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

268 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1998.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.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD