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An Investigation into the Kinetics of Quench Aging in an Mg-Zn Alloy

Janbozorgi, Mohammad | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49023 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali
  7. Abstract:
  8. Special features of magnesium such as low density, perfect bio-compatibility in the human body, and close elastic modulus to the human bone have increased the use of Mg alloys in the bio-medical applications. Nevertheless, low mechanical properties and poor corrosion resistance of Mg alloys have limited their bio-medical applications. The addition of alloying elements together with the use of precipitation hardening and plastic deformation are the most effective methods to improve the mechanical properties and increase the corrosion resistance of Mg alloys. Thus, the present research with the aim of improving the mechanical properties of an Mg-Zn alloy by Ca and Gd additions together with rolling and aging treatments was carried out. At first, the microstructure and shear strength of Mg-2Zn-1Ca-1Gd (wt.%) alloy in the as-cast and as-rolled conditions were evaluated, and then, the effect of solution and aging treatments on the microstructure and shear strength of this alloy was investigated. The solution treatment was performed at 500°C, 550°C and 600°C, and the aging treatment was applied at 125°C, 150°C and 175°C. In order to accomplish the pre-aging and pre-solution treatments, the temperatures of 70°C and 350°C were used, respectively. The results of microstructure examination showed the formation of Ca2Mg6Zn3 and Mg3Gd2Zn3 secondary phases in the microstructure of all samples. The shear strength was decreased at about 21 % during solution treatment due to the dissolution of casting precipitates and increase in grain size, whereas, the precipitation process and morphological changes after aging treatment were led to increasing the shear strength (32 %). In addition, the solution-treated and peak-aged samples at one-step treatment exhibited the same grain size due to the presence of Ca2Mg6Zn3 phase in the grain boundaries. The shear strength was improved at about 11 % using two-step progressive aging treatment due to increase in morphological changes of Ca2Mg6Zn3 phase, whereas, decrease in volume fraction of Ca2Mg6Zn3 phase after two-step progressive solution treatment was led to weakening the shear strength (10 %). Moreover, the solution-treated samples of two-step treatments were more susceptible to age hardening compared with the solution-treated sample of one-step treatment due to the presence of more heterogeneous nucleation sites in two-step samples. Finally, the kinetics of precipitation hardening of the alloy was determined with the JMAK model, and a theoretical equation was derived to estimate the hardness values after performing the aging treatment at a given time and temperature
  9. Keywords:
  10. Precipitation Hardening ; Quench Aging Kinetics ; Rolling Process ; Magnesium Alloy ; Magnesium-Aluminum-Zinc Alloy ; Microstructure

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