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An Investigation Into the Annealing Behavior of Severely Deformed Copper

Bisadi, Ghasem | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42165 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. Mechanical properties of commercial purity copper sheets are improved and grain size of material is refined into Nano scale after using a severe plastic deformation technique called constrained groove pressing (CGP). Since there are high levels of stored energy and non-equilibrium grain boundaries in nanostructure, annealing behavior and thermal stability of material should be investigated. Although there is not a clear distinction between annealing phenomena, it is essential to determine the intervals of annealing phenomena in order to control their sequences on mechanical properties and nanostructure. Also mechanisms of annealing phenomena should be determined to realize annealing behavior completely. However, some models are presented to investigate annealing phenomena till now. Because of some practical problems, in this study the modified JMAK model is utilized to investigate annealing phenomena. In this research, copper sheets are annealed in 150 °C to 400° C for 2 to 5200 seconds after CGP from one to four passes. Then kinetics and activation energy of annealing phenomena are determined and evaluated by utilizing Vickers indentation hardness test, Differential Scanning Calorimetry test (DSC) and assessing microstructure with optical microscope. Results represent that continuous recrystallization cannot occur even after four passes of CGP. Also, determining of annealing phenomena kinetics represents that recrystallization rate is increased with increasing annealing temperature and number of passes. Sometimes no recovery is seen for the high number of passes. Microstructural examination verifies the results of modified JMAK model and describes that nucleation occurs on old grain boundaries and shear bands. Calculated Avrami exponents confirm constant nucleation rate in copper. Activation energy of recovery introduces migration of vacancies as main mechanism in first stage of annealing and represents its independency on number of passes. Also, activation energy of recrystallization introduces migration of high angle grain boundary (HAGB) through core dislocation diffusion as main mechanism in second stage of annealing. Furthermore, grain boundary diffusion and migration of vacancies are as main mechanisms in grain growth, third stage of annealing. Results of DSC depict occurrence of recrystallization near 120°C and represent that recrystallization temperature is decreased with increasing pass numbers.
  9. Keywords:
  10. Copper ; Nanostructure ; Severe Plastic Deformation ; Annealing

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