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Study of Microstructure, Strengthening Mechanisms and Hot Deformation Behavior of Ultrafine-grained Al6063- Al203 Nanocomposites

Asgharzadeh, Hamed | 2010

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41311 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Simchi, Abdolreza
  7. Abstract:
  8. In this study, Al6063-Al203 nanocomposite powders were synthesized by reaction mechanical milling method. Nanometric reinforcement particles were formed via high- energy ball milling under a controlled oxygen containing atmosphere. Morphological and microstructural evolutions of nanocomposite powders were investigated by using X-ray diffraction (XRD), thermal analysis (DTA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods at different milling times. The results showed that mechanical milling stages were accelerated and the time for reaching steady- state condition was decreased by an increase in the oxygen content in the milling atmosphere. The in-situ synthesized reinforcement particles were amorphous Al203 with an average size of 25 nm that were homogeneously distributed in the nanostructured Al alloy matrix. The volume fraction of these particles was increased by increasing the partial pressure of oxygen in the milling vial. Investigation of the compressibility of the powders showed that compressibility of nanocomposite particles is higher than nano- and micro- structured ones at initial stages of compaction. Nevertheless, densification rate of nanocomposite powders is lower than nano- and micro-structured ones at higher pressures due to their inferior plastic deformation capacity. Hot powder extrusion at 450 OC with an extrusion ratio of 14:1 was utilized for consolidation of powders and billets with high density (>99% theoretical density) were produced. Microstructure of nanocomposite billets was investigated by optical microscopy, SEM, TEM and electron backscatter diffraction (EBSD) methods. The results revealed that a bimodal Al grain structure containing almost dislocation-free nanostructured grains as well as ultrafine grains with a relatively high density of dislocations was formed. Large fraction of high-angle grain boundaries (HABs) was observed in the microstructure of nanostructured Al6063 and Al6063-Al203 nanocomposite. The results of tensile and compression tests at room temperature revealed that a significant improvement in yield stress and ultimate strength of coarse-grained Al6063 was achieved by grain refinement and introduction of reinforcement particles into the Al matrix. Strengthening mechanisms were evaluated by Hall-Petch, Bowen and 0rowan models. It was found that 0rowan mechanism has an imperative effect on the yield stress of nanocomposite material through interaction of dislocations with nanometric precipitates and reinforcement particles. Hot deformation behavior of nanocomposite material was investigated by iso-thermal compression test at temperatures of 300-450 OC
    and strain rates of 0.01-1 s-1. The results showed that nanocomposite material has higher flow stress compared to nano- and micro-structured Al6063 at high temperatures. The stress-strain data was analyzed by Arrhenius and hyperbolic sine type equations. The activation energy for hot deformation of nanostructured Al6063 and Al6063-Al203 nanocomposite was determined to be 225-291 kJ mol-1. Microstructural observations on hot-deformed specimens revealed that dynamic recovery and dynamic recrystallization was occurred. The dynamic recrystallization mechanism was determined to be continuous dynamic recrystallization in that small grains surrounded by high-angle boundaries were formed by subgrains rotation.


  9. Keywords:
  10. Microstructure ; Hot Deformation ; Strengthening (Metal) ; Aluminum Oxides ; Aluminum Matrix Nanocomposite ; Metal Matrix Nanocomposite ; Aluminum Alloy 6063 ; Reaction Milling

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