CANCELLED HEMI Seminar: Huseyin Sehitoglu (UIUC)

In this presentation, we overview the importance of plastic deformation via slip in shape memory alloys (SMAs) utilizing results from the literature and our own work. The plastic flow affects the irrecoverability and hysteresis in shape memory alloys and typically initiates from austenite-martensite interfaces. Its occurrence in all SMAs including NiTi, CuZnAl, NiTiCu and Fe based SMAs is well-known, but the stress levels to trigger dislocation-mediated flow are not well established. Because these alloys have a cubic structure in the austenitic phase, we analyzed the spreading of the dislocation core and developed a theory to predict the critical resolved shear stress (CRSS). The theory is compared with experiments on single crystals and it predicts the tension-compression asymmetry and crystal orientation dependence of CRSS.  We outline future directions to predict the transformation stress utilizing similar concepts of  shearing on conjugate planes. Ideally, the design of new shape memory alloys must include a rapid assessment of their CRSS for slip and CRSS for transformation. A wide gap between the two, ie. high slip stress relative to transformation stress, would result in a large superelastic window and the achievement of high transformation strains close to theoretical levels.

Professor Sehitoglu received his BSc degree from City, University of London in 1979 and his MS  and PhD degrees from Theoretical and Applied Mechanics at University of Illinois in 1981 and 1983 respectively. He served as Director of Mechanics and Materials program at the National Science Foundation during 1991-1993. He received the Nadai Medal from ASME, Marcus Grossman Award from ASM (American Society of Metals International), Honorary Membership from DVM, German Association of Research and Testing and an honorary doctorate degree from Aalto University, Finland. He was the Head of Mechanical Science and Engineering Department at Illinois during 2004-2009. He is currently the Nyquist Chair in MechSE Department. He is the Founding Editor of the new Journal Shape Memory and Superelasticity published by ASM. He was one of the early researchers on thermo-mechanical fatigue and has advised a large number of PhD students of which 11 are in academic positions as professors. In more recent years, he has worked on shape memory materials. His areas of research are fatigue of materials, twinning and phase change in metals, shape memory materials and high temperature mechanical response of materials. The entire listing of his papers can be found at the website http://html.mechse.illinois.edu/publications/