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Investigation of the Microstructural Characteristics and Mechanical Properties of Extruded Microtubes Made from Magnesium Microalloy X0 for Absorbable Vascular Stents Applications

Validoust, Mehrnoosh | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 57227 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Akbarzadeh Changiz, Abbas
  7. Abstract:
  8. Magnesium and its alloys have attracted significant attention in the field of medical applications, particularly biodegradable stents. However, due to inherent limitations such as weak mechanical properties and high corrosion rates, they have not been widely used. Alloy X0 has emerged as a suitable substitute for magnesium alloys with rare earth elements due to its corrosion resistance and cost-effectiveness in biomedical materials. In this study, the direct extrusion method was used to produce microtubes with suitable mechanical properties from microalloy X0 magnesium. At first step, hot compression tests were condueted on the homogenized microalloy X0 at four temperatures of 250, 300, 350, and 400 °C and two strain rates of 0.1 and 0.01 s^(-1) to determine the appropriate strain, temperature, and strain rate for hot extrusion. constitutional equations, material constants and activation energy for dynamic recrystallization were calculated. The activation energy for dynamic recrystallization of this alloy was determined to be 266.6 kJ/mol. Experimental results showed that the alloy undergoes dynamic recrystallization at temperature of 400 °C. At next step, hollow cylindrical samples with a diameter of 5.5, inner diameter of 2.5, and length of 23 millimeters were machined and drilled. The extrusion process was carried out at three temperatures of 300, 350, and 400 °C with a constant strain rate of 0.16 mm/s and an extrusion ratio of 1.6:1 for all three samples, resulting in the production of microtubes with a thickness of 0.5 millimeters and acceptable dimensional quality. Optical microscopy (OM), scanning electron microscopy (SEM), and Vickers hardness (HV) testing were conducted to study The microstructure, and hardness evolution. The grain size of the cast, homogenized, and extruded samples at temperatures of 300, 350, and 400 °C was calculated. The average grain size after direct extrusion at 350 °C was 9 micrometers, while for the other conditions, it reached to 27 and 11 micrometers. In all samples, a significant reduction in grain size compared to the homogenized initial sample was observed. Experimental results indicate a bimodal structure for most samples, indicating that the driving energy required for complete dynamic recrystallization was not provided. This is attributed to the low strain applied to the samples. The mechanical properties of the samples were examined using microhardness testing. Extrusion at 300 °C exhibited the highest hardness value of 49 Vickers, attributed to work hardening and increased density of dislocations in the microstructure. The extruded sample at 350°C showed a homogeneous structure and finer grains, indicating better conditions for the subsequent tube drawing process among other extruded conditions
  9. Keywords:
  10. Direct Extrusion ; Hot Compression ; Dynamic Recrystallization ; Microtube ; Biodegradable Magnesium Alloys ; Coronary Stent

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