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Investigating the Effects of Ca Additions and Thermomechanical Processing on the Microstructure, Mechanical Properties and Degradation Behavior of Mg-Y Based Alloys

Jorati, Sina | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57940 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Alizadeh, Reza; Aashuri, Hossein
  7. Abstract:
  8. In this study, the effects of calcium addition and T4 heat treatment at 450°C for 36 hours on the microstructure, mechanical properties, and corrosion resistance of the Mg-2.5Y alloy were comprehensively investigated. The studied alloys included the base Mg-2.5Y alloy and the Mg-2.5Y-0.2Ca alloy, which were prepared through casting. Microstructural analysis using optical microscopy, scanning electron microscopy, and X-ray diffraction revealed that the addition of 0.2 wt% calcium significantly reduced the grain size from 732±34 µm in the base alloy to 475±12 µm. After heat treatment, the dissolution of dendritic branches and secondary phases (Mg24Y5 and Mg2Ca) resulted in finer, more equiaxed grains, reducing the grain size to 589±28 µm in the Mg-2.5Y alloy and 393±11 µm in the Mg-2.5Y-0.2Ca alloy. The addition of calcium improved the ultimate shear strength in the as-cast condition, increasing it from 101.6 ± 3 MPa in the base alloy to 107.6 ± 4 MPa in the Mg-2.5Y-0.2Ca alloy (a 6.5% increase). This improvement was attributed to the formation of Mg2Ca precipitates alongside Mg24Y5, grain refinement, and solid solution strengthening in the calcium-containing alloy. After heat treatment, the ultimate shear strength decreased due to the dissolution of precipitates and softening effects, reaching 95.3 ± 2 MPa in the Mg-2.5Y alloy and 101.1 ± 2 MPa in the Mg-2.5Y-0.2Ca alloy. Consistent with the shear punch test results, Vickers hardness testing showed that calcium addition and heat treatment respectively increased and decreased hardness. The highest hardness value was observed in the as-cast Mg-2.5Y-0.2Ca alloy at 36.7 ± 3 HV, while the lowest was recorded in the heat-treated Mg-2.5Y alloy at 24.8 ± 1 HV. Electrochemical tests demonstrated that calcium addition promoted the rapid formation of a protective surface film and reduced corrosion current density. After calcium addition, the corrosion current density decreased from 6.11 × 10⁻⁵ A/cm² in the base alloy to 5.71 × 10⁻⁵ A/cm². Furthermore, heat treatment enhanced the corrosion behavior of both alloys due to the dissolution of precipitates and microstructural homogenization, reducing the corrosion current density to 1.62 × 10⁻⁵ A/cm² in the Mg-2.5Y alloy and 1.04×10⁻⁵ A/cm² in the Mg-2.5Y-0.2Ca alloy. EIS results corroborated these findings. Finally, hydrogen evolution tests indicated that heat-treated Mg-2.5Y-0.2Ca alloy exhibited superior performance in PBS solution, showing the lowest hydrogen release rate over 240 hours
  9. Keywords:
  10. Mechanical Properties ; Corrosion Resistance ; Biodegradable Implants ; Thermomechanical Processing ; Magnesium-Yttrium Alloy

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