Twinning Studies via Experiments and DFT-Mesoscale Formulation

1. Twinning Studies via Experiments and DFT-Mesoscale Formulation Huseyin Sehitoglu, University of Illinois at Urbana-Champaign, DMR 0803270 The purpose of this investigation is to understand the twinning mechanisms in B2 type alloys that exhibit shape memory, hence improve properties that lead to better actuation, sensing and damping applications. Our recent focus has been on NiTi alloy. 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. Figure 2 TEM image of NiTi in the martensitic state, (a) the self accomodating structure, (b) after deformation showing the presence of system (dashed line). Figure 1 Stress-strain response of NiTi in the martensitic phase showing the different twin systems 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 Ezaz, T.., H.Sehitoglu, W. Abuzaid, H.J.Maier,  Higher Order Twin Modes in Martensitic NiTi- The (201) Case, to appear in Materials Science and Engineering A, 2012 Twin Energy Barriers are shown for three twin modes in Table 1. The Type II-1 (transformation twin), (001) , (100) and planes respectively in NiTi. We note that unstable slip energy barriers are much higher for slip compared to twinning.

2. Twinning Studies via Experiments and DFT-Mesoscale Formulation Huseyin Sehitoglu, University of Illinois at Urbana-Champaign, DMR 0803270 Twin formation mechanism for the case showing the combination of shear and shuffle. The peak in twin energy barrier is 61 mJ/m2. Figure 3 (a) Shear and shuffles associated with creation of the twin (shuffles are shown with arrows) with orange cross showing the motif units; viewing direction is (b) PES (potential energy surface) for 3rd to 4th layer twin formation. The GSFE curve for the case shows rather high energy barriers exceeding 1300 mJ/m2. The major finding is that we are showing the conditions for optimizing shape memory alloys; a high slip resistance combined with ease of twinning is conducive to design of higher performance shape memory materials. Figure 4 (a) Shears associated with slip, (b) atomic position of possible dislocation glide in the system. The red curly parenthesis point to the short near neighbor distance once the atoms are moved a displacement of , (c) the GSFE curve associated with slip displaying substantially high energy barriers. Broad Impacts (1) Professor Sehitoglu was the co-organizer and co-chair of the Special Workshop on Shape Memory Alloys, Istanbul, Turkey. Students and faculty from US participated in this workshop. Special funds were made available for minority and women students to attend. (2) International Collaboration with German and Russian researchers continued. Professors Hans Maier from Germany, and Professors Yuriy Chumlyakov and Irina Kireeva from Russia are the collaborators. (3) Graduate Students, T. Ezaz, M.Sangid and W. Abuzaid presented their work in Materials Interest Group Seminars open to graduate and undergraduate students at University of Illinois. (4) Graduate students (W. Abuzaid, M. Sangid) presented their work to industrial groups (during the Fracture Control Program Short Course) particularly focusing on twinning, slip, and digital image correlation.