Low temperature shape setting of NiTi

Shape memory alloys, when subjected to elevated temperatures in deformed state with martensite phase prevented from the reverse martensitic transformation by external constraint, exhibit a unique deformation/transformation process coupling martensitic transformation with dislocation slip and twinning. In case of NiTi this scenario occurs at temperatures between 100 °C - 250 °C depending on the microstructure. The process has been called Low Temperature Shape Setting /LTSS/ because it can be used to set a new shape of NiTi component at relatively low temperatures with or without modifying its functional properties.

In this talk, we will summarize results of long term systematic investigations of the LTSS deformation mechanism carried out by thermomechanical testing with in-situ evaluation of: i) electric resistance, ii)  synchrotron x-ray diffraction, iii) digital image correlation, iv) unique ex-situ thermomechanical test and v) transmission electron microscopy of microstructure in LTSS treated NiTi wires.

It will be claimed that, besides the low temperature shape setting capability, LTSS is responsible for strain drift during actuation as well as for frequent malfunction of NiTi elements due to accidental overheating and can be used to imprint a very large two way SME in a simple one step processing manner.