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Improvement of Mechanical Properties of a Mg-Zn alloy using the Micro Alloying Elements

Cheraghi Heyvedi, Hamid | 2014

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 46073 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali
  7. Abstract:
  8. The development of new wrought magnesium alloys for automotive industry has increased in recent years due to their high potential as structuralmaterials for low density and high strength/weight ratio demands. However, the poor mechanical properties of the magnesium alloys have led tosearch a new kind of magnesium alloys for better strength and ductility.In this research,a new type of magnesium alloy based on Mg-Zn-Si-Ca system has been developed using the permanent gravity casting process. For comparison, an alloy without Siby the same method was also produced. The effects of trace Si addition on the microstructure and mechanical properties in magnesium alloy with composition of Mg–1.5Zn–0.3Ca during castingand homogenization was evaluated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and standard ambient hardness and tensile testing. The results showed that Si exhibited a very low solubility in solid magnesiumand formed eutectic thermally stable Mg2Siprecipitates with a fish-bone shape morphology in Mg(α) matrix. The Si-bearing alloy exhibited a considerably high strength but lower ductility than the non-Si alloy at homogenized condition. The effects of rolling parameters including rolling temperature and reduction ratio per pass onmicrostructure, strength,and ductility in the Si-bearing alloy were studied to specify the best condition for rolling. By assessing the mechanical properties and optical microscopy, It was found the optimal rolling temperature and reduction ratio per pass for the Si-containing alloy are 400℃ and 25% respectively. Indeed the microstructures show that these optimal conditions is related to more grain refining and homogenizing of rolling microstructure. The optimal conditions were also operated for without Si alloy. It was considered that Si-bearing rolled alloy probably due to effect of PSN and strengthening of Mg2Si precipitates, has a finer grains and considerably higherultimate tensile strength than other (even elongation values approximately equal). Moreover, further annealing of the Si-bearing alloy and other alloy at high and low temperature for comparing the stability of microstructure was investigated. High temperature annealing led to a considerable increasing at elongation in Si-bearing alloy despite not too much decreasing strength, nonetheless in Si-without alloy both of strength and elongation decreased. In addition, Si markedly increased the thermal stability of the alloy structure
  9. Keywords:
  10. Mechanical Properties ; Scanning Electron Microscopy (SEM) ; Magnesium Alloy ; Silicon Without Magnesium Alloys ; Silicon Bearing Magnesium Alloys ; Magnesium-Silicon Precipitation

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