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The Effects of Stress-Assisted Heat Treatment on the Microstructure and Phase Transformation Behavior of Ni-Rich Ni-Ti Shape Memory Alloy

Rahmanian, Rowshan | 2009

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39761 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Asgari, Sirous
  7. Abstract:
  8. Ni-Ti shape memory alloys have attracted most of the research in the field of shape memory alloys both because of their complicated and diverse phase transformation characteristics and their optimum properties for various applications which span such a properties as shape memory effect, pseudoelasticity, excellent mechanical properties and biocompatibility. The key properties of these alloys is generated through a thermoelastic martensitic transformation the occurrence of which is extremely sensitive to the metallurgical conditions of the alloy e.g. composition and microstructure. This has led to the development of various procedures implemented in either fabrication, processing or heat treatment stages among which heat treatment is of special importance, because it can be conducted even after the production of the parts. In the present study heat treatment procedures has been conducted on a Ni-Rich Ti- 50.23at.% alloy, in both stress-assisted and stress-free practices. The effects of such a treatment on the microstructure and phase transformation behavior of the alloy have been studied through TEM, XRD and DSC methods. Moreover, owing to the fact that most of shape-memory-based applications specifically in sensors, actuators and smart materials demand repeated heating and cooling of the alloy below and above the transformation temperatures, as a complementary part of this research thermal cycling has been applied on the alloy so that the effects of such cycles on the transformation behavior and durability of transformation characteristics can be studied. The results indicate that stress-free heat treatment facilitates the martensitic phase transformation, while implementation of stress-assisted heat treatment suppresses the transformation and splits its path in two distinct stages. Thermal cycling promotes the development of dislocation structures and induces the formation of intermediate R-phase transformation. The total transformable volume of the material reduces gradually with cycling. These trends have been carefully studied in each specimen and a comparison has been made between stress-assisted and stress-free heat treated samples.
  9. Keywords:
  10. Nickel-Titanium Shape Memory Alloy ; Stress Assisted Heat Treatment ; Martensitic Transformation ; Two Stage Martensitic Transformation ; Thermal Cycle ; Nickle Rich Alloy

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