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Investigation of Microstructural Characteristics and Mechanical Properties of Microalloyed Magnesium X0 via Severe Plastic Deformation Process with Cyclic Closed Die Forging

Kord Taminim, Mohammad | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56654 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Akbarzadeh, Abbas
  7. Abstract:
  8. Nowadays, the application of magnesium alloys as biodegradable implants in the medical industry has garnered significant attention. However, the low mechanical properties and rapid degradation rate of pure magnesium implants in the biological environment have hindered their widespread use for clinical applications. Magnesium-calcium alloys, due to their low degradation rate and promotion of faster bone healing, have become attractive choices for orthopedic applications. Nevertheless, the micro alloy X0 used in this study lacks sufficient strength and flexibility, limiting its potential applications. Utilizing severe plastic deformation methods to create ultrafine microstructures can greatly enhance the mechanical properties of magnesium alloys several-fold compared to their initial states. In this research, to improve the mechanical properties, the cyclic closed-die forging process was performed on three sample categories under isothermal and temperature reduction conditions at temperatures of 250, 300, 350, and 400 ℃, and strain rates of 0.01 and 0.05 s-1 under one, three, and five passes. The smallest average grain size in the isothermal and temperature reduction samples decreased by 97% and 96%, respectively, compared to the homogenized samples, reaching values of 8 and 10 microns, respectively. Due to the significant reduction in grain size and the achievement of a bimodal structure through dynamic recrystallization, the highest compressive strength increased by 122% compared to the homogenized sample, reaching 313 MPa, and the fracture strain increased by 26%, resulting in 22.9%. The average hardness also increased by 157%, reaching 76.3 Vickers after applying five passes of deformation at a temperature of 300 ℃ and a strain rate of 0.01 s-1. Furthermore, microstructural observations indicated that increasing the strain rate could accelerate the rate of refinement and homogenization of the microstructure by increasing the susceptible sites for recrystallization nucleation. Based on the values of compressive strength and fracture strain of the forged samples, it can be concluded that the magnesium-calcium micro alloy possesses desirable mechanical properties for use in orthopedic implants or stabilizing fixtures such as screws or bone plates
  9. Keywords:
  10. Magnesium Alloy ; Dynamic Recrystallization ; Severe Plastic Deformation ; Ultra Fine Grain Structure ; Closed-Die Forge ; X0 Magnesium Alloy ; Cyclic Closed Die Forging

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