Customization of SMA Wire Properties by Applying Short Pulse Joule Heating

Microstructure of thin NiTi SMA wires after cold drawing consists of highly deformed partially amorphous microstructure that depends significantly on the degree of the final cold work. The functional properties of the wires are set by final heat treatment which is actually a thermo mechanical heat-treatment during which the microstructure of the wire partially recovers through a sequence of complex recovery and recrystallization processes. An alternative method of heat treatment of thin SMA wires using the Joule heating /FTMT-EC – Final TermoMechanical Treatment by Electric Current/ has been recently proposed as a powerful tool for lab scale compatible industrial processing. The method in its single pulse form can be, however, also used for a kind of physical simulation of nonconventional heat treatment SMA wire by electric current.  Main features are the following: a) thermo mechanical treatment with FTMT-EC method is extremely fast (in millisecond range) compared to conventional heat treatment (minutes), b) adjusting of the microstructure is precisely repeatable and predictable, c) new thermo mechanical properties can be achieved. In this work, customization of the material and thermo mechanical properties of SMA and steel wires using the FTMT-EC method will be discussed with help of supplemental TEM and in-situ synchrotron x-ray diffraction experiments.