Zn alloy through grain refining, and although more unifrom distribution of secondary phases. Homogenization heat treatment at 300 °C caused partial dissolution of the secondary phases in the Mg matrix, which led to a slight decrease in mechanical properties and improved corrosion behavior due to the reduction of the galvanic corrosion effect. The Mg-8Li-4Zn alloy, due to the presence of β-Lithium phase with bcc crystal structure, had high ductility and lower strength than the Mg-4Li-4Zn alloy. Application of hydrothermal coating on the Mg-8Li-4Zn alloy showed to decrease the corrosion rate considerably. This process significantly increased the corrosion resistance of this alloy and introduced it as a suitable candidate for medical stent application.
  9. Keywords:
  10. Multidirectional Forging ; Biodegradation ; Coating ; Mechanical Properties ; Magnesium-Lithium Alloy ; Microstructure ; Biodegradable Magnesium Alloys

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