Investigation into the Effect of Adding Sr on Microstructure, Mechanical Properties and Biodegradability Behavior of Biodegradable Mg-Zn-Sr Alloys

Gerashi, Ehsan | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55189 (07)
  4. University: Sharif University of Technolog
  5. Department: Materials Science and Engineering
  6. Advisor(s): Alizadeh, Reza
  7. Abstract:
  8. Biodegradable implants are new generation of implants that require no secondary surgery for their removal. Mg exhibits a great potential to be used as the biodegradable implant. However, mechanical properties and biocorrosion behavior of Mg are not suitable for being used as biodegradable implants. In this study, effects of Sr additions, heat treatments (T4 and T6), and multi-directional forging on the microstructural evolution, mechanical properties and biocorrosion behavior of Mg-4Zn-xSr alloys, and also the effect of hydrothermal coating on the biocorrosion behavior of Mg-4Zn-0.3Sr alloy were investigated. Potentiodynamic polarization and hydrogen evolution methods were used to investigate the biocorrosion of the alloys. Furthermore, shear punch and hardness tests were employed to determine the mechanical properties. When the addition of Sr to Mg-4Zn (base) alloy was 0.3 wt%, mechanical properties and the biocorrosion behavior of the base alloy were improved, as the result of obtained grain refinement (the grain size was reduced from 69.4 to 54.1 µm, the USS value increased by 6.5 MPa, and the current density was reduced by 1 µA/cm2). However, further increase of Sr content did not improve biocorrosion and mechanical properties, as the precipitates coarsened and formed a network-like shape, which deteriorated the properties. The addition of Sr resulted in the formation of new binary and ternary precipitates of Mg17Sr2 and Mg70Zn25Sr5 in addition to the precipitates present in the base alloy (MgZn2 and Mg4Zn7). It was shown that T4 treatment improved the degradation properties, as it reduced the precipitate content and decreased the micro-galvanic effect. However, T4 treatment deteriorated the mechanical properties slightly due to the softening effect caused by the dissolution of the precipitates. On the other hand, during T6 treatment, new tiny precipitates were formed, which significantly improved mechanical properties and indeed did not have any significant effects on the degradation properties of the Mg-4Zn-0.3Sr alloy. Furthermore, multi-directional forging significantly reduced the grain size, and the precipitates became smaller and dispersed more uniformly after this process. Therefore, the USS value increased (by 16 MPa) and the current density reduced to 1.91 µA/cm2 after multi-directional forging. Also, a hydrothermal coating was synthesized on the surface of the Mg-4Zn-0.3Sr alloy. Coating synthesis for 5 h at 160 °C made a homogeneous and crack-free coating, which increased the corrosion resistance dramatically (the current density reached to 0.48 µA/cm2). Besides, biocompatibility assessments showed that Mg(OH)2 coated samples have a better cell attachment and cell viability in comparison to the bare alloy
  9. Keywords:
  10. Biocompatibility ; Heat Treatment ; Mechanical Properties ; Multidirectional Forging ; Magnesium Alloy ; Strontium ; Coating ; Hydrothermal Method ; Biodegradable Magnesium Alloys

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