Phase transformations in NiTi at high stresses and temperatures - beyond the strain recoverability limitsThursday (17.05.2018) 09:00 - 09:45
Tensile deformation of medical grade NiTi wire was investigated in thermomechanical loading tests from -100°C to 350°C. Supplemental in-situ electrical resistance, synchrotron x-ray diffraction, dig-ital image correlation and ex-situ TEM studies were employed to characterize the defor-mation/transformation processes taking place in the wire deforming at various temperatures. A spe-cial attention was paid to high temperature (>100°C) - high stress (>1GPa) conditions, at which martensitic transformation does proceed but inelastic strains are not reversible anymore. In addition to the dislocation slip accompanying the reverse martensitic transformation on unloading or heating under stress , there is a reverse martensitic transformation into twinned austenite (B2=>)B19´=>B2T proceeding in thermomechanical cycles at high stress and temperature conditions . Since both the dislocation slip and B19´=>B2T transformation bring about irrecoverable plastic strains, they destroy the superelastic and actuation functionalities of the NiTi wire. Nevertheless, it still shows phase reversibility in closed loop thermomechanical cycles. The coupled transformation-plasticity renders NiTi excellent ductility and allows for its low temperature processing and shape setting.
 L. Heller, H.Seiner, P. Šittner, P. Sedlák, O. Tyc, L. Kadeřávek, On the plastic deformation ac-companying cyclic martensitic transformation in thermomechanically loaded NiTi, 2017, sub-mitted to Acta Materialia
 L. Heller, P. Šittner, P. Sedlák, H.Seiner, O. Tyc, L. Kadeřávek, P. Sedmák, M.Vronka, Cou-pled martensitic transformation with plasticity in NiTi at high temperatures and stress, 2017 submitted to Acta Materialia