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 Title: The Effect of Stress State and Precipitation on Stress-Induced Martensitic Transformations in Polycrystalline and Single Crystal Shape Memory Alloys: Experiments and Micro-Mechanical Modeling Author(s): Gall, Kenneth Allen Doctoral Committee Chair(s): Sehitoglu, Huseyin Department / Program: Mechanical Engineering Discipline: Mechanical Engineering Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Engineering, Metallurgy Abstract: To properly understand stress-induced transformations in aged NiTi single crystals, a micro-mechanical model is used to quantify the stress fields outside perfectly coherent Ti$\sb{11}$Ni$\sb{14}$ precipitates in NiTi. Using the model, the decrease in the orientation dependence of $\sigma\sb{\rm cr}$ is linked to the unique orientation relationship that exists between the coherent precipitates in NiTi and the martensite correspondence variant pairs. As experimentally observed the model predicts that peak aged NiTi single crystals loaded under tension along the (100) orientation will show a dramatic decrease in $\sigma\sb{\rm cr}$ while those loaded along (111) will show less of a decrease in $\sigma\sb{\rm cr}.$ The effect of different aging treatments is properly explained by extending the local stress field model to the case of semi-coherent precipitates. In addition, the depletion of Ni in the matrix surrounding the precipitates is found to contribute to changes in $\sigma\sb{\rm cr}$ and M$\sb{\rm s}$ for aged NiTi alloys. Issue Date: 1998 Type: Text Language: English Description: 268 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1998. URI: http://hdl.handle.net/2142/83968 Other Identifier(s): (MiAaPQ)AAI9904460 Date Available in IDEALS: 2015-09-25 Date Deposited: 1998
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