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Effect of Cryogenic SPD with Various Strain Conditions on Development of Nano and Bimodal Structures in Al Alloys

Mohebbi, Mohammad Sadegh | 2015

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 47449 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Akbarzadeh, Abbas
  7. Abstract:
  8. Amount of strain has been always considered as the primary parameter affecting the microstructural and strength evolutions during Severe Plastic Deformation (SPD). However, these evolutions unavoidably saturate in large strains under deformation with constant conditions, i.e. constant deformation temperature, strain rate and strain path. In this study, effects of these factors on the microstructural and flow behavior of Ultra-Fine Grain (UFG) aluminum (AA 1050 and AA 5052) processed by Accumulative Roll-Bonding (ARB) are investigated. The flow behavior and its time-dependent aspect are evaluated up to large strains by Plane Strain Compression (PSC) and Stress Relaxation (SR) tests. Moreover, in order to understand the results of mechanical properties and the governing mechanisms, macrostructure, microstructure and texture of the specimens are investigated by optical microscopy, Electron BackScatter Diffraction (EBSD) and TEM analyses. The results revealed two important features of the mechanical behavior of UFG aluminum. First, when the grain refinement by high strain rate ARB is exceeded the saturation level corresponding to low strain rate PSC, a flow softening would occur at the beginning of the test until the preferred steady state deformation is established. Second, due to higher effective (or thermal) component of the flow stress and enhanced grain boundary slidingas well as recovery of the internal (athermal) componentof the flow stress, the degree of stressrelaxation is increased by increase of the number ofARB cycles of AA 1050. This viscoplastic nature is also revealed by strain rate sensitivity within a large range of strain rate by the SR test. Effects of deformation temperature are investigated by rolling process at ambient and cryogenic temperatures. It was shown that at both temperatures similar microstructural and strength evolutions are developed in annealed specimens because due to their coarse grains, dynamic restoration plays no important role in them. In fact, for this metals, the amount of deformation, and not its condition, is the determining factor. In contrary, for SPDed specimens in which grain refinement has been almost saturated, instead of the strain, the deformation condition is the determining factor, therefore cryo-rolling is effective on higher levels of grain refinement and strength. Meanwhile, the strain path, which is investigated by direct, cross and pseudo cross rollings, shows interesting effects, so that the ability to achieve enhanced grain refinement by cryo-rolling is more pronounced by cross or pseudo cross rollings in comparison to direct rolling. This is due to very strong texture created during direct rolling which causes a decreased fraction of high angle boundaries. During annealing of UFG AA 1050, bimodal grain structures are formed through secondary recrystallization in areas with no shear texture. Another form of bimodal grain structure is also created during annealing of UFG AA 5052 through partial recrystallization. EBSD analyses show that in specimens with initial strong texture including only a few components, rotation of crystal orientations during annealing drastically intensifies the texture components and consequently reduces the fraction of high angle boundaries. Therefore, cross rolled and pseudo cross rolled specimens with weaker texture components show more resistance against annealing
  9. Keywords:
  10. Ultra-Fine Grained Aluminum Sheet ; Flow Stress ; Plane Strain ; Strain Rate Sensitivity ; Bimodal Grain Size ; Severe Plastic Deformation ; Cryogenic Deformation ; Plane Strain Compression Test

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