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Study of Morphology and Effectiveness Mechanism of Mg2Si Phase on Mechanical Properties of Magnesium Alloy AZ91

Piryaee, Mohammad Mahdi | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42613 (07)
  4. University: Sharif University of Technology
  5. Department: Material Science and Engineering
  6. Advisor(s): Varahram, Nasser; Aashouri, Hossein
  7. Abstract:
  8. AZ91 alloy (Mg-9Al-1Zn-0.2Mn) is the most important Magnesium casting alloy and because of its high strength to weight ratio, is increasingly demanded in automobile and aerospace industry as the best alloy of the present century. Poor creep resistance of AZ91 alloy is attributed to poor elevated temperature properties of Mg17Al12, which has melting point of 437°C. The softening of the Mg17Al12 phase at elevated temperature is the main cause of grain boundary sliding. in this investigation, The effects of addition of Si element on the microstructure and mechanical properties of AZ91 alloy have been studied as a solution to produce high temperature resistant magnesium alloys. The results show that addition of Si leads to formation of Mg2Si with octahedral and Chinese script morphology. Preferential growth in <100> direction leads to formation of Mg2Si with octahedral morphology. Non-equlibrium cooling rate and presence of alloying elements such as Al and Zn lead to limitation of growth in <100> directions, which subsequently causes Mg2Si develop into other morphologies, such as hopper, truncated octahedral.Also Si increases stability of Mg17Al12 phase due to locating in Mg17Al12 lattice and replacing Al atoms. Also, Effects of Mg2Si phase on tension and creep properties have been studied. Generally, the results show Mg2Si decreases secondary creep rate and increases creep life of the AZ91 alloy. According to creep mechanism, addition of silicon at temperature range 130-150 °C and 30-50 Mpa leads to considerable increase of creep resistance, however in the range of 180°C and 50-70 Mpa it also increases the creep resistance. Secondary creep rate of alloy reduced from 5.17×10-81/s to 2.17×10-81/s by Si addition at 150°C and 50Mpa. Presence of Mg2Si prevents grain boundry sliding at grain boundries and diminishes softening effect of Mg17Al12 phase. However effect of coarsening and softening of Mg17Al12 phase at 180°C overcomes the benefical pining and strengthening effect of Mg2Si phase. Study of stress exponent and activation energy shows Mg2Si phase causes to prevention of moving of grain boundries and particle hardening which subsequently causes stress exponent to reduce from 2.87 to 1.1 . so that in the presence of Mg2Si the creep mechanism changes from grain boundary sliding and dislocation creep to grain boundary sliding and diffusion creep. The Mg2Si phase with Chinese script and octahedral leads to improvement of creep resistance, however deteriorates the tensile properties particularly, the ductility
  9. Keywords:
  10. Thermal Stability ; AZ91 Magnesium Alloy ; Creep Strength ; Creep Mechanism ; Magnesium-Silicon Morphology

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