Simulation of coupled transformation and plasticity in NiTi at elevated temperatures and stressesWednesday (16.05.2018) 12:35 - 13:00 Part of:
Description of mechanisms of plastic deformation on high quality NiTi superelastic wires is far from trivial, as basic understanding of these mechanisms is still missing. The mechanisms qualitatively change with temperature - from functional behaviors derived from martensitic transformation at low temperatures (<50oC) to temperature range, where large plastic deformation is generated alongside the martensitic transformation (100-250 oC) and, finally, to conventional plastic deformation of austenite beyond the MD temperature (> 300 oC) .
In this contribution we present a description of the observed phenomena based on phenomenological thermomechanical modeling. An original thermomechanical model for description of reversible behavior of NiTi SMA  capturing transformation and reorientation processes was further extended to cover also irreversible processes associated with plastic deformation. Conventional approach describing plastic deformation by metal plastic deformation models with considering phase dependent yield surface was found to be insufficient to describe the observed evolution of plastic strain especially for the temperature range 100-220oC where large plastic deformation is coupled with martensitic transformation. For accurate description of the observed phenomena, another TRIP-like mechanism connected to deformation twinning in martensite was necessary to introduce, which generates a large plastic strain during reverse phase transformation.
 L. Heller, P. Šittner, P. Sedlák, H.Seiner, O. Tyc, L. Kadeřávek, P. Sedmák, M.Vronka, Coupled martensitic transformation with plasticity in NiTi at high temperatures and stress, submitted to ActaMat.
 Sedlák, P. Frost, M., Benešová, B., Ben Zineb, T. and Šittner, P.,“Thermomechanical model for NiTi-based shape memory alloys including R-phase and material anisotropy under multi-axial loadings”, Int. J. Plast., 39, 132–151 (2012).