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Characterization of Transformation-Induced Defects in Nickel Titanium Shape Memory Alloys

Bowers, Matthew
http://rave.ohiolink.edu/etdc/view?acc_num=osu1417649766

Abstract Details

2014, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Shape memory alloys have remarkable strain recovery properties that make them ideal candidates for many applications that include devices in the automotive, aerospace, medical, and MEMS industries. Although these materials are widely used today, their performance is hindered by poor dimensional stability resulting from cyclic degradation of the martensitic transformation behavior. This functional fatigue results in decreased work output and cyclic accumulation of permanent strain. To date, few studies have taken a fundamental approach to investigating the interaction between plasticity and martensite growth and propagation, which is vitally important to mitigating functional fatigue in future alloy development. The current work focuses on understanding the interplay of these deformation mechanisms in NiTi-based shape memory alloys under a variety of different thermomechanical test conditions. Micron-scale compression testing of NiTi shape memory alloy single crystals is undertaken in an effort to probe the mechanism of austenite dislocation generation. Mechanical testing is paired with post mortem defect analysis via diffraction contrast scanning transmission electron microscopy (STEM). Accompanied by micromechanics-based modeling of local stresses surrounding a martensite plate, these results demonstrate that the previously existing martensite and resulting austenite dislocation substructure are intimately related. A mechanism of transformation-induced dislocation generation is described in detail. A study of pure and load-biased thermal cycling of bulk polycrystalline NiTi is done for comparison of the transformation behavior and resultant defects to the stress-induced case. Post mortem and in situ STEM characterization demonstrate unique defect configurations in this test mode and STEM-based orientation mapping reveals local crystal rotation with increasing thermal cycles. Changes in both martensite and austenite microstructures are explored. The results for several different thermomechanical histories are discussed and a new mechanism of austenite grain refinement is proposed with support from ab initio calculations and crystallographic theory.
Michael Mills (Advisor)
Peter Anderson (Committee Member)
Yunzhi Wang (Committee Member)
Ronald Noebe (Committee Member)
177 p.
Materials Science

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Citations

  • Bowers, M. (2014). Characterization of Transformation-Induced Defects in Nickel Titanium Shape Memory Alloys [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1417649766

    APA Style (7th edition)

  • Bowers, Matthew. Characterization of Transformation-Induced Defects in Nickel Titanium Shape Memory Alloys. 2014. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1417649766.

    MLA Style (8th edition)

  • Bowers, Matthew. "Characterization of Transformation-Induced Defects in Nickel Titanium Shape Memory Alloys." Doctoral dissertation, Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1417649766

    Chicago Manual of Style (17th edition)

osu1417649766
109
© 2014, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.