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Investigation of Toughening Mechanisms in Al/Al3Ti In-Situ Hot Extruded Composites

Rezaei, Alireza | 2014

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 46561 (07)
  4. University: Sharif University of Technology
  5. Department: Materials science and Engineering
  6. Advisor(s): Madaah Hosseini, Hamid Reza
  7. Abstract:
  8. In this study in-situ Al/Al3Ti was fabricated by mechanical alloying, hot extrusion and subsequent heat treatment. First, initial pure powders of Al and Ti were mechanically alloyed with 5 weight percentages of Ti for 40 hours. The as-hot extruded samples containing 5 wt% of initial Ti particles, were exposed to heating at 600 oC for various time intervals up to 10 hours. Microstructure analysis by OM and SEM proved that in-situ Al3Ti phase was formed as a layer around Ti particles by solid reactive diffusion between Al and Ti particles. After 10 hours of heating, all the Ti particles were consumed and transformed to Al3Ti. Mechanical properties of samples were investigated by Brinell hardness, tensile test and three point bending test. Three point bending test of notched samples was conducted to measure the fracture toughness of composites. The results revealed that the hardness of composites increased by lengthening the the heating duration. The tensile strength of Al-5wt% Ti composites increased as a result of the extension of the heating period up to a point of 6 hours; but since this point thereafter, the tensile strength decreased due to the porosity and the voids generated by reason of formation of Al3Ti. The tensile strength of the 6-hours heated sample was 2.53 times greater in comparison with that of the pure Al sample. Fracture toughness of Al-5wt% Ti composites declined by the increase of the heat treatment length, ranging over an interval of 13.5-25.9 MPa.m1/2. The main reasons of the fall in composites toughness were: the rise in volume fraction of Al3Ti, reduction of the mean free path between the particles and gradual elimination of core-shell structure. The crack blunting and crack deviation phenomena were detected in composites possessing core-shell structure. These two phenomena cause an increase both in toughness and in energy consumption level before the fracture. Through studying of the SEM fractography images, it was noted that there was a shift in fracture mechanism from particle fracture to particle decohesion from Al matrix as a result of the extension of the heating duration
  9. Keywords:
  10. Fracture Toughness ; Mechanical Alloying ; Hot Extrusion ; In-situ Metal Matrix Composite (MMCs)

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