Loading...

Development of Nanostructural Al-Mg-Si Alloys using ECAE and Ageing Processes

Vaseghi, Majid | 2010

928 Viewed
  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41036 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali
  7. Abstract:
  8. The manufacture of ultra high strength materials has always been a target for aerospace and transportation industries. Currently, the limitation of energy resources even makes this goal more serious. Nowadays, more than 50% of total extrusion products are made from Al alloys and around 90% of them are the 6000 series alloys. Therefore, regarding to high strength, low weight, and hardening aluminum AA6000 alloys capabilities can play a major role in fulfilling this task. Over the last decade, a number of techniques collectively referred to as severe plastic deformation (SPD), have emerged as a promising approach for the production of bulk ultrafine-grained (UFG) nano-structured materials. Recently, equal-channel angular pressing/extrusion (ECAP/ECAE) became the most popular process among various SPD processes, aiming at bulk submicron or even nano-structured materials and resulting in excellent mechanical properties. In this research the dynamic ageing behavior of two Al-Mg-Si alloys in warm equal channel angular pressing (ECAP) was investigated in order to develop an ultra high strength Al alloy. Tensile tests were carried out over wide ranges of temperature and strain rate to evaluate the dynamic strain ageing conditions. ECAP processing was then experimentally performed at temperatures from room temperature up to 200˚C under various strain rates ranging between 10-4 and 10-1 s-1. An upper bound analysis for predicting the dynamic ageing was developed and its results were compared with experiments. The theoretical strain ageing region was found to be in the temperature range of 90-260˚C, which is in good agreement with the experimental observations in the temperature range of 75-175˚C. Followed by ECAP, Vickers microhardness measurement on the cross-sectional planes and microstructure examinations were undertaken using transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). Precipitations were examined using differential scanning calorimetry (DSC). It was found that the combination of the ECAP process with dynamic ageing at both 100 C and 150 C leads to a significant increase in hardness. The hardness value of the sample produced with the highest shear strain (4.4) was increased from 70.8 HV (as-solution treatment) to 163HV. The lowest average grain size was measured as ∼160 nm after 4 passes. The microhardness results showed that the hardness distribution is more homogenous after four passes by using pressing route BC. But if the deformation temperature was considered, it may be deduced that performing of ECAP at 100ºC may provide the most homogeneous microstructure after multi-pressing as long as the total pressing passes are times of four. It means that the lower temperature (in the range of warm working) is the most favorable for achieving an ultra-fine grained material with more homogenous microstructure. The fraction of high angle boundaries in the microstructure was calculated to be 0.72. Using DSC measurements it was found that the ECAP processing accelerates many steps of the ageing sequences. Activation energies were determined by measuring transformation temperatures. The minimum and maximum β” precipitation activation energy was calculated as 7.6 and 93.9 (kJ/mol), respectively. These data show that increasing the dislocation density via a multiplication number of passes causes solute atoms to diffuse quickly in the direction of dislocations and grain boundaries and therefore, forming new precipitations needs more activation energy. A comparison with the published data on the same alloy processed by ECAP at room temperature and statically aged, suggested several advantages in incorporating dynamic ageing with ECAP. These advantages consist of the ability to attain better grain refinement, increased hardness and the potential for saving time and energy
  9. Keywords:
  10. Precipitation Hardening ; Aluminum Alloy ; Nanostructured Materials ; Microhardness ; Equal Channel Angular Extrusion (ECAE) ; Dynamic Strain Aging ; Aluminum-Magnesium-Silicon Alloy

 Digital Object List

  • محتواي پايان نامه
  •   view

 Bookmark

No TOC