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Microstructure Optimization of In-situ Al-Al3Ti Nanocomposite Fabricated by Mechanical Alloying & Hot Extrusion for Improving the Fracture Toughness

Basiri Tochaee, Ensie | 2016

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 48538 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Madaah Hosseini, Hamid Reza; Seyyed Reihani, Morteza
  7. Abstract:
  8. The aim of this research was to fabricate an in-situ Al-Al3Ti nanocomposite from pure microsized aluminum and titanium powders by mechanical alloying and hot extrusion without any usage of nanometric particles or applying secondary heat treatments. The focus of the present research was fabricating a nanocomposite material with high strength, modulus and hardness accompanied with high fracture toughness that could be comparable with aluminum alloys in terms of fracture toughness. Mechanical alloying and hot extrusion techniques were employed to produce in-situ fully dense Al-Ti composites. Platenary and high energy vibratiory mill was used to produce composites with different percent of titanium and milling time. Mechanical alloying was performed in order to homogeneous distribution of titanium in aluminum, formation of solid solution and refinment of aluminum grain size. The morphology of milled powders was studied by Scanning Electron Microscopy (SEM). The milled powders were then consolidated by hot extrusion at 530 ℃. The microstructure of the extruded composites was examined by Optical Microscopy (OM) and Field Emission Scanning Electron Microscopy (FESEM). Moreover, the density and hardness of samples were investigated. Three point bending test of the single edge notched beam (SENB) samples was applied to examine the fracture toughness behavior under the quasi-static condition. In order to find the effective mechanism of fracture, the crack propagation paths and the fracture surfaces were studied using optical microscopy and SEM, respectively. Finally thermal and structural stability of these composites was examined at 500 ℃ and 600 ℃ . The results showed that the morphology of the milled powders has a significant impact on the final mechanical properties of the extruded samples. The best bonding between particles corresponds to the powders with equiaxial morphology. The high energy vibratory mill was used to fabricate nano composite . The results indicated the improved distribution of particles with steady state equiaxial morphology and the formation of in-situ nanometric intermetallics in the ultrafine Al matrix in this sample. Accordingly, the highest density, the best mechanical properties and fracture toughness were obtained. The use of low energy planetary mill led to the formation of flattened particles, low density and subsequently, poor mechanical properties in other composite samples. The results indicated that after extrusion a fully dense Al-Al3Ti nanocomposite with the crystallite size of 200-400 nm is produced in which the Al3Ti particles with the size of 50-500 nm were homogeneously distributed. The mechanical properties of samples such as modulus, flexural strength and hardness was significantly increased to 130 GPa, 910 MPa and 136 HV, respectively. These results are the consequence of refinement of Al matrix grain size, distribution of Al3Ti nano-particles and the absence of voids. The high density of grain boundries and particle-matrix interfaces provide a lot of sites for nucleation of dimples which leads to the delay in crack initiation stage. Accordingly, the fracture toughness of the nanocomposite sample obtained K_Q=27.4 MPa√m which is a high value in comparison with the conventional Al composites. On a macroscopic scale, the fracture surface was brittle and flat, while SEM images showed dimpled fracture surfaces confirming microvoid coalescence as the main fracture mechanism. In addition, the good agreement between the experimental results of fracture toughness and the Rice & Johnson model predictions attained which confirms that this model is applicable for describing the fracture toughness of these composites
  9. Keywords:
  10. Aluminum Matrix Nanocomposite ; Mechanical Alloying ; Hot Extrusion ; Fracture Toughness ; Intermetallic Compounds ; Metal Matrix Composite (MMC)

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