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Investigation of Solidification Rate and Alloying Elements (Cu, Ni, Cr) on the Microstructure and Mechanical Properties of Al-20Si-5Fe Alloy

Rajabi, Mohammad | 2009

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 39360 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Davami, Parviz; Simchi, Abdolreza
  7. Abstract:
  8. In this research, Al-20Si-5Fe-2X (X=Cu,Ni,Cr) alloys were produced by gas atomizing and melt spinning processes. Microstructure and growth morphology of powder and ribbons were investigated using optical microscopy, scanning electron microscopy and transmission electron microscopy, then, analyzed by solidification models. Powder and chopped ribbons were hot pressed at 400°C/250 MPa and microstructural variations, mechanical properties and thermal coefficient expansion were evaluated after consolidation. Results show that relatively high cooling rate in gas-atomized powders lead to refinement of the microstructure and formation of semi-equilibrium δ intermetallic phase. As the powder particle size decreases, the growth morphology changes from primary silicon + eutectic to eutectic + primary dendritic aluminum. According to LKT and TMK models, experimental results were agreed with theoretical models. Cu and Ni exhibited only marginal influence on the microstructural refinement, while Cr changed the morphology of δ-intermetallic compound and refined the size of primary silicon. In melt-spun ribbons, two different zones (featureless with faceted growth morphology and Dendritic) were observed. Higher cooling rate in ribbons compared to powders extended solid solubility of alloying elements and suppressed the formation of any intermetallic phases. After hot consolidation, solubility of silicon and alloying elements at the matrix decreased and consequently, coarsening of Si and precipitation of iron intermetallic compounds in nanometric scale observed. Results of mechanical testing showed higher mechanical strength of melt-spun ribbons compared to the atomized powders, particularly at elevated temperatures. An improved strength was achieved by alloying with Cu, Ni and Cr, particularly in the Ni-containing alloy. Using rapid solidification alloys lead to improvement of high temperature strength and thermal coefficient expansion in the amount of 58% and 21%, respectively, compare to conventionally cast counterpart
  9. Keywords:
  10. Aluminum-Silicon Alloy ; Rapid Solidification ; Mechanical Properties ; Modeling ; Morphology ; Growth

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