The Influence of Residual Martensite on the Two-Way Shape Memory Effect in NiTi Alloys

Historical observations into the origin of the two-way shape memory effect (TWSME) seen in NiTi alloys reveals a complex nature by which internal stress fields assist in repeatedly guiding a transition from the austenitic phase into the martensitic phase. Previous research has owed the origins of this effect to an intricate internal zones within the deformed regions of the microstructure; highlighting the importance of dislocations and precipitates within the material. The investigations here suggest an additional piece to this puzzle by showing that residual martensite left within the effected zones might also play an important role in seeding and selectively guiding the transition from austenite into martensite.

In this series of work, it is seen that the amount of recoverable TWSME is a strong function of the temperature at which the material is trained during a sharp micro-indentation process. Based off the Clausius-Clapeyron relation, it is expected that as temperatures continue past the austenite finish temperature the amount of martensite produced under the indenter will decrease. However, in this research it is also shown that as the amount of residual martensite post-indentation decreases so does the TWSME. Through this, it can be inferred that the residual martensite is a key component assisting the transition. This suggestion is bolstered through microscopy techniques such as Transmission Electron Microscopy and Laser Scanning Confocal Microscopy.