Loading...

Microstructure-Mechanical Properties Relationships in Aluminum Metal Matrix Nanocomposites Reinforced via Al3Ti and MgO phases Prepared by Reactive Friction stir Processing of AA5052 alloy with Initial TiO2 Nanoparticles

Khodabakhshi, Farzad | 2014

403 Viewed
  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 46114 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Simchi, Abdolreza
  7. Abstract:
  8. Ultrafine- and fine-grained aluminum matrix nanocomposites were fabricated by friction stir processing (FSP) of 5052 Al alloy sheets containing different volume fractions of pre-inserted TiO2 nanoparticles (2, 3, 5 and 6 vol%). Annealed and wrought (H32) sheets were processed at different rotational (w=800-1400 rpm) and traverse speeds (v=50-200 mm/min). A special design tool (concave shoulder and cylindrical pin with regular threads) without changing the rotation and traveling directions was utilized. It is shown that a homogenous distribution of the reinforcement particles throughout the metal matrix is attained at w=1200 rpm and v =50 mm/min after 4 cumulative FSP passes. Transmission electron microscopy reveals that in situ solid-state chemical reactions occur between the Al-Mg alloy and the ceramic nanoparticles upon the severe plastic deformation process. Consequently, MgO and Al3Ti nanophases with an average size of about 50 nm are formed. Crystallographic orientation relationships between Al3Ti and MgO phases with the aluminum matrix are (semi-coherent) and (non-coherent), respectively. Deformation-assisted interfacial reaction and deformation-assisted solution-precipitation are suggested as the dominant formation mechanisms for the new phases. The structure of precipitates and grains are also changed due to the applied severe plastic deformation, which ultimately affect the mechanical properties of the processed sheets. While the microstructure of the annealed aluminum alloy consists of a coarse grain structure, large (Fe, Mn, Cr)3SiAl12 particles and small Mg2Si precipitates, FSP results in a deformed grain structure containing rod-like Al-Fe-Mn-Si precipitates, and cuboidal (~100 nm) and spherical (~5 nm) Mg2Si particles. It is also shown that the initial thermo-mechanical history of the Al sheet strongly influences the microstructural features of the Al alloy after FSP. Occurrence of a static recrystallization prior to a continuous dynamic recrystallization is detected for the wrought Al-Mg alloy. Besides, a discontinuous dynamic recrystallization process is involved for the annealed sheets after FSP. Examinations of the mechanical properties of the Al sheets indicate significant improvement in Vickers hardness and tensile strength with increasing of the TiO2 concentrations up to 3 vol%. Although higher hardness values are attained at higher TiO2 concentrations, the tensile strength is remarkably reduced. Fractographic studies determine a change in the fracture mode from completely ductile rupture for the Al alloy before and after FSP to a ductile-brittle fracture for the nanocomposites. Tensile testing at different strain rates reveals that friction stir processing decreases the strain rate sensitivity and work hardening of the annealed Al-Mg alloy without and with TiO2 nanoparticles while for the wrought alloy opposite results are obtained. Evaluation of wear characteristics by a pin-on-disk setup displays an enhanced wear resistance and friction coefficient by incorporating of the nanoparticles in the metal matrix. Examination of the hot deformation behavior of the nanocomposites by means of uniaxial tensile test at temperatures of 300-450 ºC and strain rates of 0.001- 0.1 s-1 yields an apparent activation energy of 137 and 456 kJ mol-1 for and , respectively, which are about 30% and 77% higher than that of annealed Al-Mg alloy at the mentioned temperature ranges. It is also found that a post-annealing treatment can precede the solid-state reactions between the metal matrix and TiO2 nanoparticles; hence, the ductility of the nanocomposites is significantly enhanced (more than three times) without deteriorating the tensile strength and hardness. The highest strength and ductility are obtained for the nanocomposites annealed at 400ºC for 3 h. The effect of submerged friction stir processing under water-dry ice (~ -25°C) and liquid nitrogen (cryogenic temperature) on the microstructural features of the Al alloy is also elaborated. Ultrafine-grained nanocomposites with an average grain size of <200 nm are produced. The formation of the ultrafine-grained structure is accompanied by significant improvement in the yield strength (~170 MPa) and Vickers hardness (~165 HV). By employing experimental and theoretical models, a relationship between the microstructure and mechanical properties is established. Contributions of different mechanisms in the strengthening of the examined materials are theoretically elaborated. The role of grain boundaries and dislocations are found to be more effective on the strengthening that Orowan looping and solid solution hardening
  9. Keywords:
  10. Nanocomposite ; Microstructure ; Mechanical Properties ; Aluminum-Magnesium Alloy ; Friction Stir Welding ; Titanium Dioxide

 Digital Object List

 Bookmark

No TOC