Twinning Studies via Experiments and DFT-Mesoscale Formulation Huseyin Sehitoglu, University of Illinois at Urbana-Champaign, DMR 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 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. Ezaz,T., H. Sehitoglu, H.J.Maier, Energetics of Twinning in Martensitic NiTi, Acta Materialia, 59, 15, ,2011 Ezaz,T., H. Sehitoglu, Coupled Shear and Shuffle Modes During Twin Growth in B2-NiTi,Applied Physics Letters,98, , Ezaz, T. H. Sehitoglu, Type II Detwinning in NiTi, Applied Physics Letters,98,14, 2011 u x /b

Twin formation mechanism in (001) and (100) is found to be different even though these two twins are conjugate of each other. Twinning partial forms in (001)[100] system and aids the nucleation and growth of twins. However, no twinning partial is possible in the (100) plane, and twinning occurs with a combination shear and shuffle mechanism only. Atomistic details on the left show the exact movement during the twin growth process. The energy barrier calculations for slip and twin systems in (001)[100] (top) and (100)[001] (bottom) respectively are shown for NiTi martensite. The slip energy barrier (GSFE) and the twin energy barrier (GPFE) share the same path up to the formation of twinning partial in the top-right curve pointing to the existence of the twinning partial during twin formation in (001) plane. However, slip and twin energy paths are different for the (100)[001] twin. Shuffles of atoms reduce the barrier upon required shear during twin formation in (100) plane and facilitate twinning growth. 4 layer twin u x / a[100]= mJ / m 2 50 mJ / m 2 u x / a[001]=0.20 Twinning Studies via Experiments and DFT-Mesoscale Formulation, Huseyin Sehitoglu, University of Illinois at Urbana-Champaign, DMR (001) twin (100) twin Ezaz,T., H. Sehitoglu, H.J.Maier, Energetics of Twinning in Martensitic NiTi, Acta Materialia, 59, 15, ,2011