PSI - Issue 2_A

O. Tyc et al. / Procedia Structural Integrity 2 (2016) 1489–1496 Author name / Structural Integrity Procedia 00 (2016) 000–000

1495

7

superelastic fatigue in the literature. Why the wires exhibiting low transformation strain show opposite trend is not clear at the moment. There is considerable decrease of the forward transformation stress and dissipated energy per cycle taking place upon cycling of all wires suggesting that microstructure of all wires changes significantly upon cycling. Surprisingly, the wires showing best fatigue performance exhibit same decrease of transformation stress upon cycling as the wire with the worst fatigue performance. This suggests that the microstructure evolution upon cycling is probably not the dominating factor controlling the fatigue performance. The furnace treated NiTi wires (exhibit recovered and precipitation hardened microstructure) show better fatigue performance than the electropulse treated wires having nanosized but partially recrystallized microstructure. The poorer fatigue performance of the latter can be due to the different microstructure but equally well it can be ascribed to larger transformation strains of electropulse treated wires. Fatigue fracture surfaces of the NiTi wires observed in SEM revealed the most likely reason for the preliminary fatigue failure of all wires. Major fatigue cracks causing the wire failure nucleated preferentially at nontransforming inclusions on the wire surface. It is assumed that cracks nucleate and propagate when the martensite band front of localized deformation passes over inclusions located at or near the surface. The larger the superelastic strain, the more pronounced is the incompatibility between the phase transforming matrix and hard inclusions leading to the nucleation and growth of cracks at inclusions and the shorter is the fatigue life. Why this is not the case for the furnace treated wires showing the low transformation strains remains unclear. Acknowledgements We kindly acknowledge the financial support from the student grant SGS ČVUT no. SGS16/249/OHK4/3T/14 and the Czech science foundation through project no. 16-20264S. References Figueiredo, A., M., Modenesi, P., Buono, V., 2009. Low-cycle fatigue life of superelastic NiTi wires. International Journal of Fatigue 31, 751-758. Eggeler, G., Hornbogen, E., Yawny, A., Heckmann, A., Wagner, M., 2004. Structural and functional fatigue of NiTi shape memory alloys. Materials Science and Engineering, 24-33. Maletta, C., Sgambitterra, E., Furgiuele, F., Casati, R., Tuissi, A, 2014. Fatigue properties of a pseudoelastic NiTi alloy: Strain ratcheting and hysteresis under cyclic tensile loading. International Journal of Fatigue 66, 78-85. Pelton, A.R., 2011. Nitinol Fatigue: A Review of Microstructures and Mechanisms. Journal of Materials Engineering and Performance 20, 613 617. Otsuka, K., Wayman, C.M., 1998. Shape Memory Materials, University Press, Cambridge, pp. 284. Kollerov, M., Lukina, E., Gusev, D., Moson, P., Wagstaff, P., 2013. Impact of material structure on the fatigue behaviour of NiTi leading to a modified Coffin–Manson equation. Materials Science & Engineering A 585, 356-362. Launey, M., Robertson, S. W., Vien, L., Senthilnathan, K., Chintapalli, P., Pelton, A. R., 2014. Influence of microstructural purity on the bending fatigue behavior of VAR-melted superelastic Nitinol. Journal of the mechanical behavior of biomedical material 34, 181-186. Malard, B., Pilch, J., Sittner, P., Delville, R., Curfs, C., 2011. In situ investigation of the fast microstructure evolution during electropulse treatment of cold drawn NiTi wires. Acta Materialia 59, 1542-1556. Delville, R., Malard, B., Pilch, J., Sittner, P., Schryvers, D., 2010. Microstructure changes during non-conventional heat treatment of thin Ni–Ti wires by pulsed electric current studied by transmission electron microscopy. Acta Materialia 58, 4503-4515. Pelton, A.R., Dicello, J., Miyazaki, S., 2000. Optimisation of processing and properties of medical grade Nitinol wire, Min Invas Ther & Allied Technol, 107–118. Sedmák, P., Šittner, P., Pilch, J., Heller, L, Kopeček, J., Wright, J., Sedlák, P., Frost, M., submitted 2016. Grain resolved analysis of localized deformation in NiTi wire under tensile load, Science. Zurbitu, J., Santamarta, R., Picornell, C., Gan, W.M., Brokmeier, H.-G., Aurrekoetxea, J., 2010. Impact fatigue behavior of superelastic NiTi shape memory alloy wires. Materials Science and Engineering A 528, 764-769. Sedmák, P., Šittner, P., Pilch, J., Curfs, C., 2015. Instability of cyclic superelastic deformation of NiTi investigated by synchrotron X-ray di ff raction. Acta Materialia 94, 257–270. Mahtabi, M.J., Shamsaei, N., Mitchell, M.R., 2015. Fatigue of Nitinol: The state-of-the-art and ongoing challenges. Journal of the mechanical behavior of biomedical material 50, 228–254. Hartl, D. J., Lagoudas, D. C., 2007. Aerospace applications of shape memory alloys, special issue paper 535, DOI: 10.1243/09544100JAERO211 Rahim, M., Frenzel, J., Frotscher, M., Pfetzing-Micklich, J., Steegmüller, R., Wohlschlӧgel, M., Mughrabi, H., Eggeler, G, 2013. Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys. Acta Materialia 61, 3667-3686.