Twinning Studies via Experiments and DFT-Mesoscale Formulation Huseyin Sehitoglu, University of Illinois at Urbana- Champaign, DMR 0803270 Identification of different twin systems in martensitic NiTi is accomplished utilizing digital image correlation (DIC) . DIC captures the high local strain field near the deformation twins. The results are also confirmed with TEM. The angles between (001) and (100) and (100) and twin are measured to be 97.8 0 and 18 0 respectively. 3 layer Twin 4 layer Twin 5 layer Twin Twin Migration Energies (TME) are shown for three twin modes, Type II-1 (transformation twin), (001) and (100) planes respectively in NiTi. TME is calculated using density functional theory. TME points to the energy barrier during the twin growth process and can be utilized for twin migration stress calculations. Theoretical twin growth stress is calculated using twin migration energy and compared with theoretical slip stress. We note that twin migration stress is lower compared to slip, and this is the fundamental reason why shape memory behavior is observed in NiTi. The twin migration stress of 165 MPa is consistent with experiments. Deformation by slip produces irrecoverable plastic strain whereas twinning deformation is recoverable. Evolution of different types of twins during deformation in the martensitic state is shown. Type II twins are visible from an austenitic transformation, and undergo a detwinning process at the beginning of the deformation. Afterwards, three twin systems (001), (100) and have been experimentally observed. (20 1) 60 50 40 30 20 10 0 Twin Migration Energy TM (mJ/m 2 ) 5.0 4.5 4.0 3.5 3.0 (001) (100) Type II-1 Ezaz,T., H. Sehitoglu, H.J.Maier, Energetics of Twinning in Martensitic NiTi, Acta Materialia, 59, 15, 5893-5904,2011 Ezaz,T., H. Sehitoglu, Coupled Shear and Shuffle Modes During Twin Growth in B2- NiTi,Applied Physics Letters,98, 241906 , 2011. Ezaz, T. H. Sehitoglu, Type II Detwinning in NiTi, Applied Physics Letters,98,14, 2011 (20 1) u x /b