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Friction-Stir Processed Al-Al3Ti-MgO Hybrid Nanocomposites

Sahandi Zangabad, Parham | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49299 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Simchi, Abdolreza
  7. Abstract:
  8. In recent years, investigation of mechanical properties of micro- and nano-structure materials, and peculiarly nanocomposites have been attracted a wide variety of researches. In this regard, friction-stir-processed (FSPed) metal matrix nanocomposites have been considered an important case with appropriate mechanical properties especially for fabrication of ultra-fine grained Al alloys and Al-matrix nanocomposites. In this research, Al matrix hybrid nanocomposites were fabricated via multi passes-friction stir processing (FSP) of Al-Mg alloy (AA5052) with different amounts (~2 and 3 vol%) of pre-placed TiO2 (30 nm) particles nanoparticles. It was revealed that employing of cumulative FSP up to six passes with 100% overlap and without changing in the rotational and traveling directions between subsequent passes leads to more homogenous dispersion of incorporated initial TiO2 powders through the Al-Mg matrix and considerable developments in the volume fraction of reinforcing phases produced as in situ. Grain structural evolutions and in situ phase’s formation during FSP were monitored by using optical microscopy (OM), electron back scattering diffraction (EBSD), and transmission electron microscopy (TEM) techniques. In situ formation of Al3Ti (50 nm), and MgO (50 nm) particles nanophases within the fine grained Al matrix (<2 µm) as a result of solid-state chemical reactions was noticed. Universal tensile loading and uniaxial stress-controlled tension-tension fatigue testing’s (R=0.1) were utilized to assess the tensile flow and fatigue rupture behaviors of the prepared hybrid nanocomposites, and the results compared to annealed and processed alloys without reinforcing particles. Fatigue strength at 107 cycles for the processed nanocomposites with 2 and 3 vol% of TiO2 nanoparticles was increased up to 28 and 32% respectively, as compared to the annealed Al-Mg base metal beside of significant tensile strength improvements. Fractographic analysis illustrated the ductile fracture behavior with deep-equiaxed dimples on fracture surfaces for annealed and FSPed alloy whereas a ductile-brittle fracture mode with finer shallow dimples for the processed nanocomposites counterparts in tensile and fatigue loadings. Micro and macroscopic fracture features like striation, micro-cracks, micro-voids, and clam shell markings displayed the fatigue crack propagation step for the FSPed nanocomposites
  9. Keywords:
  10. Fatigue ; Microstructure ; Fractography ; Titanium Dioxide Nanoparticles ; Hybrid Nanocomposite ; In-Situ Nanocomposite

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