Microstructure and functional superelasticity property changes in thin NiTi wires heat treated by electric current

Abstract

High energy synchrotron X-ray diffraction, transmission electron microscopy and mechanical testing were employed to investigate the evolution of microstructure, texture and functional superelastic properties of 0.1 mm thin as drawn Ni-Ti wires subjected to a nonconventional heat treatment by controlled electric current (FTMT-EC method). As drawn Ni-Ti wires were prestrained in tension and exposed to a sequence of short DC power pulses in the millisecond range. The annealing time in the FTMT-EC processing can be very short but the temperature and force could be very high compared to the conventional heat treatment of SMAs. It is shown that the heavily strained, partially amorphous microstructure of the as drawn Ni-Ti wire transforms under the effect of the DC pulse and tensile stress into a wide range of annealed nanosized microstructures depending on the pulse time. The functional superelastic properties and microstructures of the FTMT-EC treated Ni-Ti wire are comparable to those observed in straight annealed wires.

Effect of plastic slip on thermomechanical behavior of NiTi polycrystals investigated by micromechanics modelling

“…While the superelastic response of the 125W/12ms treated wire ( Fig. 1a) with nanosized microstructure (~50nm) is quite stable [3], unrecovered strain increases up to 3% upon tensile cycling of the 125W/16ms treated wire with grain size ~400nm (Fig. 1b).…”

“…On the other hand, other SMA alloys which are more resistant against plasticity as NiMnGa, CoNiAl or CuAlNi frequently fail by intergranular fracture in polycrystalline state. , dislocation activity on {110} slip systems (c,d) and internal stresses (d) (extinction contour along grain boundaries due to strain mismatch between grains) in superelastic NiTi wire subjected to 10 tensile cycles (Fort Wayne Metals #1 0,1mm thin wire heat treated by electric current pulse 125W/12ms (a) and 125W/16ms (b,c,d) [2,3]). …”