P. Sittner, Institute of Physics Academy of Sciences of the Czech Republic, Prague, Czech Republic; J. Pilch, Institute of Physics of the ASCR, v.v.i., Praha, Czech Republic; V. Gartnerova, B. Malard, Institute of Physics of the ASCR,v.v.i., Prague 8, Czech Republic
Physical processes taking place during the final heat treatment of cold worked NiTi elements gives them parent austenite shape, microstructure and functional properties as shape memory or superelasticity. Since the heat treatment in large extent controls the functional properties of NiTi, these processes are of significant application interest but, surprisingly, very little is known about them.
In this work, we will discuss the activity of physical processes taking place during the heat treatment performed on as drawn thin NiTi wires (d=0.1 mm) by passing electric current of constant power through them. The discussion is based on the results of: i) in-situ electrical resistance measurements during annealing using dedicated experimental equipment allowing to measure simultaneously the evolution of stress, strain, electric resistance, temperature on the NiTi wire exposed to high temperatures and stresses induced by electric power pulse, ii) in-situ synchrotron powder diffraction experiments during the heat treatment using this equipment at ESRF Grenoble and iii) ex situ high resolution transmission electron microscopy observations.
It is shown that the heavily deformed microstructure of as drawn NiTi wires changes into the annealed microstructure (~10-50 nm grain size) via a sequence of physical processes involving phase transformation, plastic deformation and diffusion. Depending on the temperature, time ( depending on parameters of the pulse of constant electric power) and applied mechanical constraint, the kinetics of individual processes can be suppressed or promoted which results in various functional behaviors of the NiTi wires as well as in various quality of the shape setting.
Summary: It is shown that the heavily deformed microstructure of as drawn NiTi wires changes into the annealed microstructure (~10-50 nm grain size) via a sequence of physical processes involving phase transformation, plastic deformation and diffusional processes taking place during heat treatment made by passing electric current of constant power through the wire. The kinetics of individual processes can be suppressed or promoted in which way the functional properties of the NiTi wires can be set as required.
