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Esham MS MSE Thesis 2017.pdf (9.22 MB)

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The Effect of Nanoscale Precipitates on the Templating of Martensite Twin Microstructure in NiTiHf High Temperature Shape Memory Alloys

Esham, Kathryn V
http://rave.ohiolink.edu/etdc/view?acc_num=osu1494251602171757

Abstract Details

2017, Master of Science, Ohio State University, Materials Science and Engineering.
Shape memory alloys (SMAs) are a class of materials that undergo a diffusionless, martensitic transformation. Due to their unique shape memory and superelastic properties, there is interest in implementing these alloys as lightweight, power dense actuators in high temperature environments like aerospace. The traditional SMA NiTi will undergo stable transformation up to 100C, but will not sustain a dependable lifetime beyond this. One method of extending the transformation temperatures is creating a ternary system based on NiTi. With the proper heat treatment and aging conditions, this third element will form optimally sized nanoprecipitates. In this work, the NiTiHf precipitation phase of interest is the H-phase. These precipitates work in two-fold: they stabilize the material over its lifetime by suppressing fatigue and ratcheting and raising transformation temperatures, yet they allow twinned martensite lathes to form. The mechanisms of this duality are not well defined. This work seeks to understand this phoneme using phase field finite element analysis. Phase field modeling incorporates the interfacial energy between the austenite and developing martensite front into the free energy. From this, the relationship between the martensite and austenite over time can be explored spatially and temporally to determine how the twinned martensite microstructure develops over time. This work incorporates several new features into the phase field investigations of the NiTiHf system. These simulations quenched the austenite system from 350K to 150K. The first feature is the introduction of an elliptical precipitate as non-transforming elements within the mesh. These inclusions influence show some templating of the microstructure, depending on the ratio of their major and minor axes. A precipitate with major axis 0.40 and minor axis 0.05 was chosen to represent the H-phase. It was rotated from 0 to 90 degrees within the matrix to investigate the interaction of orientation and the directional slip systems. This shows that the 45 degree angle allows the continuous inclusion of the precipitate into the twinned structure, while the 0 degree angle templates the microstructure to form twins that accommodate the precipitate along its major axis. Three cases: no precipitate, with a precipitate oriented at 0 degree and a precipitate orientated at 135 degree were explored. Next, plasticity was introduced to the system by varying the shear strength of the austenite from 450 MPa down to 100 MPa. While the shear strength did not have significant templating effects, it did introduce plastic strain development.The location of elements related to specific features was explored to determine if the element saw specific martensite or plastic strain development over time. The general concepts for this exploration are provided, but results here do not give statistically significant information. The introduction of strain misfit between the precipitate and matrix showed significant templating of the twin structure with the selection for one dominate variant and the earlier development of martensite. Slow cooling and a thermal cycle of the material were introduced next and showed limited templating without and significant templating with the misfit strain. Future work on this project should explore variations of the misfit strain, which is difficult to determine experimentally. The thermal cycling can also be extended to investigate effects of residual martensite.
Peter Anderson (Advisor)
Michael Mills (Committee Member)
156 p.
Materials Science
shape memory alloys, martensite twinning, martensite, austenite, SMAs, phase transformation, precipitate, misfit strain, thermal cycle, martensite formation, martensitic transformation

Recommended Citations

Citations

  • Esham, K. V. (2017). The Effect of Nanoscale Precipitates on the Templating of Martensite Twin Microstructure in NiTiHf High Temperature Shape Memory Alloys [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1494251602171757

    APA Style (7th edition)

  • Esham, Kathryn. The Effect of Nanoscale Precipitates on the Templating of Martensite Twin Microstructure in NiTiHf High Temperature Shape Memory Alloys. 2017. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1494251602171757.

    MLA Style (8th edition)

  • Esham, Kathryn. "The Effect of Nanoscale Precipitates on the Templating of Martensite Twin Microstructure in NiTiHf High Temperature Shape Memory Alloys." Master's thesis, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1494251602171757

    Chicago Manual of Style (17th edition)

osu1494251602171757
490
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